News Release Distributed 08/06/10

By LSU AgCenter Horticulturists Dan Gill, Kyle Huffstickler and Allen Owings

August means that many landscape plants around Louisiana are showing the adverse affects of the hot, humid growing conditions that we experience during the 120-150 days of summer.

It’s important to consider summer heat extremes in determining the success of your landscape plants. We all know it’s critical. But now we have some new information that will simplify the process of selecting landscape plants based on their ability to tolerate Louisiana summers.

In many areas of the country, summer growing conditions are just as important, if not more important, than winter conditions in determining long-term hardiness and survivability for many plant species. We mostly think about selecting plants for their ability to overcome cold weather in the wintertime. Louisiana is different.

Louisiana is located in USDA hardiness zones 8 and 9. Hardiness zones indicate the average minimum temperatures that occur during winter in different geographical regions of the country. Hardiness zone 8 has a minimum temperature of 10-20 degrees, and zone 9’s average minimum temperature is 20-30 degrees.

Since the Hardiness Zone Map was published in 1960, plants have been recommended for different areas of the country based on these zones. For example, crape myrtles are classified as hardy in zones 7-9, which indicates they will do fine across all of Louisiana but may see some cold damage starting in northern Arkansas, Tennessee and Oklahoma where hardiness zone 6 begins.

But it’s just as important to be thinking about how hot our Louisiana summers are when selecting landscape plants.

In addition to the Hardiness Zone Map, we now have the Plant Heat-Zone Map developed by the American Horticultural Society. This map designates 12 different zones in the United States and classifies areas of the country based on the average number of days per year when the temperature is above 86 degrees. Why 86 degrees? This is the temperature where cellular proteins in plants start becoming damaged.

Louisiana is located in heat zones 8 and 9. Zone 8 annually has 90-120 days above 86 degrees, while zone 9 includes 120-150 days in this temperature range. The coastal portions of the state, portions of northwest Louisiana and the area near the Mississippi River in northeast Louisiana are in zone 8. The rest of the state sits squarely in the middle of zone 9.

With this information, we should be selecting plants based on their ability to tolerate these hot growing conditions.

Visual symptoms of heat stress on landscape plants include withering flowers or flower buds, drooping leaves, loss of green foliage color (bluish-gray color), diminished root growth and increased attractiveness to insects.

We can do some things to reduce plant stress symptoms caused by temperature extremes. Keeping the plants well irrigated will aid in plant survivability. It also is important to make sure plants are growing in soils with the right pH and are in the sun/shade situation most conducive for their optimum growth. Planting at times of the year when the plants will be least stressed will also help.

We are starting to see ornamental plants recommended not only for their hardiness zone areas but also for their heat-zone areas, so be aware of this method to select plants for different geographical areas and add this selection method to your plant purchasing efforts.

Visit LaHouse in Baton Rouge to see sustainable landscape practices in action. The home and landscape resource center is near the intersection of Burbank Drive and Nicholson Drive (Louisiana Highway 30) in Baton Rouge, across the street from the LSU baseball stadium. For more information, go to www.louisianahouse.org and www.lsuagcenter.com/lyn.

Rick Bogren