Understanding USDA Hardiness Zones and meaning of Frost Tender with respect of Zone 8

Question

I live in Rural Auckland, and some early mornings in Winter the temperature drops to -4°C (although on a cloudy night temperatures above +4°C are more common). I would have thought that temperatures of < 0°C 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 0°C.

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 than -4&degC as I understand it], but also claim the plant is "frost tender" This plant grows in areas I would expect to get much colder than where I am (and indeed I am in USDA hardiness zone 9)

Similarly, I have a Hass Avocado tree growing in a sheltered area on 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?

Answer 1

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....