This rose bush is an example of how summer drought and heat can create problems for many plants in the landscape. (Photo by Allen Owings. Click on photo for downloadable image; right click to save.)

News Release Distributed 06/07/13

By Allen Owings, LSU AgCenter horticulturist

HAMMOND, La. – June is here, and that means summer will officially start in a couple of weeks. In reality, Louisiana typically experiences summer-type growing conditions from early to mid-May to as late as late September or early October.

Although we don’t know what the next three to four months will bring, we commonly see hot conditions with daily highs in the low to mid-90s with warm nighttime temperatures. Although we have seen more drought-type trends with less warm-season rainfall the past few years, summer humidity and moisture can still cause problems.

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

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 located in U.S. Department of Agriculture hardiness zones 8 and 9. A hardiness zone indicates the average minimum temperature that occurs during the winter in different geographical regions of the country. Hardiness zone 8 has average minimum temperatures of 10-20 degrees, and the zone 9 average low temperatures are 20-30 degrees.

Since the Hardiness Zone Map was published in 1960, experts have been recommending plants 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 in northern Arkansas, Tennessee or Oklahoma, where hardiness zone 6 starts. But it is just as important to be thinking about our hot Louisiana summers when selecting landscape plants.

In addition to hardiness zone maps, we now have a Plant Heat Zone Map. Developed by the American Horticultural Society, the map identifies 12 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 when cellular proteins in plants start becoming damaged.

Where is Louisiana located in the heat zone ratings? Louisiana is located in zones 8 and 9. Zone 8 has 90-120 days annually above 86 degrees, while zone 9 has 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.

Are we 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.

What can we do to reduce plant stress symptoms due to temperature extremes? Keeping plants well irrigated will aid in survivability. It is also important to make sure we have plants growing in the right soil pH and growing in the sun or shade location most conducive for their optimum development. Planting during the times of the year when the plants will be least stressed also will help.

We are starting to see more ornamental plants recommended not only for their hardiness zone but also for their heat zone. So be aware of this technique for selecting plants and add this method to your plant purchasing efforts.

You can see more about work being done in landscape horticulture by visiting the LSU AgCenter Hammond Research Station website. Also, like us on Facebook. You can find an abundance of landscape information for both home gardeners and industry professionals at both sites.

Rick Bogren