Meet David W. Wolfe, Cornell plant scientist and expert on the impacts of climate change on agriculture. Wolfe is also the author of an absolutely delightful book about the soil, Tales from the Underground: A Natural History of Subterranean Life, that ought to be required reading for all gardeners.
In the 2011 Timber Press title The New American Landscape: Leading Voices on the Future of Sustainable Gardening, Wolfe wrote the chapter on gardening and climate change. This week, I called him up to ask about the new USDA hardiness zone map, after I looked up my own zone and got a result that in no way corresponds with current reality. Zone 5a? Maybe 20 years ago, but not now. (For more about the map, see Elizabeth's post here.)
Q: The old zone map was based on a 13-year average of winter minimum temperatures from 1974 to 1986. The current map is based on a 30-year average that reaches all the way back to 1976. In a time of rapid winter warming, how does looking at this longer time scale make any sense?
A: First, hats off to the USDA for doing a new map. We haven't had an official one since 1990 and a lot of nurseries and commercial growers have been waiting a long time for a new map. Why the USDA chose a 30-year average, I don't know. It means we can't really compare it to the past map.
And since the spans overlap, a good percentage of the data in the new map was already in the old map. This dampens the perception of change.
Of course, there is the argument that looking at a bigger swath of time is more reliable. On the other hand, we know we are on a trajectory, so a longer time span will dilute more recent changes. A 15 year-map would have been preferable in my opinion.
That said, the map still shows a significant shift towards higher average minimum winter temperatures in the Northeast and Ohio, as well as other places. The map corroborates changes we are seeing in the living world, as plants bloom earlier, as insects appear earlier, as their ranges move northwards.
For conservative gardeners, this map is fine.
Adventuresome gardeners, on the other hand, would probably prefer something else. In fact, in 2002, the New York Botanical Garden planted a test garden in the Bronx of plants hardy only to a zone warmer than its zone on the 1990 map. These plants are all doing fine.
Q: Though I'm delighted that warming winters make similar experiments plausible in my own yard, your chapter in The New American Landscape makes it clear that climate change is not all fun and games. We're all probably aware that warmer winters mean new pests, but you mention other effects, such as "de-hardening" during winter warm spells.
A: Well, when you say "pests," your readers probably think of insects, but warmer winters are also really great for deer. Without a long period of snow cover when they can't feed, they have better survival rates, produce more young, and are likely to want more food from your garden.
About "de-hardening": The buds and stems of perennial plants go through a physiological hardening process so they are protected from winter damage. If they "de-harden" during a warm spell, then the threshold for winter damage is at a higher temperature. In the winter of 2003-2004, we had a very warm December in upstate New York and then cold temperatures in January. It caused millions of dollars' worth of damage to new plantings of European wine grapes It wiped them out down to the ground. In late winter, a warm period can cause a premature leafing out and frost damage.
Many perennials also have a chilling requirement that may not be met in future, so they may not be able to produce a good amount of flowers and fruit. Apples, for example, have a long winter chilling requirement: 1200 hours with temperatures below 45 degrees. So do our native Concord grapes. On the other hand, European wine grapes, Vitis vinifera, have much less of a chilling requirement. We may be growing more of those.
Q: In The New American Landscape, you recommend "cautious exploration" with less hardy plants on the part of gardeners. Why not wild experimentation?
A: Actually, gardeners can lead the way here, figuring out how we can take advantage of the opportunities offered by a warming climate, because it's not their entire livelihood at stake, as with farmers. Maybe we need VIctory Gardens in a new context, that of climate change.