By R L Hathaway
Soil Conservation Centre, Aokautere, Ministry of Works and Development,
Palmerston North.

Most growers agree that the provision of a well-designed shelter system is very important for the production of high quality horticultural crops in most parts of New Zealand. Considerable effort may be spent in determining the optimum design and species combination for the area and crop type to be protected, but of equal importance to design is shelterbelt management, especially in the first few years after planting.

Failure to apply the correct management during this period can result in slow growth rates, uneven tree size, poor low branching, unsatisfactory tree form, and gaps. Gaps in shelterbelts caused by tree deaths are particularly to be avoided, as it is very difficult to re-establish trees in the shelterbelt after it has reached a reasonable size.

Shelterbelts should be managed with the same care as the crop if maximum production of high quality crops are to be obtained.

Weed control

Probably the most difficult task in managing young shelterbelts is achieving adequate weed control within them without damaging or restricting tree growth. It is essential that newly established shelterbelts are kept weed free for at least the first two to three years after planting, and in the case of slow growing species, this may need to be extended to four or five years. Once trees have reached the desired height, an annual clean up is generally all that is required.

Some people have the impression that if they pour water on a shelterbelt, they do not have to worry about weed control. But, it’s no good providing the one without the other in shelterbelt management — the two have to go together, says scientist Bob Hathaway. Particularly when planting small seedling trees, such as casuarina or cryptomeria, weed control should come first. And shelterbelt irrigation must be planned and installed right from the start, instead of discovering in the February the year after planting that the trees will need more water than they are getting.

Herbicides

The use of herbicides is the most common means of controlling weed growth around newly planted shelter trees. This is also the most common cause of damage. Herbicides can be of two types: contact (including translocated) and residual.

Contact herbicides are used primarily to dessicate or kill existing weed or grass growth. They have no long term effect, and most are inactivated once in contact with the soil. They therefore have no effect on new weed seedlings which have germinated or emerged after herbicide application. Commonly used contact herbicides are paraquat, diquat, and glyphosate.

Residual herbicides control weeds by their effect on germinating weed seeds. They can be present and active in the soil for many months, and often can be absorbed through the root system of shelterbelt trees. Thus, it is of utmost importance when using residual herbicides to apply the chemical at the recommended rate for the tree species concerned.

Examples of the residual herbicides in common use are simazine, oxadiazon (“Ronstar”), diuron and terbumeton/terbuthylazine (“Caragard”). Some herbicides have limited contact action as well as a residual effect, e.g., diuron at higher rates, terbumeton/terbuthylazine, and amitrole/ ammonium thiocyanate (“Weedazol TL”).

Recommended application rates: It is not possible to give detailed recommendations for all species and soil types for the wide range of herbicides which are available. The rates given below have generally been found to be safe around most tree species used for horticultural shelterbelts. On heavy soils and soils with a high organic content, higher rates may be tolerated without damage to the trees. See Table 1.

Many cases of damage to poplars and willows have occurred when simazine or “Caragard” have been applied at rates higher than those in the table. If higher rates are required in order to achieve adequate weed control, it is suggested that a trial be carried out before large scale application. It may be more desirable to apply the herbicide at lower rates and at more frequent intervals rather than risk damage to the shelterbelt species.

Mulching

The use of black polythene film is a very suitable method of weed control when establishing shelterbelts of poplar and willow species from unrooted cuttings or stakes. Sawdust and bark can also be used very effectively as a mulch for weed control and to conserve moisture.

When these organic mulches are used, herbicides (especially residual types) should not be applied because it is likely that feeding roots from the shelterbelt trees will be growing very close to the surface of the mulch such that they can readily absorb toxic amounts of the chemical.

Additional nitrogen applications are necessary when mulching with sawdust.

Cultivation

Cultivation provides optimum growing conditions for young plants but is often not practicable on a large scale. A recent trial conducted at Aokautere to compare the effect of weed control methods on the growth of the hybrid willow clones “Tangoio” and “Makara” in a shelterbelt gave the following results two years after planting as cuttings. See Table 2.

Cultivation can sometimes result in mechanical damage to the stems of young trees which allows the entry of pathogens. For example, push-hoe damage to the stems of hybrid willows in the first year after planting resulted in Pythium infection in several districts. Great care must be taken not to damage the stems of plants, particularly near the soil surface.

Irrigation

Nearly all shelterbelt species respond to irrigation, even on sites with a moderately high rainfall. Irrigation is of most benefit during the establishment phase, and should be installed when the trees are planted. Irrigation design and installation is a large subject in its own right which cannot be dealt with here, and generally requires specialist advice.

The quantity of water to be applied can depend to some degree on the climate and soil type. The currently used general rule is 25 mm of water per m2 per week (i.e. 25 l/m2/week), either from irrigation or natural rainfall. Thus a typical two or three year old tree with a root spread of 1.5 m radius will require 175 l/tree/week.

This correlates reasonably well with measurements made by Dr W.R.N. Edwards at Aokautere of water use by four year old trees of Salix matsudana and Populus “Flevo”, grown in lysimeters, of 30 and 50 l/day respectively. Trees planted closer than 3 m in a row will probably require less water a tree; an application of 75 l per metre of row, in weeks where 25 mm of rainfall has not been reached, should be sufficient.

