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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.

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First, a word on how USDA Hardiness Zones are determined. USDA Hardiness Zones are based on the Average Annual Extreme Minimum Temperature. So, the average of the coldest temperature on the coldest night, that's where we're looking at. If you're thinking that's around -3.5 to -5 C that likely puts you in Zone 9a according to this map

The shelter is adequate because it traps in heat. The ground under the shelter is warm still. The shelter traps that heat in while it escapes outside.

Depending on exact species, citrus can tolerate a few degrees below freezing. In particular the blossoms are very tender, but the tree itself can handle a bit more cold with minimal effect.

Avocado depends on variety. Some cultivars can handle down to -9 C, others will barely tolerate 0. Putting a cover on it on cold nights might make the difference.

You probably could keep the plants alive over the winter with a polytunnel. However they won't be terribly vigorous until it warms up again, those species like it warmer.

Frost tender plants can absolutely be kept alive through sub-zero nights. Covering them up overnight can get you through a moderate frost. I've used everything from old bed sheets to cardboard boxes. Anything that traps heat in will help. Interestingly, on bigger things that are you can't reasonably cover (like your citrus trees) you can actually save them by spraying them with water. Water is very resistant to temperature changes. For a leaf that is soaked from irrigation to freeze first the water on the leaf has to freeze before the leaf can even begin freezing. If you're just a degree or two under freezing it can make the difference.

For a more thorough discussion on frost, I recommend this guide from Cornell University.

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Edible Tree Crop Farm was Dick Robert's pioneering permaculture site in Nelson, NZ before the term existed. A sub tropical climate was created with the use of North (equatorial) facing hillsides, ponds for thermal mass and heat reflection, wind breaks to divert cold winds, rocks etc.

A description of micro climates can also be found here. https://treeyopermacultureedu.wordpress.com/chapter-5-climatic-factors/microclimate/

Sepp Holzer's book should be in the public library and gives more practical advice.

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  • Thank you for this. Very interesting reading. I'm sorry I could not accept 2 answers (I accepted the other answer because it answered more of my question, but both answers are valuable to me, and together provide a better picture then either single one). – davidgo Apr 4 '16 at 10:21
  • Mine was just an FYI, and also we need to increase the number of answers per question to get this site out of beta! – Graham Chiu Apr 4 '16 at 10:28

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