Daniel Gill, Merrill, Thomas A. | 6/1/2007 2:16:47 AM
By Dan Gill
LSU AgCenter Horticulturist
Adequate moisture is critically important to landscape plants during hot weather, but too much rain or excessive watering also can bring problems. Wet soil combined with high temperatures can create stressful conditions for bedding plants, vegetables, shrubs and even trees – especially those just planted this year.
When the soil is saturated with water, pore spaces in the soil, which normally hold air, are filled with water. Since the roots of plants get the oxygen they need from the air in those spaces, the roots can literally drown in a soil that stays waterlogged over an extended period.
In turn, a sick root system leads to a sick plant. Plants in such situations often lose vigor, look wilted, turn yellow, are stunted and may even die.
Wet soil conditions also encourage fungus organisms that live in the soil to attack the roots or crown of a plant, thus causing rot. The crown is the area where the stem of a plant enters the soil. These disease organisms can cause die-back, inflict severe damage or even kill plants. Worse yet, once infection occurs, little can be done to help a plant affected by these disease organisms.
Plants with succulent stems such as impatiens and begonias, those that like cooler temperatures such as geraniums and dianthus and those that prefer drier, well-drained soils such as Indian hawthorns are particularly susceptible.
Gardeners can take steps to help alleviate the situation. For one thing, adjust your irrigation systems that are on automatic timers. I often see sprinklers unnecessarily watering at homes or businesses the day after a heavy rain simply because the timer turned them on. Turn off the automatic timer if the weather is wet, and turn the system on only when drier conditions occur.
You should always keep your beds well mulched to control weeds and maintain soil moisture. If you find that your beds are staying too wet, however, the mulch can be pulled back from around plants or removed entirely to allow the soil to dry faster. Just make sure you keep weeds under control while the mulch is off.
Plants affected by wet soils or root rot may look wilted even though the soil is moist. A plant showing these symptoms immediately after a period of prolonged heavy rain may benefit from soil aeration in its root zone. Using a garden fork, drive the tines straight down into the soil and pull straight out in numerous places around the plant. This provides air to the roots and encourages the soil to dry faster. Just be sure you don’t dig with the fork.
Fungus diseases that attack the foliage of many plants also are encouraged by rainy weather. Black spot on roses is prevalent even on fairly resistant varieties, and control is nearly impossible if it rains every afternoon. Cercospora leaf spot on crape myrtles can cause the leaves to turn yellow or red and drop off. The disease is not fatal, and the trees will recover without sprays, but flowering may be diminished.
Other pests such as snails and slugs thrive and reproduce rapidly during rainy weather. These pesky critters chew holes in the leaves and flowers of plants and are particularly fond of soft-leaved plants such as impatiens, begonias and hostas, among many others. Try not to let their populations get out of control. Toads in your garden, feed on slugs and should be left alone. There also are numerous baits on the market that will help control snails and slugs. You can even place a bowl up to its rim in the ground and fill it half full of beer to attract and drown many snails and slugs.
Frequent rains can leach available nutrients from the soil in the landscape. You should evaluate your landscape plantings with this in mind and fertilize, if needed. Plants rapidly growing now such as lawn grasses, summer bedding plants and tropicals like ginger and hibiscus are especially vulnerable.
Bark Lice Alert
Turning to another issue, during the summer silvery webbing may appear in patches on the bark of trees. These webs are caused by tiny insects called bark lice, which are common in Louisiana.
According to LSU AgCenter entomologist Dr. Dale Pollet, their proper name is psocids, and the webbing they produce on the trunk and branches is to protect themselves from environmental conditions and any predators that may come along. The webbing looks alarming and sometimes ghostly in appearance as it spreads on the tree from the ground to the upper branches.
Here’s the good news: Bark lice are in no way harmful to the trees. The insects feed on organic debris lodged in the bark, such as molds, pollen, fragments of dead insects and similar materials.
The small, soft-bodied creatures are about 3 millimeters to 6 millimeters in length and may or may not have wings. They will be active until fall. (They usually go away about October, if not before.) And once the bark lice begin to die, the webbing will break up and disappear.
Since they won’t injure your trees, no control is necessary. But if you can’t stand the appearance of the webbing, you can sweep it off with a broom or blast it off with water. Most of us just leave these creatures alone, however, and let them clean other things off the bark of the trees.
Get It Growing is a weekly feature on home lawn and garden topics prepared by experts in the LSU AgCenter. For more information on such topics, contact your parish LSU AgCenter Extension office or visit our Web site at www.lsuagcenter.com. A wide range of publications and a variety of other resources are available.