Pruning, trimming

Poplars and willows established from cuttings, or as rooted trees cut back after planting, need to be trained to a single leader or stem in the first season after planting. Up to four or five shoots may develop from the top of the cutting. These should be left to grow at least until January of the year following planting, when the straightest and tallest leader should be selected, and all the remaining shoots cut back.

They can be pruned right to the base of the shoot, or to a distance of 20 cm or so above the base. By adopting the latter technique, a number of new shoots will develop from the remaining section of stem, and fill in the base of the shelterbelt quite effectively. This is very desirable, especially in single row shelterbelts.

It may be necessary to remove double leaders from the dominant stem in the second year after planting. The aim of all training in poplar or willow shelterbelts is to maintain a single dominant leader on each plant.

Corrective pruning may also be necessary with evergreen species, particularly Casuarina species, to maintain a single dominant leader. Trees should not be topped until they have reached their final desired height.

Regular side trimming of most species will be required from the third year after planting to maintain a narrow shelterbelt of the optimum density. With fast growing species, such as Salix matsudana, S. matsudana hybrids and Populus “Flevo”, side trimming may need to be carried out each year to maintain the desired form. Casuarina sp., Cryptomeria japonica and alders may need side trimming only every second year.

With some species, the time of the year that trimming is carried out is important. Evergreen species are normally trimmed in the autumn, in time for new growth to harden off before winter frosts start; this is probably not critical in most horticultural areas of New Zealand.

However, poplar and willow shelterbelts should be trimmed only during mid- to late summer (February-April). There are two reasons for this. Firstly, these species are particularly susceptible to silver-leaf disease (caused by Chondrostereum purpureum) which gains entry to the tree through pruning wounds. The trees are most likely to be infected when pruning or trimming is carried out on wet days during winter, when the number of silverleaf spores in the atmosphere is at its highest. Thus winter pruning or trimming should be avoided as far as possible where there is a danger of silverleaf infection.

Secondly, poplars and willows can also be damaged by the lemon tree borer (Oemona hirta). The larvae of the borer burrow considerable distances down stems, weakening them, and often causing growth reduction and breakage.

Entry is also via pruning wounds. The adult insect lays its eggs on wounds, from where the larvae begin to bore into the stem. Because the main egg-laying period is in spring and early summer, delaying trimming until February-March will minimise the chances of entry of the larvae.

Root pruning

Competition with the crop for soil moisture and nutrients by shelterbelt species with extensive wide spreading root systems, such as poplars, willows and eucalypts, can be limited by root pruning at regular intervals. Where soil conditions are favourable roots of these species can be found up to 10m from the shelterbelt after only 3-4 years.

Ripping at a distance of 1.5-2m from the trees, and to a depth of 60cm, will reduce the lateral root spread to a large degree. Care should be taken to ensure that roots cut by the ripper are well separated to prevent rejoining. It may be necessary to make an additional run with the ripper in the opposite rip, to dislodge a complete section of root. Only one side of a shelterbelt should be ripped in any one year, with the other side being ripped every alternate year. This should avoid windthrow.

Fertiliser

Assuming a base dressing of fertiliser has been applied before planting to bring the nutrient status of the soil to a satisfactory level, additional fertiliser is not normally required in the first year. However, fast growing species such as willows, poplars and eucalypts may benefit from nitrogen applied in November, and again in January at a rate of 15 g/m2 (e.g., 30g urea or 60g calcium ammonium nitrate). Maintenance applications of an NPK fertiliser at the beginning of the growing season (August-September) for a further two or three years will usually be beneficial.

The rate required will depend to a large extent on the soil type, but an application of 50 g/m2 of a 12.10.10 or equivalent fertiliser will normally be sufficient. Fertiliser, in particular urea, should be spread evenly over the surface of the ground occupied by the root system, and not heaped up against the stem because this can cause severe damage. Urea and other nitrogenous fertilisers should be applied just before any rain to avoid losses to the atmosphere.

Good management right from the start will result in even growth in a shelterbelt, says MOWD scientist Bob Hathaway. For example, while gaps left by dead trees are usually filled in the second year of a shelterbelt’s growth, such gaps are a problem when they occur in an established belt. The problem can be solved by planting a more vigorous species in the gap, and also by using artificial shelter to help the new tree’s growth. Artificial shelter can also help where there is damage from windthrow.

Pests and disease

Mention has already been made of silverleaf and lemon tree borer with respect to trimming.
Other insect pests which can affect shelterbelt species include black beetle (Heteronychus arator) and grass grub (Costelytra zealandica), especially when the trees are young. Leaf roller larvae, of which several species are present in New Zealand, can damage many tree species, particularly eucalypts.

Eucalypts are also attacked by gum emperor caterpillars (Antherea eucalyptii) and the eucalypt tortoise beetle (Paropsis charybdis), both of which can rapidly defoliate younger trees. Some species of eucalypts are resistant to damage by Paropsis, but the gum emperor caterpillar will attack most species grown in New Zealand. Mites, particularly the two-spotted mite (Tetranythus urticae), can be a problem on Cryptomeria japonica. All of these insect pests can be readily controlled by applying the appropriate insecticide at the first sign of damage.

The effect of insect pests of shelter trees on the crop being sheltered is often of more concern than the effect on the shelterbelts themselves. Therefore a great deal of attention must be paid to controlling insects which affect both the crop and the shelter. The use of shelterbelt species which are not hosts to insects which damage the crop is obviously the best method of control, e.g., leaf roller caterpillars are very rarely found on Casuarina spp. Unfortunately, this solution may not always be practicable.

