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.
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.
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.
Rate kg ai/ha
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.
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 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.
Cuttings planted through polythene mulch
Cuttings kept weed free by herbicide (paraquat only)
Cuttings kept weed free by cultivation
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.
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.
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.
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.
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.
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.