Warmer temperatures give gardeners opportunity to grow new greenery

BY DORA GROTE | APRIL 19, 2012 6:30 AM

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Cindy Parsons wouldn't normally grow magnolia trees in her Iowa yard, but she might experiment with them this year.

"People, me included, might try to push the envelope," she said, noting the shifted growing zones. "Gardening is a lot of trial and error."

The U.S. Department of Agriculture recently reconfigured the growing zones — or plant-hardiness map — based on 30 years of temperature data from almost 8,000 weather stations.

Iowa's new mapping reflects a warming trend.

Each zone determines what types of plants can be grown in a particular area, which is printed on the back of each plant package at nurseries. Zone 1 is the coldest and ranges to zone 13 — the warmest.

Iowa is now almost all in Zone 5-5a and 5b — with some areas of Zone 4b in the north of the state.

The slight shift in Iowa's new designated zones won't have a drastic effect on local gardeners but might cause more experimenting with plants like magnolia trees — typically grown in zone 6 areas.

"The map is just a guideline, not blueprints," Parsons said, a co-president of Project Green in Iowa City. "It's just kind of officially establishing the new climate changes, but gardeners have already made those adjustments a while ago."

Atmospheric and oceanic sciences expert Scott Spak said the USDA regularly checks the actual temperatures and length of the growing season in each zone. He said the zones shifted slightly north since the last update in 1990, as average wintertime temperatures increased across Iowa and most of the continental United States.

"Some types of plants can withstand colder winters and then still grow in the summer and bear fruit," Spak said. "It's a biological designation."

He said the shift in growing zones is consistent with the increase in temperature.

"Climate scientists have found it very likely that human activities, including increased greenhouse gas emissions, have contributed to that change," Spak said.

Iowa could eventually see different plants appearing, he noted.

"If the trend continues, we may — over a period of 50 to a 100 years — see a dramatic change in what we can grow and what we can't," Spak said.

Lucy Hershberger, one of the owners of Forever Green garden center in Coralville, said the biggest change is the more detailed breakdown of the zones. Iowa City is very close to the border between 5a and 5b.

Hershberger said gardeners had already started to experiment before the official change. Some local gardeners are now growing Japanese maples and Florida dogwoods — plants normally grown 100 miles south of here.

"I'm always a little cautious when I plant trees and shrubs because I hate to see people put in a tree that won't tolerate the cold after 10 to 15 years," Hershberger said. "Perennial-wise, it opens up a lot of different things that gardeners can grow including different kinds of oriental grasses. They're not at a risk that a tree might be."

Parsons said gardeners might try different varieties of fruit trees, roses, and shrubs, but despite what zone a grower's garden falls in, everybody's yard is different.

"The plants can't read the map so they don't always respect our zones," Parsons said. "Even though the map says you're in 5a, each garden has little micro-climates."

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