Other than silverleaf, stem and foliage diseases are not a major problem in shelter species. With the use of resistant poplar varieties poplar leaf rust is now of little significance, although in districts where the Lombardy poplar (P. nigra “Italica”) is still planted, regular spraying with copper oxy-chloride may be required throughout the growing season to retain adequate foliage.

Cypress canker causes dieback and cankers in Leyland cypress in some areas, where this is a problem, this species should not be relied upon, because there is no practical means of control. Root diseases, including white crown canker (Rigidosporus lineatus), and Phytophthera root rots are serious in some areas, but there is no practical means of control other than the use of resistant species of trees.

Hares and rabbits

Population levels of hares and rabbits appear to be increasing in many horticultural areas of New Zealand, and browsing damage to newly planted trees of some shelter species is a common occurrence. Some species, such as Casuarina sp. are particularly susceptible to browsing damage, and even willows can be damaged by bark chewing.

Reducing population by poisoning or shooting is the best method of avoiding damage, but even an occasional visit by a hare can result in many young trees being damaged. Low electric fences can be used to deter both hares and rabbits, although they are not always a completely effective barrier.

Trees can be individually protected by using small netting cages, or plastic netting stretched over a wire frame. Various chemical deterrents, including systemic substances actually absorbed by the tree, are currently under investigation at Aokautere, with some showing potential.
Commercial preparations containing thiram (e.g. “Thiropel”) are available, and have proven very effective for some species. The material can be painted onto the stems, or sprayed over the foliage of young plants, where it can remain effective for up to three months.

Conclusion

It can be seen that management of shelterbelts, particularly young shelterbelts, involves many factors, and contrary to the opinion of many intending growers, it is not simply a matter of planting the trees, then standing back and watching them grow.
Shelterbelt management is equally as important as crop management and a significant input of resources is required, especially during the first few years after planting, if good results are to be obtained.

From Growing Today, November 1983.

by R. L. Hathaway
Soil Conservation Centre,
Ministry of Works and Development
Aokautere

One of the major decisions to be made in developing a horticultural property is that of which tree species or species combination to plant for wind protection. For many growers, this is a difficult decision, despite the fact that considerable experience with trees for windbreak establishment has been gained over recent years in New Zealand.

The main reason for this difficulty is that there is no single species or species combination that is suitable for all of the environments now being developed in horticulture.

Major factors that affect species performance are:

1. Susceptibility to frost damage
2. Soil texture and drainage
3. Droughtiness of the site
4. Presence of salt laden winds

Additional factors which can influence the choice of species are:

1. How soon the crop is to be planted i.e., the growth rate required of the shelter trees
2. The degree of wind reduction required
3. The period during the year when wind protection is required
4. The degree of cold air drainage required to limit frost damage
5. Host to insect pests which affect the crop
6. Competition for bees for crop pollination
7. Whether invasion of tile drains by roots is likely to be a problem
8. The effect of the root system on crop performance by competition for water and nutrients
9. The availability of irrigation.

Despite all these variables, a relatively limited range of species is planted for horticulture shelter. This article describes the major species used, and also discusses some new options.
MAJOR SPECIES are listed in Table 1.

Willows

Willows are widely used for horticultural shelter in many areas, largely because of their fast growth rate and ease of establishment. The species most commonly planted are Salix matsudana, and the recently developed Salix matsudana x alba hybrids.

Salix matsudana has been planted very widely since 1973, when the arrival of the poplar rusts in New Zealand severely limited the usefulness of the Lombardy poplar Populus nigra “Italica”).

It is fast growing, with somewhat pendulous lower branches, and develops a wide-spreading crown if now trimmed. It is not affected by any serious pest or disease in New Zealand.

Salix matsudana x alba hybrids. These new hybrids are faster growing and have less pendulous branches than 5. matsudana, and have other desirable characteristics such as longer leaf retention and very good low branch retention. The first released and most commonly planted clone is “Aokautere” (NZ 1002). This was not selected specifically for shelterbelt planting, but its rapid growth rate and narrower crown than S. matsudana were attractive to horticulturalists. Its major disadvantages are a shorter leafing period and lower drought tolerance than matsudana.
The Soil Conservation Centre of the Ministry of Works and Development at Aokautere released in 1980 a further three new S. matsudana x alba clones specifically selected for horticultural shelter planting, and these are now becoming available commercially. All three clones are faster growing than the commonly planted S. matsudana, and are most useful in situations where shelter is required as quickly as possible.

Although similar in many respects to clone “Aokautere” they have generally straighter stem form and each clone has some specific characteristics which make it useful for shelterbelt planting. They are all completely resistant, as is S. matsudana, to the weeping willow rust which recently arrived in New Zealand, and do not harbour any pests or diseases which can affect horticultural crops.

Salix matsudana x alba cl. “Tangoio” (NZ 1040).
“Tangoio” has attractive light grey foliage similar to that of S. matsudana, and a dense moderately spreading crown, which would eventually need side trimming in an orchard. Its main feature is its excellent low branch retention which makes it unnecessary to interplant with any other species.

Tree form is very straight, with good apical dominance and little forking of leaders. “Tangoio” responds well to side trimming and can be kept quite narrow yet reasonably dense. Sex is female.

