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I live in Rural Auckland, and some early mornings in Winter the temperature drops to -4c (although on a cloudy night temperatures above +4c are more common). I would have thought that temperatures of < 0c would preclude me from growing a range of plants, but this is at odds with what I am reading online and my recent experience. Not sure how relevant it is, but on cloudless nights the temperature in Suburbia 10km away virtually never drops below 0c.

I am looking at growing "Corynocarpus laevigatus" - commonly known as the New Zealand Laurel / Karaka. Sites (and labels on plants at my nursery) - which claim that the plant is USDA hardiness zone 8 [ which is a lot colder then -4c as I understand it ], but also claim the plant is "frost tender" This plant grows in areas I would expect get much colder then where I am (and indeed I am in USDA hardiness zone 9)

Similarly I have a Hass Avocado tree growing in a sheltered area in my property and it seems to be coping with the Winter cold just fine. (In fairness I bought a 4-5 year old tree and planted it in this specific location because of failures to much smaller trees in more exposed areas on my property without success - and the nursery that sold it guaranteed it would survive) I understand though that these trees can only handle temperatures to -2c, yet I have measured colder temperatures (and heavy frost) in the vicinity of the tree.

Can anyone shine any light on these apparent contradictions? Is the cold rating maybe an average over a period?

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Hardiness is not an exact science. As an example a tree that is native to Southern Siberia, Acer ginnala, supposedly hardy to USDA zone 2 shows trunk cracking and a shorter life in USDA zone 4. This may be due to it being grown further south in USDA zone 6 and sold on to nurseries located further north or not being able to cope with freezing rain or rapid temperature swings.

The possible factors that can affect hardiness can include:

  • where it was grown
  • how the weather where it is growing is like or unlike it's native habitat
  • potential to cope with outlier events: winds, freezing rain, drought, low or high temperature events
  • are you growing in a protected microclimate
  • variability within the species

As to whether a particular plant will grow somewhere it shouldn't the gold standard is to plant a large sample size and wait ten years. For residential gardeners plant it and see....

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  • I get "hardening off" plants, but in my mind freezing of the plant would seem a fairly hard limit, as save for "antifreeze" analogs, frost must, I assume, cause cells in foliage to burst and kill the plant?
    – davidgo
    Jul 20, 2021 at 2:38
  • @davidgo from wikipedia " plant hardiness has been observed to be linked to how much stress specific plants are undergoing into the winter, or even how fast the onset of cold weather is in a specific year." There's a lot going on that we don't understand...
    – kevinskio
    Jul 20, 2021 at 11:16
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    An anecdote about the importance of micro-climate... I have two neighbors who live across the street from each other. One house faces south, the other, north. There is a downhill slope to the north of both houses, with one house and a park on that slope (so, decent north wind in the winter). They both planted Acer palmatum 'Bloodgood' Japanese maples in front of their houses. One winter, it reached (lowered?) -23 F for several consecutive nights. Come spring, the tree in front of the neighbor facing north died to the ground; the other tree (facing south & protected by the house) was unharmed.
    – Jurp
    Jul 20, 2021 at 18:50
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    I would also add late winter drainage to the hardiness factors listed. Granted, it's not a factor of temperature, but it's easy to kill an otherwise winter-hardy perennial by planting it in the soil with poor drainage (clay vs. loam, for example) when the plant (rattlesnake master, many rock garden plants, for example) requires excellent/sharp drainage.
    – Jurp
    Jul 20, 2021 at 18:53
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    Exact location is a strong factor. Here in US zone 8, we had some super all time record cold last winter. The Sego palms lost all fronds , but the range of recoveries is amazing ; As of 20 July , I have a few in full leaf that look like nothing happened , I have one ( old plant) that has just put out its first leaf. A neighbor has a large old palm with no new growth. Jul 20, 2021 at 19:07

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