We're not the tropics, but an Iowa "hardiness zone" has crept in.
Minnesota is now officially a more comfortable place for many favorite garden and landscape plants -- and possibly some new ones -- but that doesn't mean grapefruit and palm trees are headed to a yard near you.
A new U.S. map of plant "hardiness zones," released Wednesday by the Agriculture Department and awaited by gardeners for more than 20 years, shows that levels of plant-killing winter cold in Minnesota have eased from border to border. That may not surprise veteran gardeners, who have experienced Minnesota's long-term winter warming trend.
"The good news is that it probably just improves people's success with a few more items," said Lew Gerten, a partner and general manager at the nursery and greenhouse operation of the same name in Inver Grove Heights.
Scott Endres, co-owner of Tangletown Gardens, which is in a very small part of south Minneapolis where the hardiness rating rose to a level matching Iowa's, said it will encourage gardeners to "push the envelope a little bit."
"It's exciting to see what we've expected for a long time," Endres said. "We don't have to have quite the zone envy gardeners in our area often have had."
The new map comes seven months after the publication of the nation's new climate normals, which showed a general warming pattern across Minnesota over the past 30 years, with a pronounced increase in the winter months in the Twin Cities. The USDA map averages only the lowest annual temperatures -- the deep-winter extremes that can kill perennial plants -- from 1976 through 2005. It is thus only a partial picture of climate change, but likewise reflects warming.
Most of the metro area, and much of southern Minnesota, saw a half-step increase on the new map, from zone 4a to 4b. More dramatic changes were indicated elsewhere in the state.
Zone 2b disappears
The coldest rating on the previous Minnesota map -- 2b in two areas of northern Minnesota -- disappeared entirely from the new map, while two small lobes of a warmer zone previously unseen in Minnesota, 5a, crept across the Iowa border into southern Minnesota; that zone also popped up in a few very small parts of the metro area, the so-called "urban heat island."
This means some perennials whose northern limit was central Iowa on the previous map might now be reliably planted in Fairmont, Minn., or parts of Minneapolis and Richfield.
In all, there are 26 plant hardiness zones across the country, divided in five-degree increments, ranging from 1a (the coldest) to 13b.
The previous map was produced in 1990 and was based on only about a dozen years of weather data, from 1974 to 1986, said Mary Meyer, horticulture professor at the Minnesota Landscape Arboretum.
"We did want a [new] map that's a better reflection of our conditions, and this does that," Meyer said. "And it includes the new years, where we've seen much milder conditions."
USDA officials said the new map is based on readings from 8,000 temperature-observing stations across the country as well as slope, terrain and proximity to large bodies of water. Experts from across the country were consulted as well to provide "ground truth" and produce a map with unprecedented accuracy, said Catherine Woteki, undersecretary for research, education and economics at the USDA.
Local knowledge still best
"I'd also like to remind gardeners that as sophisticated as this one is, maps are truly guides," Woteki added. "Nothing is better than the gardener's knowledge of his or her own garden."
So with seed catalogs arriving in Minnesota mailboxes, what will change?
Gardeners might be more confident in planting warmer-climate plants; a plant such as lavender, which most Minnesotans dig up each fall, might now hang on through the winter and return the next spring, Meyer said. Some lilies that people now tuck close to the house might thrive in more open parts of the yard, Gerten said.
In Bemidji, which had been in the coldest zone on the old Minnesota map but moved up a full notch, Tyler Olson, co-owner of Nature's Edge nursery and greenhouse, said his customers have been growing apple trees bred for the Twin Cities area for several years. He even has some grasses formerly rated as hardy no further north than mid-Iowa growing in his garden.
"We're definitely having less severe winters," he said. But he's not going to try to sell a customer a $130 tree bred for Iowa, he added.
"I think with all plants, it's still a little hard here in northern Minnesota," he said.
Bill McAuliffe • 612-673-7646