Salix matsudana x alba cl. “Makara” (NZ 1179).
The very fast growth of “Makara” makes it suitable for tall shelter, although the maximum height is not known as yet. The crown is lighter than “Tangoio” but considerably more narrow, with blue-green foliage. Stem form is straight. The lower side branches do not persist well in dense plantings, but are retained adequately in single row shelterbelts.
It is best interplanted with a shrubby species, or can be planted alternately in the row with “Tangoio”. Trees of “Makara” are generally in full leaf until early June, 3-4 weeks later than the clone Aokautere (“NZ 1002”), making it especially suitable for sheltering kiwifruit and subtropical fruit crops. Sex is female.

Salix matsudana x alba cl. “Moutere” (NZ 1184).
“Moutere” is probably the best all round willow for horticultural shelter, because it has a reasonably narrow crown, with slightly broader leaves than the above two clones, and forms a fairly dense but permeable windbreak under most conditions. The foliage is darkish blue-green and the stem form is straight. Like “Makara” this clone is generally in full leaf until early June. Growth rate is almost as fast as “Makara” and because lower side branches are retained well, ‘Moutere’ can be used in single row shelterbelts without underplanting. Sex is male.

Both S. matsudana and its hybrids are susceptible to damage from salt winds, and cannot be grown successfully in areas exposed to strong salt-laden winds. They will grow readily on a wide range of soil types, but under dry conditions will require irrigation.

Poplars

Since 1973, repeated epidemics of poplar rust have caused either death or severe reductions in growth of Populus nigra ‘Italica’ (Lombardy poplar) and P. nigra ‘Sempervirens’ (semi-evergreen Lombardy poplar). These trees can no longer be recommended for shelter planting except in areas of low humidity in the South Island, or where regular spraying with copper based fungicides is proposed.

Rust resistant species and clones selected by the Soil Conservation Centre, include Populus x euramericana ‘Flevo’, P. x euramericana “Tasman” and P. trichocarpa.

Populus x euramericana “Flevo”. This has been the only rust resistant poplar available until recently, and has been widely planted for horticultural shelter. It is also resistant to poplar leaf spot (Marssonina brunnea). However, it has a much more spreading crown than Lombardy type poplars and is suitable more for boundary shelter than for internal shelter. It rapidly grows into a large tree, and requires heavy side trimming.

It grows best on friable, well drained soils with adequate soil moisture. It is less drought sensitive than willows, and will retain its foliage through quite dry periods. “Flevo” comes into leaf in mid to late September, and loses its leaves in mid-April. It is slightly more tolerant than willows to salt laden winds.

Populus x euramericana “Tasman”. This clone together with P. trichocarpa was recently released by the Soil Conservation Centre, Aokautere, and is rapidly becoming popular for horticultural shelter. “Tasman” was bred in Holland in 1965, and introduced to New Zealand in 1973.

It has a considerably better form than “Flevo” with a straight stem and finely-branched fairly narrow crown. The growth rate is similar to or slightly faster than “Flevo”. It is highly resistant to the two poplar leaf rusts in New Zealand, but slightly susceptible to the leaf spot disease Marssonina brunnea which, although not detrimental to vigorous growth, may cause early autumn defoliation following high summer rainfall. Its leafing period and salt wind and drought tolerance are almost identical to those of ‘Flevo’.

Populus trichocarpa. This is a balsam poplar, originally selected in Belgium, and released in New Zealand in 1979. Its growth rate is reasonably rapid, and it is highly resistant to both poplar rusts and to Marssonina. Although not suitable for very exposed situations, it is promising for horticultural shelter because it retains a very good low branch structure and autumn leaf fall is not until mid-June. However, it is very late coming into leaf in the spring.

Other species

Pinus radiata
Radiata pine is normally planted where other species cannot be successfully grown because of exposure to salt-laden winds, or where very tall evergreen boundary shelter is required. Trees are inexpensive, and where correct management is practiced this species can provide a very effective shelterbelt.

Trees must be side-trimmed from an early age to prevent their opening at the base. However, this will eventually occur, and underplanting with a shade tolerant lower growing species will be necessary.

Eucalypts
Eucalyptus botryoides and E. saligna have been the main species planted, particularly for boundary shelter. This is mainly due to their fast growth rate and resistance to the leaf chewing insect Paropsis charybdis. In shelterbelts, they rapidly develop a spreading form and often suffer crown damage from strong winds.
Other Eucalyptus species which could be used as well as the above for boundary shelter are E. fraxinoides, and E. regnans. These species retain a more narrow crown form, particularly at younger ages, and are reasonably resistant to Paropsis. E. fraxinoides retains its lower branches longer than many other Eucalyptus species, and does not suffer from crown damage by wind.

Eucalypts are not suitable for internal shelter, and need underplanting when used for boundary shelter.

Sheoaks. Casuarina species.
Two species are widely planted: Casuarina cunninghamiana (river sheoak), and C. glauca (swamp sheoak). Of the evergreen species, the sheoaks are the most permeable, although regular trimming is necessary to prevent them from becoming too open. C. glauca will withstand heavier soils than C. cunninghamiana and is more resistant to infection by Phytopthora root rots.

C. glauca may be slightly more resistant to salt-laden winds though neither have high levels of tolerance.

C. cunninghamiana has considerably better form than C. glauca and is to be preferred where soil conditions are not limiting. Both species are fairly frost sensitive, and in frost prone areas should not be planted out until September. Young trees are particularly palatable to hares and rabbits.

