The Farmers' Almanac promised a "wet, wild, very cold" winter for the upper Midwest. Instead, snowdrops appeared at Olbrich Gardens before the holidays. Daffodils emerged in late February. Where are we? Atlanta? Not quite. Try Chicago. According to the recently released USDA 2012 Hardiness Zone Map, Madison is no longer in Zone 4. We are now in Zone 5, along with Chicago. Gardeners and growers follow the Hardiness Zone Map to determine which plants will thrive where they live. The map is based on the average annual minimum winter temperature, divided into 10-degree Fahrenheit zones.
The map, which hadn't been updated since 1990, clearly shows a changing climate. In a news release, USDA takes pains to point out that the new map is based on temperature data from a longer time period (1976-2005). The 1990 map was based on temperature data from only a 10-year period (1976-1986). And the shifts in each area are small: just five degrees, or one-half zone.
Bottom line: Don't start planting figs and bougainvillea just yet. But do think about making slight adjustments in your gardening habits, and be prepared for a few surprises.
Samantha Peckham, a horticulturist at Olbrich Gardens who manages the wildflower and herb gardens, says the variances they're seeing are slight, but noticeable.
"We are sticking with what has always done well here, with a few experiments here and there, like leaving some annuals in that we think might make it through," says Peckham. Petunias and marigolds might have survived one of the top 10 warmest winters ever in Madison.
But what's good for annuals is bad for the native perennials, woody shrubs and evergreens that have adapted to our cooler climate. Peckham reminds gardeners that freeze-and-thaw cycles are hard on plants, stressing their root systems, as warm spells trick them into expending stored energy too soon.
"The best winters are ones with a lot of snow, like the previous two years," says Peckham. "Plants are insulated by the snow, not damaged by sun or wind, and they are safely dormant."
Burpee Seed Co. acknowledged the cold's detrimental effect on perennials in a news release about the zone shifts (the entire gardening world, it seems, has been abuzz about this). But chairman and CEO George Ball claimed it was a boon for vegetable gardeners. "For seed customers, global warming results in slightly warmer and longer summers, which means - since annuals literally feed on heat and light - higher yields of vegetables and longer displays of flowers. In other words: good news! Certainly nothing to worry about," Ball said, according to the release.
Laura Jull, associate professor of horticulture at UW-Madison, takes a more cautious approach. "The zone map isn't a predictor of the future," she warns. "It doesn't affect when you plant your veggie seeds outside. That has to do with the last frost-freeze date."
Jull advises gardeners to be conservative when it comes to planting. Check the cold hardiness zones on plant tags, but don't take them as gospel. Out-of-state growers and mail-order companies will sometimes overestimate cold hardiness zones. Buy local, she advises, since local nurseries "have to be accurate."
Just as plants have a certain critical minimum temperature, so do insects. UW entomologist Phil Pellitteri, who grew up in Madison, is seeing species here that weren't present 35 years ago. Some of these have a detrimental effect on the garden, such as the bean-leaf beetle that chews down the leaves of green beans, and the euonymus caterpillar, which decimates the burning bush shrub. Until recently, the caterpillar couldn't survive our winters, but Pellitteri says he's watched it migrate to Madison, and now to the Dells, over the years.
"I feel like I'm in central Illinois, instead of Wisconsin," says Pellitteri. He points out, though, that not all insect migrants are undesirable.
"Mantids - they're good guys in the garden -are not native to the Midwest, but I've seen them flying overhead and at the Arboretum," he says. "Giant swallowtail butterflies are doing great."
Warmer temperatures are almost always the reason a new insect shows up further north. But the picture gets a bit more complicated when you're talking about unusual numbers of a pest that's been here a long time, such as Japanese beetles, a species that can survive cold pretty well. Regardless of why the pests are here, gardeners have choices in how to deal with them, says Olbrich's Peckham.
"In our new rose garden, we've incorporated woody perennials and trees into the beds, instead of just planting roses en masse. This minimizes the need for chemicals to fight powdery mildew and Japanese beetles. If you intersperse with other plants, the beetles can't just hop from plant to plant."
Staff at Olbrich have been working to educate themselves, and others, on sustainable gardening practices. The botanical garden is using fewer chemicals, minimizing lawn, turning to local experts on composting, and offering a variety of classes on native plants, bird-friendly gardens and low-impact gardening.
"We see not only the new USDA map, but all the other research that's out there," says Peckham. "We understand that humans are impacting the planet, and we are trying to model, to the public, that you can do things in a sustainable manner and still have a beautiful garden."