Your
YARD AND GARDEN

What's in a number?

If there is one thing you could say about our state, and there probably are at least one and a half billion things, it's that the climate runs the gamut from A to Z.

Things that do well in the southern tip freeze out completely in Rockford. Things that grow in JoDaviess County suffer during the summer in Cairo.

Plant hardiness is the key to plant survival, whether you're talking shade trees, flowering shrubs, evergreens or fruit plants. So often hardiness is a measure of winter's cold. In fact, the vast majority of hardiness problems deal with cold temperatures.

David Robson

Many garden catalogs and books list plant hardiness. The primary listing is published by the United States Department of Agriculture (USDA), which places plants in zones from 1 to 10. Zone 10 is ten degrees warmer on the average than Zone 9, which is 10 degrees warmer than Zone 8, and so forth. Since gardeners felt that 5 degrees could make or break a plant, the zones were subdivided into A and B Zones, with Zone B five degrees warmer than Zone A.

Most of us are savvy enough NOT to try growing palm trees and oranges in Illinois, at least not outdoors. We can get some good summer growth, but what remains next spring is essentially kindling or compost fodder.

For years, horticulturists and gardeners discussed how many days of subzero temperatures were needed to kill a certain plant. Unfortunately, nothing concrete was ever established. Soil types, plant growth and location all had an effect on plants. And just when someone said this plant wouldn't grow, someone in that area grew it successfully.

Seven years ago, I planted a southern magnolia in my yard, right next to a nandina. Both are really USDA Zone 7 plants, meaning they can grow in zones 7, 8, 9 and 10. However, the nandina was transplanted from a similar yard in Springfield where it had been growing for close to 30 years. The magnolia was a ten-inch seedling. In the first case, the plant had adapted for 30 years to the local environment. For the magnolia, its youthfulness helped it overcome several harsh winters.

This really isn't bragging. Anyone can try to push the boundaries of plants. Sometimes you succeed; most often, you don't. This doesn't mean the plants are as nice as they would be in Alabama or Georgia. They will lose their leaves, and, in the case of the nandina, die back to the ground. But they keep living.

Hardiness is different things to different plants. Oranges would definitely die completely in Illinois. Crape myrtle usually just freezes back to the ground, but since it blooms on second year wood, you end up with nothing but a green plant.

Azaleas and rhododendrons may have cold-sensitive flower buds. If the winter temperatures drop too low, the buds are killed. That's one of the main reasons why peaches don't do well in northern Illinois.

Unfortunately, the USDA Hardiness Zone numbers can be misleading. While they talk about a plant's sensitivity to cold, they really don't deal with summer's conditions.

Have you ever seen a Colorado blue spruce or paperbark birch growing in Arizona or Florida? Probably not. These plants actually prefer cooler temperatures during the summer and winter. Even central Illinois' summer temperatures are a little too hot for blue spruces. While the plants don't die, they never seem to get as tall and lush as they do in Canada.

For the best luck, stick with USDA Zones 3, 4 and 5 plus the A and B of each if listed. Gardeners south of Interstate 72 can attempt Zone 6 plants, but the further north you go, the less likely you'll have success.

Finally, you need to watch out for plants that can grow from Minnesota to Florida such as Flowering Dogwood. These spring blooming trees look the same in St. Paul as they do in Tallahassee. Try transplanting one to the other location and they die. There's enough variation in the plant's make-up that's been handed down over the centuries.

In other words, always be leery of plants that were grown in a location further south than yours, even though you see them growing in your area. They may be similar, but not totally hardy. Locally grown plants, or those from colder regions, are usually the best bet.

David Robson is an Extension Educator, Horticulture, at the Springfield Extension Center, University of Illinois Extension. You can write to Robson in care of Illinois Country Living, P.O. Box 3787, Springfield, IL 62708. Telephone: (217) 782-6515. E-Mail: robsond@mail.aces.uiuc.edu

18 ILLINOIS COUNTRY LIVING FEBRUARY 1999