Japanese cedar (Cryptomeria japonica).
This species is widely planted in northern districts of the North Island, where it grows moderately rapidly. It is often planted in conjunction with willows or poplars, because it is particularly shade tolerant. It can be used as the slower growing species in both boundary and internal shelterbelts. It does not perform well in the South Island.

Leyland cypress (Cupressocyparis leylandii). Several clones of Leyland cypress are available, the most commonly planted being “Leighton Green”. Other clones include “Naylors Blue”, “Green Spire”, “Haggerston Grey”, and “Castlewellan Gold”. All these clones can be affected by cypress canker, but they are more resistant than other Cupressus species.

They are relatively fast growing for an evergreen species, and eventually grow into large trees. Their major use is for boundary shelter in districts exposed to salt-laden winds. Regular side trimming is required (they are prone to heavy-side branching) but this may cause the shelterbelt to become too dense for optimum sheltering effect.

The clone with the narrowest form is “Naylors Blue” which, although somewhat slower growing than the other clones, may in fact be the most suitable for horticultural shelter on fertile sites. All clones can tolerate a wide range of soil conditions, and are quite frost tolerant.

Western Red Cedar (Thuja plicata). This evergreen species is slow growing, but useful for planting in combination with a faster growing poplar or willow species. Its major advantage is its resistance to cypress canker. It does not withstand very exposed conditions, although it is quite frost tolerant.

Alders (Alnus species). Alders are gaining in popularity for use as internal shelterbelts, for several reasons. Because they fix atmospheric nitrogen they do not compete to any extent with the crop for this nutrient and in fact can contribute additional nitrogen to the ecosystem, mainly through leaf litter. Although they are reasonably fast growing, they do not reach a very large size, and can be easily controlled by annual or biennial trimming.

They retain their lower branches very well when trimmed, and some species hold their leaves well into the autumn. This is an important factor for the protection of autumn or winter harvested crops. When trimmed regularly, their permeability is about the optimum 50%. Four species are commonly available in New Zealand: Alnus cordata, A. glutinosa, A. incana and A. rubra.

A. cordata (Italian alder) retains its leaves longer into the autumn than the other species, but can be susceptible to damage from severe frosts. It is probably the most promising species for New Zealand conditions. It is not suitable for very acid soils.

A. incana is the hardiest of the European alder species, and retains its vigour on dry soils. An unattractive feature is the production of root suckers.

A. glutinosa (common or black alder) is more tolerant of wet or clay soils. In Europe, some trees have a tendency to flower precociously at about age four years, and subsequently lose vigour. However, this characteristic has been noticed only rarely in New Zealand.

A. rubra (red alder) grows well in New Zealand, but its usefulness in shelterbelts has not been determined as yet. It has a tendency to become quite spreading at a relatively early age, but this can no doubt be controlled by trimming.

Species tolerant to salt-laden winds
Damage to shelterbelts from constant westerly winds over much of New Zealand during last summer highlighted the need for species more tolerant to salt-laden winds. Species which can be considered for areas where damage occurred are listed in Table 2. Several of these species have been commented on above.

Norfolk Island pine (Araucaria heterophylla).
Norfolk Island pine will tolerate very exposed coastal conditions, and can be considered for first line defence where radiata pine or macrocarpa are burnt by salt-laden winds. Trees are initially slow growing, and require wind protection for the first two years. They can be trimmed successfully, but need to be underplanted with a lower growing species.

Coast Banksia (Banksia integrifolia).
This species is extremely tolerant to salt-laden winds, has a moderate growth rate and forms a narrow crown. It tolerates most soil conditions except waterlogging, but is not suitable for planting in areas with heavy frost. It should be planted in conjunction with another faster growing species.

Low growing species.
A reasonable number of lower growing species are quite tolerant of salt-laden winds, although most are widespreading. An exception is Olearia traversii (Chatham Island akeake) which has a very narrow crown, and grows well alongside taller growing species. Of the remainder of the species in this group listed in Table 2, Pittosporum crassifolium (karo) is the most useful, and can be planted very successfully alongside Pinus radiata to provide bottom shelter.

Summary:
The characteristics of all the more commonly planted horticultural shelter species are summarised in Table 3. This Table enables direct comparisons to be made between species, and the most suitable species or species combination can be selected according to individual requirements.

Table 3.

Note – Table 3 above is an image file –
A larger clearer view is available by right-clicking on it (or equiv); choose open image (or equiv).

Please share your perspective, experiences or suggestions to make this information better –
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Mr R. (Bob) L. Hathaway

For some time since 1968 he had been the National Plant Materials Centre scientist in charge of the programme for introducing, breeding and selecting willows for soil conservation. Clones he has developed include the recently released fast-growing Salix matsudana x alba shelter selections “Tangoio”, “Makara” and “Moutere”.

Mr Hathaway’s other major responsibility was the selection of soil conservation species, mainly Eucalyptus, for seasonally dry hill country.

His interests included tree protection systems, tree establishment methods and the potential of poplars and willows for stock fodder and biomass.

He spoke to the Tree Crops Association’s annual conference in Palmerston North on the planning of shelter taking into consideration such effects as wind direction, land contours, shelter density and the choice of species after considering wind velocity and closeness to the coast.

From Growing Today, July 1983.

For several years, Growing Today was the journal of the NZ Tree Crops Association.
Thanks Bob, MWD, and the Growing Today team for this timeless series of shelter articles.

