Todd Davis is editor of NMPRO, Nursery Management & Production.
It looks like we’re on the verge of getting an updated version of the USDA Hardiness Zone Map. This map, published jointly by American Horticultural Society and USDA, was updated most recently in 1990. And, after years of crusades by industry folks including yours truly, it looks like the publishers will use more data than ever to produce the map.
If you don’t know, the map indicates average annual low temperatures by zone. The colder your climate, the lower your number.
The 1990 version used 1974-1986 weather data. Yes, the current version is based on just 12 years of information. In 2003 AHS was on the verge of releasing a new version that used 1986-2002 data. Why AHS chose to ignore the 1974-1986 data it already had astounded me and others. A meteorologist working on the project told me, in summary, that the map better reflects current climatic trends by including only recent information.
I told him, in summary, hogwash. How can you defend a climate map that ignores all weather that happened prior to 1986?
Luckily a few highly squeaky wheels put a halt to the 2003 version and USDA forced AHS to go back to the drawing board. And, as fast as plate tectonics, we’ll soon have a new map. And you may soon have a new Zone. And the public will be confused.
As an example, let’s look at what happened in Washington, D.C., this winter. It appears somebody leaked information (not uncommon in Washington) and put out the word that D.C. will be changing from Zone 6 to Zone 7.
Garden writers and bloggers in the area treated the news like they’d soon be enjoying a climate similar to that of Miami’s South Beach. Yes, they actually thought changing their color on the map would magically change the weather.
But when you look at reality, our nation’s capital already straddles Zone 6 and Zone 7. So it’s like saying Kansas City now has weather more like Kansas than Missouri. Some people also stated that they’ll now be able to plant more heat-loving plants in Washington, D.C. That’s not the case, either.
The USDA Hardiness Zone Map indicates nothing about high temperatures. It only reflects an area’s low temperatures (and does a pretty poor job of that, too). If a city goes from Zone 6 to Zone 7, that in no way means the summer temperatures are getting any higher. Your job is to explain to your employees and customers how the map works. I wish you luck. It’s like calculus -- some people will just never get it.
Use the USDA Hardiness Zone Map as a guide for plant selection, not a cast-in-stone guarantee of plant survival. In Zone 6 you can plant a Zone 7 plant and it may survive for more than a decade. But all it takes is one good Arctic blast and, zap, that plant is history.
So for certain things like accent plants and shrub/perennial border denizens, you can get by with cheating by a zone or maybe even two.
But for long-lived plants like shade trees, you’d better stick with your zone or even trade up a level. If that shade tree lives up to its expected lifespan, it’s guaranteed to get hit with a 50- or 100-year freeze.
Tell clients that plant hardiness is not an exact science. Don’t be afraid to sell plants that push your climate’s limits, but make sure customers know the expectations.
--Todd Davis, guest blogger
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