Plant hardiness: It’s on the map


Now that winter has arrived, you may be wondering how your plants will hold up. Will there be empty places in your landscape this spring? What can you do to protect your plants?

When selecting plants for landscapes we are very careful about “climate zones.” It’s pointless to plant perennials and woody plants that can’t survive Ohio winters. There are two ways we decide about winter hardiness. One is the USDA Hardiness Zone for each plant; the other is our personal experience.

Hardiness Zones are sections of the United States where the soil typically stays within a certain temperature range. It’s not the air temperature that kills plants; it’s when the root system gets colder than the plant can stand. Air temperatures and “wind chills” can get extremely low for short periods of time, but if the soil temperature never gets below what the plant can survive, the plant will probably live. The most effective protection from “winter kill” is to select plants from hardiness Zone 6 or below.

Southern Ohio is in hardiness Zone 6. Plants hardy in Zone 6 can survive root temperatures between 0 degrees and -10 degrees, which is about as cold as the ground ever gets in this part of Ohio. Plant labels should always have a hardiness zone number on them. The lower the number, the colder the root temperature the plant can tolerate. Remember that soil in raised beds, containers and planters gets much colder than the ground itself, so plants hardy in Zone 5 or below are a better bet.

For example, blueberry plants are hardy to Zone 4. If you look at the map, you’ll see that Zone 4 means the upper Midwest and northern New England states. Zone 4 plants tolerate root temperatures as low as -30 degrees, so it’s no problem to grow them here. On the other hand, Zone 7 plants may survive most winters in Ohio but they’ll need special protection; a sheltered location where the ground doesn’t freeze very deeply.

Most crape myrtles are hardy to Zone 7, so they’re not hardy in Ohio. We’ve found a half-dozen varieties that are hardy in Zone 6, so there are places for them in Ohio landscapes. Winter will often kill the top growth, but not the roots. We recommend planting crape myrtle in sheltered areas, away from winter wind, so even Zone 7 crape myrtles can survive some winters if they are mulched to insulate the roots. But Zone 7 plants aren’t a good investment in Ohio so we don’t plant them.

Another trick for winter survival is plenty of moisture. Winter winds will dry out the branches just like summer drought, so a deep soaking before the ground freezes can help plants survive winter. Another survival tip is to postpone pruning and shearing of shrubs and perennials until spring, since the extra foliage helps protect from wind and cold.

USDA Hardiness zones help predict winter hardiness but don’t tell us which plants can tolerate summer heat extremes. Summer heat can be a problem for many plants. For example, have you ever wondered why Frazer Fir isn’t grown in southern Ohio, so Frazer Fir Christmas trees must come from other regions? This is because extreme summer heat is just as deadly to some plants as bitter cold. If you are concerned about heat stress on your landscape, you can find maps online that divide the USA into zones based on the number of days each year with temperatures above 86 degrees.

Steve Boehme is a landscape designer/installer specializing in landscape “makeovers”. “Let’s Grow” is published weekly; column archives are on the “Garden Advice” page at For more information is available at or call GoodSeed Farm Landscapes at (937) 587-7021.

By Steve Boehme

Contributing Columnist

No posts to display