The new USDA Plant Hardiness map is out. The old version was from 1990, so now there is added information from almost two decades, plus better interaction and more detail. You can see it and read about it here.

This measure the average yearly temperature for an area, and is usually used for determining what plants will survive the winter.  The current map bumps many places up by a zone. (You can look at this map here. They are comparing changes for a map made in 2006, not adopted by the USDA, but it should be similar.)

I am currently living in a zone 7b. Previously I was in a 7a. Doesn’t sound like I’ve changed much, but winters are very different in both locations. Utah county looks like it’s mostly in a 6b situation. I’ve always said a zone 5/6, so it’s just a little warmer. Where I lived before, Cache County, has changed quite a bit. It used to be mostly 4/5, now it’s all in zone 5, even with pockets of zone 6.

I still believe in being a little reserved in choosing plants. I know a zone 5 plant will survive about any winter in Utah county. But if I decided to strically go off of the USDA map I can suddenly plant something hardy to zone 7. I might go ahead with some perennials, but I would always recommend being on the safe side with larger, more expensive plants including large trees and speciem shrubs.

For instance, last winter was pretty cold and the temperature pattern made for a lot of winter kill. Boxwoods, peaches, roses, and similarly tender plants saw a lot of damage. The hardiness rating on these plants are in the zone 5-6 range, which is well within the current USDA hardiness rating.

Because the map is based on average temperatures not lows, cold winters and different weather patterns can result in winter kill for what was supposed to be a hardy plant. There are many plants growing here that would not survive the winters back in Utah, even though I’ve apparently only changed half a zone. Experience and knowledge is much better than strict numbers.  Still, the map does provides a good starting reference point for determining plant hardiness.