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°C 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?