USDA Changes Hardiness Zones For Eastern Idaho
Recently the US Department of Agriculture revised its zoning maps for plant hardiness throughout the United States. This represents a substantial increase in the amount of data used to formulate each zone. It now covers the years from 1976 through 2005 whereas the previous edition was from 1976 through 1990. Each zone represents the average minimum temperature for an area. Keep in mind that it certainly does not represent the lowest temperature that has occurred or that will occur in the future. New zones with warmer minimum temperatures were created for Hawaii and Puerto Rico as well. Plus, new maps were adapted to be used with the internet with more extensive data which allows a much finer scale and zone choice according to ZIP Code.
As I’m sure you’ve noticed, temperatures have generally been a little warmer lately and the new zone maps reflect these changes. Eastern Idaho and Idaho has (in general) experienced some of the more significant increases in the country. The average temperatures have increased by nearly 20° from the earlier timeframe, this new data puts our area into Zones 4 and 5 rather than Zone 3. For those of us old-timers who have been around a while, I think we would agree that the deep subzero temperatures of the 70s and early 80s are very rare lately. After all, we walked to school in 10 feet of snow, uphill, both ways… 😉 Here is the link to the maps: http://planthardiness.ars.usda.gov/PHZMWeb/Default.aspx
So, what does this all mean when it comes to plant selection for our landscapes? Well, it does mean that you have a little forgiveness in planting Zone 4 trees or shrubs that would have been a sure winter kill in the past. As a professional arborist I still hesitate to recommend Zone 5 plants except in very protected areas or in areas to the south of our valley, aka. Blackfoot, Pocatello and below. Even then it is possible to have an unusually cold winter that could damage or kill plants.
But these moderating temperatures certainly do provide an opportunity to plant trees and shrubs that were too risky prior to now. I have been using the last couple of seasons to experiment with some Zone 4 trees that have done very well, this is in spite of the cold of last winter reaching about 20 below zero. An additional factor to consider is that the amount of snow cover and humidity we have been having during winter can increase a plants ability to adapt to cold temperatures. I expect to see more plants that will technically survive winter but with higher amounts of winter damage than our traditionally lower zone trees.
“Climate change” is real and has been going on for a long, long, while. But it reminds me of the market…….. past performance is no guarantee of future results! For now we are in a warmer cycle that will allow us to enjoy a few more plant species. Please remember to carefully check your zone hardiness prior to purchasing any new plants since some retailers will still want to bring in plants that will not be tolerant enough for most locations here in eastern Idaho.