I've been doing a little studying of late and it would seem that the worlds of agriculture and gardening in both Canada and the US share some common ground when it comes to zone/heat unit creep, this in spite of evidence to the contrary, especially in the past several years. I'll step right up and say that as a former Canadian resident I'm plenty enthralled with the idea, but in the case of crops and farming I've yet to figure out what happens when one is proved (disasterously) incorrect ?
I've always been a fan of palm trees and I live in envy of those a couple hundred miles south of us who can easily grow several types as ornamentals. Lately I've noticed more and more ornamental palm plantings in southern Middle TN and North Alabama including several at area hotels and restaurants including a poor, bedraggled looking specimen at the Sonic in Fayetteville, TN. There's even a palm tree nursery just south of Huntsville; I stop in every time I go by but I have yet to lay down the money and get a specimen for the yard because I don't think our climate has changed enough to risk a few hundred dollars against a series of cold winter nights. Depending on the map one studies, we're either in Zone 7A or 7B and that isn't consistently warm enough for palm trees. However we ARE consistently warm enough to grow large, awesome crape myrtles, bamboo, winter azaleas and camellias, some types of evergreen oak and huge old southern magnolias.
Really, I think it's all about knowing your land and your microclimate and what it will allow. Here in TN our Lynnville farm is slightly warmer year round AND slightly more frost prone (because it sits in a valley) than our College Grove farm which is counter-intuitive but true. In Ontario my early land was among the earliest and warmest in the area and despite government publications suggesting that we should select varietals requiring 2700 CHU or less to mature, it was easy and relatively safe to push this another 100-150 heat units on that particular farm.
Like most things in Ontario, warmth is relative and often fleeting, especially in the spring. After several years of watching me plant corn and beans in April and listening to gardening shows that proclaimed our part of central Ontario as the new zone six, my mother succumbed to the mood of the moment and brought home a japanese maple. She planted it in a sheltered spot on the southeast side of her house and it did quite nicely for a few years until one year the climate remembered we were in zone four rather than six and her japanese maple froze to the ground. The poor thing was nothing if not diligent though because when I cut it off the following spring it sent out new shoots and began to regrow. Fifteen years later it's been frozen to the ground and reborn at least three more times that I know of and it was still tenaciously hanging on, head high and ready to be frozen again, on my visit last week.
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