Growing pains: Gardeners experiment with less hardy plants
Nancy Nedveck loves the colorful blooms that grow on agastache and penstemon, two plants that attract tiny hummingbirds. Some varieties of these plants used to be too fragile to grow in south-central Wisconsin, but now Nedveck is happy to be able to offer them among the endless rows of perennials on display at her nursery, south of Oregon.
Thanks to global warming, these and other less hardy plants are surviving and even thriving in this area, said Nedveck, who has owned the popular Flower Factory for 25 years.
"The change over the past 10 years has been gradual," she said, but has accelerated during the last five, as temperatures warm and the growing season lengthens.
An ornamental known as giant silver grass, for instance, would not have bloomed in Dane County in the past but now the longer growing season gives it time to produce its fluffy silver flowers. Japanese maples, flowering dogwood and hardy gloxinia also are now thriving.
What all this means to local gardeners is that the "zone" code on seed packets and planting guides may no longer be accurate. These often rely on a hardiness map put out by the U.S. Department of Agriculture that the government hasn't updated since 1990.
The map puts south-central Wisconsin mostly in Zone 4, but Nevdeck says the area is now more of a Zone 5.
The National Arbor Day Foundation, which just recently released its own zone map, agrees with Nevdeck.
"The map confirms that much of the United States has warmed in recent years," said Mark Derowitsch, public relations manager for the foundation.
But while the zone shift gives local gardeners new courage to experiment with plants they might have shied away from in the past, experts urge caution. Seasons are variable even during a warming climate trend -- as last winter proved -- and such factors as early freezes, snow cover and "micro-climates" in one's own backyard must also be factored into planting decisions.
A zone map, in other words, "is not the be-all and end-all," said David Ellis, director of communications for the American Horticultural Society. "The map is a tool to help gardeners select plants. You have to take into account cold hardiness, heat tolerance, rain levels and other factors that affect how plants thrive and survive."
The National Arbor Day Foundation's 2006 Hardiness Zone Map differs substantially from the 1990 map from the USDA. In the foundation's map, southern Wisconsin is now solidly in Zone 5 and northern Wisconsin is in Zone 4, each one zone higher than in the past. The central part of the state is a mix of the two.
Drawing on data from 5,000 weather stations around the United States, the zones are based on average annual low temperatures using 10-degree increments. Those increments are further divided into 5-degree A and B zones; plants in A zones are able to survive lower temperatures than those in B. The Zone 4 range is from -20 to -30 degrees Fahrenheit. Zone 5 is from -10 to -20 degrees.
Karen Johannsen, co-owner of Johannsen's Greenhouses, said many local gardeners have been successful with Zone 5 perennials that are a little less hardy than Zone 4 plants.
But she warned that there could always be problems with a bad winter.
"Even though it's been warmer, we still get a 20-below plunge just long enough to kill stuff," Johannsen said.
In some areas this winter, for instance, an ice layer under the snow smothered plants and other plants rotted under heavy snow cover, she said.
And because plants are blooming later in the season, they may not go dormant, leaving them vulnerable to winterkill when a cold snap does hit, Johannsen added.
Ellis of the American Horticultural Society said his organization recommends that gardeners visit their local botanical garden to see what is growing there.
"They are usually on the cutting edge as to what can survive, and many have trial gardens for borderline plants," Ellis said.
"Our overall recommendation is to experiment a little with plants that are less hardy than the traditional zones but not to make wholesale changes to the garden."
Jeff Epping, director of horticulture at Olbrich Botanical Gardens in Madison, said he uses hardiness maps as a general guideline.
"I don't use them as the bible because I have killed plants that supposedly are hardy according to the map and I have successfully grown other plants they say are not hardy for us here," said Epping, noting that some Japanese maples in the area did not survive this past harsh winter.
He said it would be reasonable to plant a zone up or zone down.
"If you are a gardener, have fun, and use the zones as a general guideline. But half the challenge is to grow things you're not supposed to," Epping said. "We have plants here that by the books shouldn't be here. But plants don't read books."
Local gardener Jane LaFlash, a member of the board of the Wisconsin Hardy Plant Society, grows hostas, ferns, geraniums, woodland wildflowers, small trees and shrubs in what she calls her "small, very urban and sheltered yard."
"I make the assumption that the nurseries are selling things that are hardy," she said. "They should only sell stuff that is hardy here."
LaFlash said every garden is different.
"You might have a little micro-climate in your yard, or a corner of your garden, and a few miles away there is a different situation."
Ann Munson, the Dane County UW-Extension master gardener, favors a conservative approach and says local gardeners "are in denial."
"They try to buy plants that are hardy for Zone 5. Some winters that works. This winter we lost a lot of things that have been hardy for years."
Evergreens, maples and such shrubs as nine bark, for instance, suffered damage.
People using Zone 5 plants should put them in sheltered locations out of winter wind and winter sun, Munson suggested. Zone 6 plants should probably be avoided, she added.
Several experts, including UW-Madison horticulture Professor Brent McCown, also warned that global warming would likely bring more extreme weather, including periods of heavy rain followed by drought. Also, new pests and plant diseases will move farther north.
"You can plant things you wouldn't have been able to, but the bad news is that the various pests and pathogens also are shifting with these hardiness zones," said John Williams, an assistant professor in the UW-Madison Department of Geography and the Center for Climatic Research.
He also noted that scientists are predicting more extensive zone changes in the future, depending on how much greenhouse gas spews into the atmosphere: "We can expect drastic changes of hardiness zones and the plants we grow and the natural landscape around us."
