St. Tammany Horticultural Update July, August, September 2020

Annuals and Perennials

The summer gardening season in South Louisiana is one of the most environmentally stressful times on our landscape plants. As daily high temperatures exceed 85-90° F, plants must redirect energy into reducing overall water loss. During these times it is advisable to evaluate irrigation systems and techniques for maximum efficiency. Thoroughly examine flower beds for areas that dry out quicker than others. Run a system check on any irrigation system to make sure all heads are in working order and providing even coverage. Leaks in the irrigation system will also compromise their effectiveness. Replenish mulch as needed to help the soil retain moisture and cover irrigation tubing. Soaker hoses and plastic irrigation tubing degrades a faster rate when exposed to UV light. Covering with an organic mulch will help these products last longer in the landscape.

If you are looking to add or replace color in the garden look for transplants at local garden centers. However, there is still plenty of time to sow seeds and allow these seedlings to produce flowers into late summer and fall months. Some noteworthy exceptions include begonia (Begonia semperflorens cultorum), cockscomb (Celosia cristata), impatiens (Impatien wallerana), marigold (Tagetes spp.), Mexican heather (Cuphea hyssopifolia), moss rose (Portulaca grandiflora), pentas (Pentas lanceolata), periwinkle (Catharanthus roseus), purslane (Portulaca oleracea), torenia (Torenia foumieri), and zinnia (Zinnia elegans).

The practice of dead heading is defined as removal of old flowers and subsequent seed pods to improve and extend the blooming period of a plant.Dead heading can also rejuvenate plants that have been in the ground since the beginning of the warm season. Both annuals and perennials can benefit from this process. Plants like bush daisy (Gamolepis chrysanthemoides) fan flower (Scaveola aemula) ham and eggs (Lantana camara), plumbago (Plumbago auriculata), summer snapdragon (Angelonia angustifolia), and spiderflower (Cleome hassleriana) can all be dead headed to clean up their appearance and extend their productivity in the landscape.


Summer weather causes many spring-planted vegetable plants to become unproductive. Plants like bell pepper, cucumber, eggplant, and tomato will start dropping flowers during the summer months due to issues involving high day time temperatures coupled with high nighttime temperatures. Remove and dispose of unproductive plants and replant with young transplants to get a fall harvest from typical spring planted crops mentioned above. Regarding tomatoes, the most popular home vegetable garden crop, growers should look to plant heat-tolerant varieties such as Heat Wave II, Phoenix, Solar Fire, Sun Leaper, and Sun Master.

Even though many vegetable crops slow down in the summer months, there are some that prevail during hot summers. Okra (Abelmoschus esculentus) is one of those crops. Okra plants thrive all summer long and fruit can be picked well into the fall, up until cool weather presents itself. Transplants are currently available at local garden centers throughout the region. However, seeds can be still be sown at this time. Plant okra ten to twelve inches apart. Increase spacing twenty-four to thirty-six inches apart for best production. Fertilize using an 8-8-8 or a 15-5-10 product at a rate of two to three pounds for every one hundred square feet. Spread the fertilizer evenly throughout the planting area and mix it into the first four inches of soil. Okra, in general, does well in the garden and success can be experienced with almost any variety. However, there are several recommended varieties to try. Annie Oakley is a popular, spineless variety with slightly ribbed pods. Burgundy is a dwarf variety that produces burgundy red pods. Clemson is a long-time favorite due to its spineless pods.

Ornamental Grasses

Drought tolerance in landscape plants is a highly sought characteristic when looking for plant material that will survive the summer growing season with minimal care. Ornamental grasses are some the best examples of this trait and should be more utilized in landscape horticulture. Not only drought tolerance, but other traits like disease resistance, insect resistance, and adaptability to several different soil types make ornamental grasses easy to grow for all gardeners. Grasses also add texture, contrast, color, and year-around interest. Even as grasses go dormant in the wintertime, they provide interest and excitement throughout the winter garden.

Grasses come in many different sizes and can perform different uses. Smaller growing gasses work well when used as borders or ground cover plantings. Look at Hameln grass (Pennisetum alopecuroides ‘Hameln’) for a low growing groundcover grass that can be mass planted to cover the most amount of ground. While not exactly a grass, liriope (Liriope muscari), shares many of the same qualities. It is commonly used to outline and define flowerbeds by planting in a line or border. Medium size grasses help to create vertical lines in the garden. Some examples of medium sized grasses include Mexican feather grass (Nassella tenuissima) or purple fountain grass, (Pennisetum setaceum rubrum). A cultivar of fountain grass, Fireworks, is an LSU AgCenter Super Plant known for its purple, pink, and red color scheme. Pink muhly grass (Muhlenbergia capillaris) is another medium-sized native grass plant but with fine texture and bright pink flower spikes emerging in late summer. Taller growing grasses work best when used in the back of a flowerbed or as a natural border or screen plant. Some examples include maiden grass (Miscanthus sinensis) and pampas grass (Cortaderia selloana) pampas grass.

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Close up view of pink muhly grass showing the bright pink color of its flower spike.


Avoid making fertilizer application, especially high nitrogen containing fertilizers, after the month of September. Once the Autumnal Equinox (September 22, 2020) passes, the quality and quantity of day light reduces dramatically, signaling warm-season turfgrass to prepare for dormancy. Fertilizing the lawn after the end of September causes growth at time when the lawn is naturally slowing down. The only element worthy of applying at that time is potassium (K). Potassium is required for many functions within plants including the movement of water, nutrients, and carbohydrates, but most importantly it helps lower the freezing point of the liquid solution found in plant cells. While not a silver bullet, sufficient levels of potassium will help ensure a good healthy lawn going into the dormant season. Submit a soil sample to the LSU AgCenter Soil Testing Lab to determine the status of potassium in your lawn. You will find several fertilizers that contain added potassium such as Scotts Turf Builder Winterguard (32-0-10). For soils with low to very low levels of potassium look to add muriate of potash (0-0-60) as directed by the soil sample analysis.

Common lespedeza (Kummerowia striata) is a problematic annual weed in Louisiana lawns. The growth habit of this weed can be described of as low and branching, low enough to grow just under a mower blade. The leaves of lespedeza are compound and occur as three smooth leaflets with prominent veins that run almost parallel to the midrib. Seeds of lespedeza germinate in late winter/early spring when soil temperatures reach low to mid 50s. As the plant grows during the warm-season it eventually starts to harden off in the months of August and September. At this time, the stems of lespedeza harden and become somewhat woody. It is also at this time when pink/purple flowers emerge from the axils where leaves are attached to the stem.

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Close up view of common lespedeza showing leaflets and pink flowers.

To control common lespedeza in the lawn, pay attention to cultural practices that promote plant health. Mow often enough to remove 1/3 of the lawn grass each time you mow. Take soil samples to determine nutrient availability and use fertilizers that replenish those nutrients. Deep and infrequent irrigation promotes the growth of grass roots and produces a healthy lawn that can outcompete weeds. Mitigate soil compaction through the practice of lawn aerification. After addressing the appropriate cultural controls, you may find solace is using a herbicide. Fertilome’s Weed Free Zone is effective early in the season and can be used over-the-top on centipedegrass and St Augustinegrass. Once daily high temperatures rise above 85-90°F, switch to metsulfuron-methyl, which is the active ingredient in MSM Turf. It can be applied to centipedegrass, St Augustinegrass, bermudagrass, and zoysiagrass. Be sure to read and follow the product label before applying any pesticide.

10/7/2020 5:13:49 PM
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