September 2009 Articles

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Select landscape plants to fit environmental conditions

Select landscape plants to fit environmental conditions By LSU AgCenter Horticulturists Dan Gill, Allen Owings & John Young Environmental conditions matter when you select plants for your home landscape. Available space, sun or shade exposure, wet or dry locations and exposure to windy conditions are all important considerations. Choosing plants adaptable to the environment is a sustainable landscape practice. Plants should tolerate existing conditions and be hardy in the local climate zone. Louisiana’s 50-60 inches of annual rainfall is also an important consideration. Keep in mind, though, that rainfall in Louisiana is not well-distributed, so some periods during the year can be excessively dry and other times can be very wet. Louisiana is in U.S. Department of Agriculture hardiness zones 8 and 9. These zones indicate the average minimum winter temperatures. Zone 8 has an average minimum temperature of 10-20 degrees, and zone 9 has an average minimum temperature of 20-30 degrees. Plants have been recommended for different areas of the country based on these zones ever since the hardiness zone map was first published in 1960. Summer heat extremes also can determine success or failure. Louisiana 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. A heat zone map simplifies the process of selecting landscape plants based on their heat tolerance. The American Horticultural Society (AHS) released the Plant Heat-Zone Map in 1997. This was a revolutionary idea coordinated by the late Dr. H. Marc Cathey, president emeritus of AHS. The map contains 12 different zones in the United States and classifies areas of the country based on the average number of days per year than the temperature is above 86 degrees. Why 86 degrees? This is the temperature where cellular proteins in plants start experiencing damage. Louisiana is located in zones 8 and 9. Zone 8 has 90-120 days annually above 86 degrees, and 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. Cold damage to plants most often leads to rapid plant decline. Although rapid decline also can be associated with heat stress, more often than not plant decline caused by heat stress occurs over a more extended period – anywhere from a month or so to more than a couple of growing seasons. Heat stress symptoms include withering flowers or flower buds, drooping leaves, foliage color change from green to bluish-gray, cessation of root growth and increased attractiveness to insects. Other environmental factors that affect plant success or failure include water availability, oxygen exchange, light quality and quantity and day length. Water availability is important because limiting water stress goes a long way in limiting plant stress. Maintain optimal soil moisture through irrigation management. Plants have difficulty adapting to erratic periods of heavy rainfall and extended droughts. Oxygen exchange is needed for plant respiration. Roots need oxygen for this exchange. Prepare beds that allow oxygen to reach roots through the spaces between soil particles. Cultivate to keep those pores open. Light quality, quantity and day length (also referred to as the photoperiod) are important for a plant’s physiological processes. Light and carbon dioxide are needed for food manufacturing or photosynthesis. Light also affects a plant’s temperature – plants growing at their heat-zone limit may die because of environmental stress in the summer if planted in full sun or with a southern and/or western exposure where heat buildup is more likely to occur. With Permission: LSU AgCenter