Get It Growing: Understanding Hardiness Heat Zones Helps You Pick The Right Plants

Daniel Gill, Merrill, Thomas A.  |  3/31/2007 12:35:37 AM

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Get It Growing News For 04/06/07

By Dan Gill
LSU AgCenter Horticulturist

Average minimum temperatures are an important issue when choosing plants that are well-adapted to our climate. But so are the summer temperatures, since our landscape plants must be able to survive both extremes.

It wouldn’t make much sense to plant a tree, shrub or anything you expect to be a long-lived part of your landscape if it wasn’t hardy enough to survive the winter. Of course, it also wouldn’t make much sense if your choices couldn’t stand up to our summer heat.

To help you in making such decisions, there a couple of tools – known as hardiness zones and heat zones.

With regard to hardiness, let me provide some explanations.

Hardy plants are those that can reliably survive typical winter temperatures without protection. Permanent woody plants, like trees and shrubs, should be completely hardy and suffer no damage from the lowest expected temperature. Other plants may be "root hardy" – meaning they reliably return from their crown or below ground parts (bulb, tuber, rhizome, etc.).

Plants killed by typical winter temperatures are tender. Since our winters vary in their severity from year to year, tender plants may sometimes survive a series of milder-than-normal winters only to succumb to a severe freeze.

For a plant to be considered completely hardy, it must be able to tolerate the lowest temperatures an area may receive, not simply to make it through a milder-than-typical winter. A real concern when selecting a plant, then, is whether it is hardy enough to withstand the typical winter temperatures where it will be planted.

To help gardeners with this issue, the U.S. Department of Agriculture developed a Plant Hardiness Zone Map based on the average minimum temperatures experienced in different areas of the United States. The zones range from 3 to 11, starting with the lower numbers in the colder northern United Staties and the higher numbers in the milder southern part of the country. Each zone is further subdivided into northern "a" and southern "b" sections.

The state of Louisiana includes just two zones, Zone 8 and Zone 9. The northern section of the state is Zone 8a (average minimum temperature 10 to 15 degrees F). The central and some of the southern portion of the state is in Zone 8b (average minimum temperature 15 to 20 degrees F). Coastal areas of the state are Zone 9a (average minimum temperature 20 to 25 degrees F), and the southern portion of Plaquemines Parish is Zone 9b (average minimum temperature 25 to 30 degrees F).

When looking at garden catalogs offering plants, bulbs or seeds – or when reading articles in gardening magazines and plant descriptions in books – you often will see the hardiness zones listed after the plant name, such as Hydrangea Forever Pink (Hydrangea macrophylla ‘Forever Pink’) Zones 5-9. That means the farthest north this plant can reliably be grown is Zone 5. In zones farther north, this plant is likely to do poorly or die because the winters are too cold.

You can see how handy this is for gardeners around the country. It helps them avoid planting something that is not hardy enough for the zone where they garden. The farther north your garden is in the United States the more of an issue and concern cold hardiness becomes.

On the other hand, because our winters are relatively mild, cold hardiness would rarely, if ever, be a major concern if we didn’t plant so many tender tropicals in our landscapes.

It’s not a bad idea to choose plants that are hardy at least one zone north of where you garden. That way should an unusually cold winter occur your plants could endure it. So when selecting cold hardy plants for a landscape in Zone 9, make sure they are hardy to Zone 8. If you live in Zone 8, choose plants that are hardy to Zone 7.

You might think that as long as the plants you choose are cold hardy, that would be the end of it. But we know, of course, there is still one more factor – how hot it is during summer and how long the heat persists.

Extreme summer heat has a significant effect on how plants grow. In this case, the hardiness zones are not much help. Knowing that a plant is hardy enough to survive our winters does not mean it’s tough enough to survive our intensely hot summers.

The American Horticulture Society has addressed this issue by developing its Plant Heat-Zone Map. The zones are based on the average number of days per year the temperature reaches 86 degrees F or higher. The usefulness is clear when we see that most of Louisiana is in heat Zone 9 (120 to 150 days) and that most of Oregon is in heat Zone 4 (14 to 30 days).

Unfortunately, because the heat zones have not been around as long as the hardiness zones, and perhaps because much of the country obsesses more over hardiness than heat tolerance, the heat zones are still not widely used in plant descriptions.

Here in the Deep South, however, where summer heat is a major limiting factor in what plants we can grow, these heat-zones should eventually prove helpful. As time goes by you may begin to see the heat zones used more often in plant descriptions. For more information, see "Heat-Zone Gardening" by Dr. Marc Cathey, Time-Life Books.

If you ever wondered what those funny zone numbers were all about, I hope I’ve helped clear things up a bit. Don’t let the zones get you too confused.

If you still need more information, you may also rely on recommended plant lists in books written for our area or printed and Web-based materials from the LSU AgCenter (www.lsuagcenter.com) that are available through the AgCenter’s parish extension offices. You will also find that the majority of the hardy plants you find at your local nursery or garden center are well-adapted to our climate.

Get It Growing is a weekly feature on home lawn and garden topics prepared by experts in the LSU AgCenter. For more information on such topics, contact your parish LSU AgCenter Extension office or visit our Web site at www.lsuagcenter.com. A wide range of publications and a variety of other resources are available.

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Contact: Dan Gill at (225) 578-2222 or dgill@agcenter.lsu.edu
Editor: Tom Merrill at (225) 578-2263 or tmerrill@agcenter.lsu.edu

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