Hot-weather Gardening: Freshen with new flowers pruning pest control

Linda Benedict, Gill, Daniel J.  |  8/3/2007 12:13:40 AM

Roses should be pruned late August and early September. This is a Knock Out rose. (Photo by Mark Claesgens)

Marigolds are a flower you can plant during the hot days of August. They should bloom for you through the fall. (Photo by Mark Claesgens)

Zinnias can be planted to freshen your flower garden during the hot days of August. Note the powdery mildew on the bottom leaves. (Photo by Mark Claesgens)

Yards and gardens generally look a little frayed by this time of the year. The final really hot days that usually come at the end of a long, hot summer are especially hard on plants, says LSU AgCenter horticulturist Dan Gill. Worse yet, plants experiencing heat stress are weakened, and that means an increase in disease and insect problems. Be on the lookout for pests such as lace bugs, aphids, leaf hoppers, scales, spider mites and whiteflies.

Aim at whiteflies
One of the worst pests of late summer is the whitefly, a small white insect slightly bigger than a gnat. Clouds of them will fly up from a heavily infested plant when it is shaken. Both the adults and the immature whiteflies feed on the plant by sucking the sap from the foliage. Infested plants will have a sickly appearance and dull leaves. A black deposit called sooty mold often appears, and leaves may turn yellow and fall off. Sooty mold may also be caused by other sucking insects such as aphids and scale.

Whiteflies are somewhat selective in what they feed on – often infesting hibiscus, cleome, lantana, mallow, poinsettia, gardenia, Confederate rose and many bedding plants. When spraying, pay careful attention to thorough coverage on the most heavily infested plants. Repeat applications as necessary.

Plants heavily infested with whiteflies also may be cut back to reduce insect population levels. Just be sure to discard the clippings. In addition, as a last resort, low-value landscape plants – such as bedding plants – may be pulled up and disposed of.

Controlling whiteflies can be difficult, especially when the population levels get high. On ornamentals you can use Talstar, Malathion, acephate or dimethoate. Although oil sprays are not recommended for use in summer when daytime highs go above 85 degrees, highly refined paraffinic insecticidal oils, such as Bonide Year Round Oil, can be used now and are effective against whiteflies. Oils kill by suffocation and are an excellent low-toxicity insecticide. Spray in the early morning when temperatures are cooler. Check the label carefully for the safe and proper use of these pesticides and the listing of plants on which they may be used.

Listen to Horticulturist offers watering tips for late summer

Watch vegetables, too
Many vegetables in the garden now also are susceptible to whiteflies. Those include tomatoes, eggplants, okra, sweet potatoes, beans and others. Malathion, Bonide Year Round Oil, insecticidal soap and Thiodan should help reduce populations, but repeated applications may be necessary. Follow label directions carefully and note the waiting period between application and harvest.

Freshen flowerbeds
Flowerbeds past their prime and overrun with weeds can be a common sight in our late-summer landscape. The intense heat makes people reluctant to do much work outside. It’s not reasonable to expect all bedding plants to hold up from the beginning of the summer growing season in early May until its end several months later. Fortunately, nurseries are still well-stocked with colorful, heat-tolerant bedding plants that will grow vigorously from now through late October or early November (when we will plant cool-season bedding plants).

To replant your beds, first remove the old plants and put them in your compost pile. But try to avoid putting any weeds that have set seeds in the compost. Just dig those out and throw them away. You also could spray the weeds with glyphosate herbicide (various brands) to kill them before removing them. This would be especially recommended if you are dealing with tough weeds, such as Bermuda grass, torpedograss or dollarweed.

Next, spread a 1-inch to 2-inch layer of organic matter – such as compost, bagged or aged manure, landscape soil conditioner, grass clippings or peat moss – over the bed. Sprinkle a light application of any general-purpose fertilizer (following label directions) over the organic matter and then thoroughly incorporate everything into the soil. Rake it smooth, and the bed is ready to plant.

When planting late in the growing season, choose well-established plants in 4-inch or larger pots. Make sure the plants you purchase are healthy and vigorous and have been properly cared for. Avoid plants that look wilted or leggy, have poor color or show signs of insect or disease problems. This is not the time of year to nurse struggling plants back to health. Start off with the highest-quality plants you can find.

Gardeners often rip or pull apart the roots of root-bound bedding plants slightly when planting them into the ground. This encourages the roots to grow into the surrounding soil and helps the plant get established. But you should do this carefully or not at all when planting this time of year. Plants’ roots must absorb water rapidly to supply their needs when temperatures are hot, and transplants will not be able to tolerate much damage to their roots now.

Also be careful to plant the bedding plants at the proper depth. The top of the rootball should be level with the soil of the bed. Planting transplants too deep makes them more susceptible to root rot or crown rot. The fungal organisms that cause these diseases are very active in the moist, hot weather we generally see in late summer, so make sure plants are not planted deeper than recommended.

Once planting is finished, mulch with an inch or two of your favorite mulch and water the bed thoroughly. Watering is the trickiest part of planting this time of year. You may need to water the bed fairly frequently until the plants send roots out into the surrounding soil. Watch the plants carefully for wilting, and water when needed.

There are lots of choices for planting now. For sunny beds or containers, choose periwinkle, melampodium, blue daze, purslane, portulaca, pentas, torenia, perennial verbena, salvias, sun-tolerant coleus, lantana, zinnia, marigold, abelmoschus, globe amaranth, cosmos, balsam and celosia. For shady and partly shady beds and containers, choose impatiens, begonias and coleus.
Listen to Take precautions when gardening in the heat

Read Don’t be your plants’ worst enemy

Don’t forget tropicals
If you still intend to add tropical plants to your landscape, do it now so they will have time to get established before fall. Tropicals love the heat and are not stressed out by it like so many other plants. Feel free to plant tropical hibiscus, cannas, gingers, elephant ears and other tropicals in the landscape now.

Read more, hear more and see more at our Get It Growing news service.

The LSU AgCenter is one of 11 institutions of higher education in the Louisiana State University System. Headquartered in Baton Rouge, it provides educational services in every parish and conducts research that contributes to higher living standards and the economic development of the state. The LSU AgCenter does not grant degrees nor benefit from tuition increases. The LSU AgCenter plays an integral role in supporting agricultural industries, enhancing the environment, and improving the quality of life through its 4-H youth, family and consumer science programs.

(This AgCenter Lead was updated on Aug. 19, 2011 by Rick Bogren.)

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