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Plant Mappers Push to Keep Up with Climate Swings

NPR | By Ketzel Levine

Published June 27, 2007 at 1:40 PM EDT

The USDA's Plant Hardiness Zone Map helps gardeners, growers and planners determine which plants will best suit a climate across the United States. The USDA hopes to release a new map by the end of this year.

Tony Avent stands at his garden in Raleigh, N.C.

Tony Avent has been an adviser on a long-awaited revision of the USDA Plant Hardiness Zone Map. The map divides the country into climate zones based on average low temperatures, and is a much-consulted guide for professional and amateur gardeners who want to know which plants will thrive in their region. Avent, of the well-known Plants Delight Nursery in Raleigh, N.C., talks here about the history of the map, its strengths and shortcomings, and how a changing climate may soon bring about yet another revision of the map.

Ketzel Levine: I've always figured that those of us who care about the zone map were plant geeks. True?

Tony Avent: I'd thought so to, but during our map meetings, I was surprised at the diversity of people that use the map: universities, academics, forestry people, fruit growers. There were people from all over.

As long as I've been gardening, people have been complaining about the zone maps. You, too?

Definitely. I remember there was a great controversy between the first hardiness map from the Arnold Arboretum — which used 40 years of climate data — and the new USDA map, which used only 13 years. And this is the map the baby boomers have grown up with, the 1990 map.

I think we typically see our climate running 15 to 20 years hot, 15 to 20 years cold. And that 1990 map happened to use data from a particularly cold period. It became controversial because it dramatically changed the zones downward from the Arnold Arboretum map.

So one day you were happily zone 7, and the next you were zone 6... 10 degrees colder?

Exactly. Now fast-forward to 2003, and a new map by the American Horticulture Society (AHS). Those folks did something very bad, they actually deleted the half zones!

For example, they got rid of zone 7 "a" and "b."

Right. Most people might think a half zone is not a big difference, but it's a dramatic difference. Fortunately, there was enough uproar that the map was withdrawn.

The other problem was that the 2003 map captured a much warmer period, and indicated that people could grow less hardy plants that they couldn't before. But one cold winter and those plants would all be dead!

Any other ill-fated maps since?

Last year, the Arbor Day Foundation also got rid of half zones and moved everybody upward a zone. For example, Chicago went from zone 5 to zone 6. If people in Chicago had been planting trees according to that map, they would all have been killed this past winter.

So inevitably the question is, how will the new USDA map avert all these problems?

The USDA formed a group of stakeholders, including myself, who reached a consensus on what kind of criteria we wanted on the map. We've returned to "a" and "b" zones, and we'll be using a 30-year data set. Then once this map's out, the process will begin again on an even more detailed version.

Including?

We intend to include factors like the number of days over a certain temperature, humidity, night temperatures and other factors that affect heat adaptation. After all, hardiness is not just about winter lows.

In other words, a plant may do just fine in Raleigh during winter, but die during the summer. Like Hebe, which loathes your humidity.

Right.

So when's the next map due?

The best we can do now is keep our fingers crossed for year's end.

So let's wrap up with some advice for gardeners. What's there to learn here? How much should we rely on zone maps at all?

A map is only the first tool one should look at when selecting plants. Other things to consider are a plant's moisture needs, tolerance for humidity, amount of sun, and summer temperatures.

This is pretty conservative advice from a man whose tagline reads, "I consider every plant hardy until I've killed it myself three times!"

Well, people often limit themselves because they buy a plant and kill it, and assume it's not good for their climate. That's not necessarily the case. Many times there are more factors involved. So often it's a microclimate, a soil condition, just something else that you haven't got right. Don't be afraid to experiment!