Growers establishing a shelterbelt often make one of these four mistakes, says National Plant Materials Centre* scientist, Bob Hathaway.

• Inadequate weed control:
This problem particularly affects the new horticulturalist or part-timers who haven’t time for weed control. All new shelterbelts should be kept free from weeks for the first two years, either by cultivation or herbicides. Spraying with herbicides is the main method.

• Irrigation not organised until trees are already suffering from drought stress:
Many plant their shelterbelts before deciding to have irrigation. By the time they get organised the trees have already suffered stress. Growers should think of irrigation before or at the time they plant because irrigation is most important in the first year after planting.

• Trees that are planted too close to fences being damaged by stock.
Cattle and horses are particularly eager to lean over fences and eat shelterbelt trees. The minimum distance from the fence should be at least 1½m, preferably 2m, Mr Hathaway says.

• Shelterbelts not planted before the crop:
In most situations, the shelterbelt should be planted at least two years before the crop. Those who plant their shelterbelt and crop in the same year may find the lack of shelter affects the rate of growth and exposes the crop to physical damage, especially such crops as kiwifruit.

Most horticultural crops are long-term projects so there is no real advantage in planting at the same time as shelterbelts.

From Growing Today, July 1983
* National Plant Materials Centre? Ministry of Works? Gone the way of increasing numbers of valuable taxpayer assets – you can hire the smirking services of off-shore corporates instead; dumb stupid kiwi voters, serves you right…?

Scientist Bob Hathaway says four questions are usually asked about planting shelterbelts. Answers to these form part of the series of articles Mr Hathaway has written about shelterbelts.

• What are the best species to plant in my area and situation?
Salt-laden or very strong winds are the main limitation on what to plant. Otherwise you can grow almost anything. The situation and crop chosen also influence choice.

• What distance apart should shelterbelt trees be planted — within the row and between the rows — in a multi-row shelterbelt?
Distances depend on the species and whether the shelterbelt is an internal or boundary shelter. It also depends on how quickly the shelter is needed — “if it is needed yesterday you can plant closely to get the desired effect, but it is not usually the best way to go about it.”

• What is the best method of weed control?
There is no single answer to this question. All soils respond differently to different herbicides; different weed species require different treatment depending on their situation; and the tolerance of the shelterbelt trees must be considered.

• For poplars and willows, should I plant cuttings or rooted trees, and, if rooted trees, should they be cut back after planting?
Many people ask this question. Cuttings are satisfactory provided they have irrigation. Full-size trees can be planted in areas where there is some degree of protection.

In very exposed situations or where there is no irrigation, rooted trees cut back after planting give the best results — a more even line of trees with less tree deaths.

From Growing Today, July 1983

By Lynsey King – from 1983, timeless advice.

Five summer months of strong salt-laden winds have taken their toll on shelter-belts.
Scientist, Bob Hathaway has been studying the damage the winds have wrecked, particularly on poplar and willow shelter-belts in the Manawatu.

Strong summer winds normally last only a month or so, but last summer’s abnormal conditions has taught scientists several lessons.

One lesson is that there are serious limitations as to where these particular species can be planted. Mr Hathaway says eight kilometres from the coast is a safe distance. The willows and poplars have suffered leaf-burn and die-back of tree leaders which means the trees are actually smaller than when they were planted, he says.

The trees can be cut back to re-shoot from live material but the damage has set them back at least a year. However, those planted in less exposed areas have proved “quite satisfactory”.

The casuarina (she-oak) species used in shelter-belt tests has also been found to be inadequate in exposed coastal conditions.

This species has also been burnt by the salt winds, but to a lesser degree than the willows and poplars. But, although the Casuarina glauca planted was thought to be quite tolerant, it has not stood up to severe coastal conditions. This has shown mainly through burning and poor growth rates.

“It will be necessary to use more resistant species such as Pinus radiata and, possibly, Leyland cypress for exposed western boundaries,” he says. Pinus radiata is very tolerant to salt winds but has not been favoured because of its large eventual size.

Management of the trees is essential. Side-trimming is needed to keep them dense at the base and prevent them opening up. As they grow they will need topping to prevent shade and competition with crops.

Leyland cypress is not as tolerant as Pinus radiata but can probably be used in slightly less exposed situations. Although it is slightly susceptible to cypress canker, recent experiments in the Manawatu have shown the species can live with the disease. “They’re vigorous enough to overcome it, but it will always be there.”

The MWD plant materials centre at Aokautere is co-operating with the Ministry of Agriculture and Fisheries in selecting suitable shelterbelt species for land exposed to strong salt-laden winds. The MAF trials on its Manawatu land deal with irrigation and fertiliser requirements to maximise growth rates. The MWD centre’s role is advisory in selection and management of species for these difficult sites.

Bob Hathaway . . . studying the damage the summer’s salt-laden winds have made on poplar and willow shelter belts.
The shape of the tree-top in the background might tell us something about prevailing winds.

– from Growing Today, July 1983
MWD, Aokautere: Ministry of Works and Development. Gone, like evermore of the taxpayers’ most valuable assets into the hands of profiteers.

First part of a series from Growing Today, July 1983
by R.L. Hathaway
National Plant Materials Centre,
Aukautere

Provision of adequate shelter from wind is essential for the successful production of most horticultural crops in New Zealand.

Good shelter not only results in improved yield and quality of crops; it often determines whether or not the crop can be grown at all.

