Gardeners in a new zone

Premium ContentClick on Premium Content on the Featured navigation bar for information.

The pulse of gardeners already has quickened with the lengthening days and arrival of seed catalogs.

But now hearts are skipping beats.

That's because the U.S. Department of Agriculture has updated, for the first time in 22 years, its map of where different plants can be expected to survive the winter.

Not surprisingly, a warmer world means an expanded list of plants likely to survive the seasons in Nebraska and Iowa, meaning more choice, more variety and more beauty.

But the map of hardiness zones doesn't factor in the effects of ongoing climate change and the Midlands' extreme weather, which make planting decisions more complex than they might appear.

"This definitely reflects climate shift," said Justin Evertson, a forester with the Nebraska Forest Service. "But we're still in the middle of the country where we have extreme weather and cold snaps. If anything, we may have more trouble growing things."

Nebraska's state climatologist agreed.

"It becomes more complicated," Al Dutcher said. "Hardiness zones don't address heat stress in summer, precipitation, survivability of extreme events."

Here's why gardeners are praising the map:

» Better data. More years, temperature records from more weather stations, better accounting for terrain and greater detail.

The new map, released Wednesday, covers the 30 years from 1976 through 2005. The 1990 version covered 13 years.

For the first time, terrain has been taken into consideration, including elevation, major bodies of water, valleys and ridge tops. This particularly has improved accuracy in mountain states.

» It's user-friendly. Gardeners can locate their zone by ZIP code on the interactive website. It can be found at www.omaha.com/nancy. Go to the Jan. 28-30 almanac entry.

Climate change has been so apparent that gardeners already have been testing the limits of their official hardiness zones, even before the new map was published. That's why plants more suitable to climates farther south —such as rose of Sharon, bald cypress, flowering dogwood and Japanese maple — have become increasingly common in places like Omaha.

David Courard-Hauri, a Drake University professor who studies climate change, has been testing the limits of Des Moines' climate for several years. He grows peach and cherry trees, considered more suited to a warmer region.

"Not every outcome of climate change is going to be entirely negative for people," he said. "The bottom line is that climate change is bad news — it's small consolation we have tastier peaches."

Generally, the new map reflects an upward shift in zone temperatures of about 5 degrees.

Several climate and horticulture experts said the map is but one guide, and gardeners should consider:

Flowering dogwood/Nebraska Statewide Arboretum

» Extreme cold

The hardiness map is based on an average of extreme lows but doesn't reflect record cold. Omaha is classified as being in a zone with lows of 10 below zero to 15 below zero. However, nine times during the period covered by the map, lows have dropped well below that. Record low temperatures on those dates ranged from 21 degrees below zero to 23 degrees below zero.

» Early leaf budding/flower budding

As magnolia trees often remind gardeners, leaves can survive a freeze that flower buds can't. If you are planting a "Southern" tree or bush for its flowers, study up a little.

» Late leaf shed

A warming world could mean earlier, heavy snows such as the October 1997 snowstorm. Southern trees may hang on to their leaves longer than native trees, making them more susceptible to broken limbs.

» Summer heat.

Extreme heat stresses plants, especially the warming world's higher overnight temperatures. The map doesn't look at high temperatures.

Japanese maple/Nebraska Statewide Arboretum

» Precipitation changes.

Iowa, for example, is projected to become drier as it warms.

» Generational planting.

Hardwood trees can live 100 years or more, so a sapling thriving in today's climate might not tolerate the climate into which it matures.

"If this warming persists in shifting north, people looking at a time frame that exceeds a lifetime could be making an investment in an uncertain future," Dutcher said.

In general, gardeners should continue to look at the micro climate of the site for a plant. This includes soil type, exposure to wind and sun, moisture levels and local influences of temperature (cool air pools in low areas).

Also, within a species, different types, known as cultivars, vary in hardiness. So, for example, those purchasing a flowering dogwood should look for one bred to tolerate the Midlands' low temperatures.

"The moral of the story is this map is one guide to a plant's hardiness," said Brian Kinghorn of Kinghorn Gardens. "It's extremely helpful to someone like me. But there are these multiple factors that will come into play, whether a plant will live or die, or thrive."

* * * * *

« Metro/Region

Share