The USDA has spoken: Yes, it's a little warmer, almost everywhere. But unless you're a gambler -- or lucky enough to live in the new Zone 5 microclimate -- don't rush out to buy a Japanese maple.
The Plant Hardiness Zone Map released by the USDA last week has generated lots of buzz in plant circles, but it doesn't change the landscape for most gardeners in the Twin Cities area, which is still Zone 4. It does, however, confirm what many experts have been noticing.
"We're having milder winters. We don't need a zone map to tell us that," said John Daniels, vice president of nursery production/wholesale for Bachman's. "It's just telling us officially what we knew."
Average winter low temperatures are creeping up, said Mark Seeley, a climatologist with the University of Minnesota Extension. The downtown areas have not seen minus-25 since February 1996, he noted, although outlying suburbs have plunged that low much more recently.
Many seasoned gardeners say the new zones are hardly game-changers. Much of the metro area warmed from Zone 4a to 4b (a five-degree difference), but most plants aren't rated for subzones, so it doesn't change what gardeners can safely plant, Daniels said.
And although the new map reflects higher average temperatures, averages include extremes, he noted. "We'll still have some cold winters. So my message continues to be to use caution."
"I'm not going to do anything different as a gardener," agreed Kathy Zuzek, a University of Minnesota Extension educator in horticulture. Hardiness is about more than temperature, she said. Even if it doesn't get super cold in January, the rate of temperature drop can affect a plant's dormancy and ultimate mortality, she said.
But the revised zones might encourage more experimentation and success with certain plants, such as redbuds, she said. Even those rated Zone 4 don't reliably bloom every year for many gardeners. "Those are the kinds of plants that might do a little better. A few degrees can make a big difference," she said.
Welcome to Zone 5
The biggest news is that a small patch of the metro area, including parts of south Minneapolis, Bloomington and Richfield, is now officially Zone 5. That does open up a whole new category of plants that gardeners with zone envy have long coveted, such as Japanese maples and certain hydrangeas and roses.
"I'm not surprised," said Debbie Lonnee, horticulturist with Bailey Nurseries. "How lucky for the inner cities! They can get away with some things."
What makes this little urban oasis more temperate?
Many factors could be involved, said Seeley. Microclimates are a complex phenomenon, influenced by slope, elevation, soil conditions, proximity to bodies of water and urban "heat islands" caused by human activity.
The metro area's new Zone 5 happens to include several big manmade, heat-generating entities, including the Mall of America, both Minneapolis-St. Paul International Airport terminals and busy interstate highways, Seeley noted. The downtowns, which are still rated Zone 4, may have more building density but also more vertical disparity, with low pockets between tall buildings. "You can get a canyon effect, with settling of cold air in those pockets," he said.
The new zones are based on data collected at weather stations, he added, which are susceptible to variables. "The world's not perfect, and it's difficult to find a perfect position for every sensor," he said.
Scott Endres, co-owner of Tangletown Gardens, located in the new Zone 5, has long suspected that the area surrounding his shop represents a milder microclimate. It helps explain why some gardeners in the area have had great success with Zone 5 plants, such as Japanese maples.
"What is exciting is it gives a little more license to more conservative gardeners," Endres said. "They'll think, 'If the USDA says it's OK, maybe I can try it.'" He's been experimenting with Zone 5 plants all along. "We're not going to do anything different. We're known as the guys who push the envelope." So does that mean they'll start pushing Zone 6 plants? "That's a stretch," he said.
The new map is not likely to change plant sales policies. "We're not going to consider heat islands in our warranty policy," said Daniels. (Bachman's flagship store in Minneapolis is now located in Zone 5, but its growing range in Farmington is Zone 4.)
"We haven't figured out how we'll deal with being Zone 5," said Endres. "Maybe we'll have to card everybody," he said with a laugh.
Pros and cons
Climate change definitely has an upside for those who like to garden. "Whether it's good or bad, the jury's still out, but it's good for gardeners," Endres said. "There's a little more on our palettes we can use."
But warmer winters are more complicated when it comes to the effect on native ecosystems. "It will have a very significant impact on native plants," said Lee Frelich, director of the Center of Forest Ecology at the University of Minnesota.
The USDA's revised plant zones are not a surprise to ecologists, he said. "The only surprise to me is that it took 'em so long to update the map. If anything, it's a little too conservative."
Minnesota is where many native plants reach the southern or northern part of their range limits. Those limits are already changing, he said. Shagbark hickory, a tree that Frelich grew up with in southern Wisconsin, now thrives in the Twin Cities area, he said. Black spruce trees, which occur in the northern suburbs, are moving farther north. Yellow oaks and honey locusts, once native only in the state's southeast corner, are now thriving well north of that. "It's good for one group, bad for another," Frelich said.
It's no longer cold enough in the Boundary Waters to keep maples out of the arboreal forest, he said. Once maples take hold, they will displace spruce, which, in turn, will displace moose. "Moose won't live in a maple forest."
On the plus side, we'll probably have more pretty flowers. For example, he's seen forsythia shrubs that used to bloom only below the snow line. Now those bushes are blooming all over, Frelich said.
On the negative side, we'll probably have more pests. Some bugs that haven't been able to survive Minnesota winters, such as adelgids that destroy hemlocks and balsams, may soon be able to. "If winters warm up, we'll have new plants, and also new pests," he said. "Bugs, birds and trees -- things are responding."
Kim Palmer • 612-673-4784