A large proportion of horticultural crops are grown in coastal areas, which are often more windy than inland areas. Most areas in N.Z. are affected by the prevailing westerlies, which lower temperatures and increase evaporation and transpiration losses. These factors result in reduced growth rates, lower yields, and lower sugar levels in fruit crops.

Occasional very strong winds from the easterly quarter can result in serious physical damage to crops and trees.

Briefly, the benefits of shelter are:

1. Minimising physical damage to leaves, blossom, fruit, branches etc.
2. Improvement of microclimate, with higher temperatures, higher humidity, and reduced evaporation and transpiration.
3. Improvement in pollination by bees which are encouraged to work more often.
4. Increasing the range of crops that can be grown.

The greatest sheltering effect for a given input of resources will only be achieved if an integrated system is planned and implemented. Consider both present and future requirements of the crop, as well as management and shelterbelt replacement. The cumulative effect of shelterbelts on wind reduction is often underestimated but is an important factor in reducing wind velocities.

Shelter system design will depend to a large extent on the crop to be grown. The first step is to decide how much shelter is required by that crop to ensure a good crop performance. In other words, what level of exposure can the crop withstand. This will depend on several factors:

1. Susceptibility of the crop to wind damage. This can include mechanical damage, such as branch breakage, fruit rub, desiccation of foliage etc. For example, kiwifruit is much more susceptible to wind damage than grapes or vegetable crops and requires a much greater degree of shelter for economic production.
2. Frost susceptibility of the crop. Highly sheltered areas are more prone to cold air ponding; this can result in more severe frost damage than would occur in areas with adequate cold air drainage.
3. The period during the year when wind protection is required. Some crops need protection in early spring (e.g. stone fruits) or late autumn and early winter (e.g. kiwifruit and tamarillos). This affects the selection of shelter species to be included in the system, particularly with regard to deciduous versus evergreen species.
4. Whether the crop is wind or insect pollinated. Wind pollinated crops require a certain level of air movement for good pollination. However, with insect pollinated crops a higher level of shelter is desirable to encourage bees to work. Overseas work has shown that fruit set in pip fruit crops can be considerably greater in the most sheltered area of the orchard (e.g. Smith and Lewis, 1972).
5. The self sheltering ability of the crop.
   This factor applies mainly to larger trees, the effect being quite small initially, but increases as the trees approach maturity.
6. The degree of shelter required will depend to a large extent on the exposure and windiness of the site. For example, in more windy areas of New Zealand it may be necessary to reduce mean wind velocities by 50% or more to obtain an acceptable level for the production of many horticultural crops, while in less windy areas a reduction in mean velocity of 25% may be all that is required.

Shelterbelt performance is directly related to its height, length, and permeability.

HEIGHT: The distance downwind over which a shelterbelt is effective is proportional to its height.
Some reduction in wind speed, leeward of the shelterbelt, can be detected up to a distance of 20 times the height of the windbreak.

LENGTH: Where the wind strikes the shelterbelt at right angles a continuous length of at least 24 times the height of the shelterbelt is regarded as the minimum. There is an increase in wind speed around the ends of and through gaps in the belt.

PERMEABILITY: The relationship between permeability, extent of sheltered area, and reduction in wind velocity has been the subject of many investigations and is still debated. It is generally accepted that Fig 1 approximates the wind profiles of shelterbelts of different densities, although other factors such as the flexibility of the barrier, windward surface roughness and atmospheric turbulence can affect profiles.

Shelterbelts of medium density result in the greatest sheltered area, although wind reduction very close to the shelterbelt is not as great as with dense or very dense belts. Very dense belts result in turbulence in the lee of the belt which can sometimes cause greater damage than unhindered wind flows. This turbulence is caused by wind shear at the top of the barrier, where the air flow is deflected sharply upwards, and the velocities of the underlying layers are rapidly reduced.

The upper layer of air quickly descends to mix violently with the lower layers. To decrease turbulence, permeable shelterbelts are required; it has been found that medium dense belts with a permeability of 40-50% are generally the most satisfactory. In addition, vertical changes in permeability should be gradual.

From Fig 1, it can be seen that a moderately dense shelterbelt (40-50% permeability) will give a wind reduction at ground level of 50% at a distance of 8H from the shelterbelt, where H is the height of the shelterbelt. The height of the shelterbelt should be taken as the height above the top of the crop. Thus to achieve a wind reduction of at least 50% over a planting of 5m tall fruit trees, with shelterbelts 15m tall, the shelterbelts should be spaced no further than 80m apart.

Alternatively shelterbelts 10m tall spaced 40m apart would provide the same degree of shelter.
If a wind reduction of 25% is required, 10m tall shelterbelts placed 65m apart, or 15m tall shelterbelts placed 130m apart would be satisfactory. If a wind reduction greater than 60% is required, dense belts placed no further apart than 25m for 10m tall belts, or 50m for 15m tall belts are required. For taller fruit trees it is obvious that shelterbelts must be either taller or more closely spaced.

For kiwifruit shelter, the current Ministry of Agriculture and Fisheries recommendation is 35m between north-south shelterbelts (i.e. giving protection from easterlies and westerlies), and 80-120m between north-south shelterbelts. Close spacing between north-south shelterbelts is essential in some districts to obtain adequate protection during the establishment period, a distance of 50-60m between shelterbelts 10m tall should be satisfactory in less windy districts.

