Allen D. Owings, Trawick, Robert C., Bush, Edward W.
Environmental conditions should influence your selection of plant materials for a home landscape. Size of the planting area is important, as are the other site characteristics, such as sun or shade exposure, wet or dry locations and exposure to windy conditions. Selected plants should tolerate existing conditions and should be hardy in the appropriate climatical zone. Louisiana’s 50-60 inches of annual rainfall are also an important consideration in selecting landscape plants.
In Louisiana, we are located in USDA hardiness zones 8 and 9. This hardiness zone indicates the average minimum temperature that occurs during the winter in different geographical regions of the country. Hardiness zones 8-9 indicate average minimum temperatures of 10-20 and 20-30 degrees, respectively. 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 in northern Arkansas, Tennessee or Oklahoma where hardiness zone 6 starts. Check with your local county agent for the average frost and freeze dates for your area in Louisiana.
Consider summer heat extremes in determining the success of your landscape plants. We all know it is 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 growing conditions, in determining long-term hardiness and survivability for many plant species.
In 1997, the American Horticultural Society (AHS) released the Plant Heat-Zone Map. This was a revolutionary idea coordinated by Dr. H. Marc Cathey, president emeritus of the society. The AHS Plant Heat-Zone 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. 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, and 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.
Cold damage to plants most often leads to rapid plant decline. While rapid decline can also 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 over a couple growing seasons.
What other environmental factors may play a role, and how can we minimize these problems and maximize success? Humidity, water availability, oxygen exchange, light quality and quantity, daylength (photoperiod), wind movement, soil conditions and available nutrients all play a role in success with your garden plants through the summer but also at other times of the 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. How can plants adapt without irrigation when we receive 4-inch rainfalls in a 24-48 hour period then go through 30-day droughty periods at other times of the year? Very hard conditions for plant adaptation.
Oxygen exchange is needed for plant respiration. Roots need oxygen to breathe. Ideal bed preparation that leads to recommended pore space between individual soil particles is needed.
Be aware of light quality, quantity and daylength, also referred to as photoperiod. These are important as plants undergo their physiological processes during the summer. Light and carbon dioxide are needed for food manufacturing (photosynthesis). Light also affects the plants temperature - plants growing at their heat-zone limit may die due to 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.