On behalf of Nick E:
thanks for the blogletter, this is starting to have a life of its own which must be very pleasing to you.
Hot air rises, cold air sinks. Jeffrey Feint’s frost damage in one place but not another, has the hallmark of cold air ponding.
I would encourage Jeffrey to take a wide-angle view of his surrounding landscape and imagine hedgerows, plantations and physical landscape features as the walls of dams to store cold air in “lakes.” Is your lower orchard within such a reservoir? Does the height of your frost damage approximate the top of such a barrier?
If so, and the barrier to cold air drainage is plant material, it may be feasible to breach the dam by removing vegetation to let the cold air of a still night drain away. This work should be part of control of your microclimate.
Other work is in initial site selection for aspect to sun and wind, and selection of sloping ground to encourage air movement, which in itself is a form of frost protection.
There is no time of year that New Zealand is not subject to unpredictable polar outflows that stream Antarctic air masses over the country. Frosts will result, anywhere. If you do not have the opportunity to remediate your environment, plant your orchard elsewhere or restrict your choice of varieties to those that resist frost damage. It may be possible to top-work your trees with suitable plant material grafted “in situ.” Not easy with walnuts!
Cheers, nicK Empson