The more I look into it, the more confused I'm getting about frost protection. Any advice to help me get a grip on the issue would be appreciated.
I have a property in Dairy Flat, which is rural Auckland, New Zealand. Our temperatures tend to get 4 degrees centigrade colder then weather forecast (but not when there is lots of cloud), and I measured - probably inaccurately, a minimum of -3.5 degrees C last year - which came along with ice forming in water outside (and a burst hosepipe).
A large local tree seller not far away advised they got down to -5 degrees C - which killed their outdoor trees, but the trees which they place in the sheltered area were unaffected - the sheltered area being a large hothouse like structure covered in shade cloth down to 40cm above ground level.
(I understand that structures on the property and area can create micro-climates which make a big difference, but I am struggling to work out out how to accurately predict micro-climates around my property, not having had sufficient knowledge to observe these over the past year I've been at the property)
I'm struggling to understand -
How a sheltered but not waterproof area can provide such a high degree of protection as advised by the tree company - and if this is only beneficial to large plants like trees, or if it can be applied to plants like tomatos ?
What US hardiness zone would be comparable to Rural Auckland (Knowing this would appear to open a world of knowledge applicable to the USA I think) ?
- Why a hosepipe in an area near a small, enclosed citrus fruit orchard (about 6 trees) would freeze and burst but all of the citrus trees were fine ?
- What this means for my avocado avocado trees (which are 1 year old, in a semi-sheltered environment and have been ravaged by sheep and thus have very limited foliage going into autumn)
- If using an unheated 6m x 3m polytunnel will provide protection sufficient to allow me to grow tomatos, eggplants, zuchini, Cucumbers and the like through winter ?
- Can frost-tender plants survive sub-zero temperatures if protected from moisture, and how to work out down to what level ?
Any advice, help, resources or pointers would be appreciated.