Having determined the optimum height, spacing and permeability for shelterbelts in the system, consideration must then be given as to how this can be achieved.

Main shelterbelts for protecting horticultural crops should be oriented in a north-south direction to minimise shading. Where prevailing winds are from the west or damaging easterlies occur, as is the case in most horticultural districts, this arrangement can often be achieved. Long narrow blocks provide the most protection, with the long dimension at right angles to the wind.

Where protection is required from winds which strike the north-south shelterbelts at an angle, shorter blocks with more frequent east-west belts are required to prevent funnelling of the wind down the length of the block. It may also be necessary to place the north-south belts closer together than is required where the wind strikes at right angles to the shelterbelt.

Gaps for access through shelterbelts should be avoided as far as possible in north-south shelterbelts, or in those exposed to the most damaging winds. The positioning of access points is more critical where the wind does not strike the shelterbelt at right angles. In this situation, access points in east-west belts should be positioned on the most sheltered side of each block.

In many situations, the orientation of property boundaries, topographic features, the presence of tile drains, electric power or telephone lines or irrigation races prevent the adoption of the ideal layout and a compromise must be made. Tile drains are a difficulty encountered on many properties, and roots of many species, not only poplars and willows, will cause blockages.

Undulating land presents further problems, and the placement of shelterbelts for optimum effect on this type of country is much more difficult. On gently rolling land, wind flow follows the contour fairly closely, thus shelterbelts should be placed on ridges and upper slopes to take advantage of the extra height obtained.

On steeper land, turbulence, and eddying can be much more severe than on flat country, and no general shelterbelt pattern can be recommended. Intersecting shelterbelts across valleys are often necessary to limit funnelling. Advantage should be taken of any naturally sheltered areas.

If drainage is present or planned, a section of sealed pipe is necessary at any position where a shelterbelt crosses a tile drain. Where drains run parallel to shelterbelts, these should be at least 8m away from the trees. Lateral root growth can be controlled by deep ripping alternate sides of the shelterbelt every second year.

Double-row shelterbelts comprising a fast growing species for tall shelter, such as radiata pine, poplar, willow, or eucalypt, in combination with a slower growing species, generally evergreen, to provide bottom shelter, and to reinforce the sheltering effect of the fast growing species in time, are the most satisfactory for main north-south shelterbelts. In addition, the second row provides a safety factor should any deaths occur in the taller species. The slower growing species should be chosen carefully to be fully compatible with the faster growing species.

It needs to be either quite shade tolerant, or else planted sufficiently far away from the fast growing species not to be suppressed (1½-2m).

Side trimming on the inside of the fast growing species may also be necessary. The slower growing species should always be planted on the northern side of east-west shelterbelts, and on the windward side of north-south shelterbelts, to minimise over-topping.

The use of dense evergreen species such as radiata pine or Leyland cypress in east-west shelterbelts may be undesirable because of the heavy shade they cast during winter when the sun is at a low angle. In this situation single rows of deciduous species are preferred.

Internal north-south shelterbelts consisting of single rows of deciduous species can sometimes be satisfactory in areas where wind velocities are not high. However, it is likely that they will become open at the base in time, and may need underplanting. In general, at least every second north-south shelterbelt should be a double row.

In some districts boundary shelter may require three or more rows to break the full force of the wind. Where unproductive land is available on boundaries, it may be possible to establish small woodlots of tall growing species, particularly on southern and western boundaries.

Temporary shelterbelts of fast growing poplar or willow species may assist in establishing the crop trees earlier than would otherwise be possible. These can be planted in an intermediate position between permanent shelterbelts, and removed at a later stage when the permanent shelterbelts are tall enough to shelter the total area. A further row of crop trees can then be planted in their place.

Deciduous species can sometimes be included in the same row as evergreen species to obtain the required permeability, or to allow for cold air drainage on frosty sites. It is however difficult to avoid competition effects between the two species unless the evergreen is particularly shade tolerant (e.g. Cryptomeria japonica).

(Density can be controlled by side trimming and pruning. In areas subject to severe frosts, it may be necessary to cut holes in the bottom of downslope shelterbelts which have become too dense, to allow cold air to drain away.

In many of the more exposed districts of New Zealand now being considered or undergoing horticultural development, tall shelterbelts of satisfactory density throughout can only be achieved by planting two rows of trees. Single-row windbreaks of species such as Populus Flevo, Salix matsudana, or Casuarina species may appear to be moderately dense under light wind conditions, but when subject to strong winds, large gaps can appear which increase the permeability from 40% to nearer 80%.

Satisfactory density can be achieved by single rows of evergreen species such as Leyland cypress which are regularly trimmed, although these can often become too dense.

It can be seen that in planning a shelter system, many compromises must be made, especially where live trees are used. Because of the variability in site conditions, and differing requirements of many horticultural crops, there can be no single design to suit all situations. Artificial shelter can minimise many of the compromises which must be accepted with live shelterbelts, but at the expense of very high cost, including the possibility or replacement after a relatively short period.

Species selection, irrigation requirements, and the use of artificial shelter are also important aspects to be considered in detail at the planning stage.

REFERENCES
Caborn, J.M. 1965: Shelterbelts and windbreaks. Faber & Faber Ltd, London. 288p

Smith, B.D. and LEWIS, T. 1972: The effects of windbreaks on the blossom-visiting fauna of apple orchards and on yield.

Ann. Appl. Biol. 72: 229-238.