News Release Distributed 04/26/12

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

Keep environmental conditions in mind when you select plant materials for a home landscape. The size of the planting area is important along with other site characteristics, such as sun or shade exposure, wet or dry locations, and exposure to windy conditions. Selecting the proper plants based on regional adaptability to climate and environmental conditions is a sustainable landscape practice.

Selected plants should tolerate existing conditions and should be hardy in the appropriate climatic zone. Louisiana’s 50-60 inches of annual rainfall are also an important consideration in selecting landscape plants. We need to keep in mind, though, that Louisiana rainfall is not well distributed, so some times during the year we can be excessively dry while at other times we are very wet.

Louisiana is located in U.S. Department of Agriculture hardiness zones 8 and 9. Hardiness zones indicate the average minimum temperature that occurs during winter in different geographical regions of the country. Hardiness zone 8 indicates average minimum temperatures of 10-20 degrees, and hardiness zone 9 indicates average minimums of 20-30 degrees. Ever since the hardiness zone map was published in 1960, experts have been recommending plants for different areas of the country based on these zones.

Besides hardiness, you also need to consider summer heat extremes in selecting landscape plants. We know this is critical, but now we have some new information that can simplify the process of plant selection based on their ability to tolerate Louisiana summers. Summer growing conditions are just as important, if not more important, than winter growing conditions, in determining long-term hardiness and survivability for many plant species.

In 1997, the American Horticultural Society released the Plant Heat-Zone Map. This was a revolutionary idea coordinated by the late H. Marc Cathey, president emeritus of the society. The Plant Heat-Zone Map contains 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 where cellular proteins in plants start becoming damaged.

Louisiana is located in heat zones 8 and 9. Zone 8 has 90-120 days annually above 86 degrees, while zone 9 includes 120-150 days in that temperature range. 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.

Most plants suffer heat-stress damage, but it occurs more gradually than cold-weather damage. Heat stress occurs over an extended period – anywhere from a month or so to over a couple of growing seasons. Withering flowers or flower buds, drooping leaves, loss of green foliage color (bluish-gray color), diminished root growth and increased attractiveness to insects are some indications of stress. Many Louisiana ornamental plants had heat-stress symptoms last fall after our unusually hot, dry summer.

Environmental factors other than heat may affect our plants, but we can minimize these problems and maximize success. Humidity, water availability, oxygen exchange, light quality and quantity, day length (photoperiod), wind movement, soil conditions and available nutrients all play a role in success with your garden plants not only through summer but also at other times of year.

Water availability – or limiting water stress – goes a long way in eliminating plant stress. Maintaining optimal soil moisture levels is critical. It is hard to do without proper irrigation management. Plants cannot adapt without irrigation when we receive a 4-inch rainfall in a 24- to 48-hour period then go through 30-day droughty periods at other times of the year. These are difficult conditions for plant adaptation.

Plants need oxygen exchange for respiration; roots need oxygen to breathe. This requires ideal bed preparation that creates sufficient pore space between individual soil particles.

Be aware of light quality, quantity and day length, also referred to as photoperiod. These conditions are important as plants undertake their physiological processes during summer. Plants need light and carbon dioxide for food manufacturing (photosynthesis). Light also affects the plants’ temperature – plants growing at the limit of their heat zone may die due to environmental stress in summer if planted in full sun or with a southern and/or western exposure where heat buildup is more likely to occur.

When you head for the garden center to select new plants for your landscape, evaluate your planned location and keep these factors in mind.

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.lsuagcenter.com/lahouse or www.lsuagcenter.com/lyn.