Persistent wet weather can create conditions for Entomosporium leaf spot on Indian Hawthorn. (Photo by Allen Owings, LSU AgCenter)

For Release On 06/05/15

By Dan Gill
LSU AgCenter Horticulturist

Adequate moisture is critically important to landscape plants, but too much rain or excessive watering can bring problems of its own. We’ve seen a wet spring and early summer this year, and it has caused a variety of problems. Wet soil can create stressful conditions for bedding plants, vegetables, shrubs and even trees – especially those planted this year.

When the soil is saturated with water, pore spaces that normally hold air are filled with water. Because plant roots 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. A sick root system leads to a sick plant. Plants in this situation often lose vigor, look wilted, turn yellow, become stunted or even die.

Wet soil conditions also encourage fungus organisms that live in the soil to attack the roots or crown of plant, causing rot. These disease organisms can cause dieback or severe damage or even kill plants. Once infection occurs, little can be done to help a plant.

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 beck from around plants or removed entirely to allow the soil to dry faster. 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 shrub. Do not dig with the fork. This provides air to the roots and encourages the soil to dry faster.

Fungal diseases that attack the foliage of many plants are also encouraged by rainy weather. Entomosporium leaf spot was very bad on Indian hawthorns and photinia shrubs this spring. 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 cause the leaves to turn yellow or red and drop off. The disease is not fatal, and the trees will recover without treatment, although 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. Try not to let their populations get out of control.

Toads in your garden feed on slugs and should be left alone. Numerous baits on the market 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, may benefit from fertilizer applications now.

Bark lice alert

During 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.

The proper name is psocids, and the webbing they produce on trunks 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 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 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. Once the bark lice begin to die, the webbing will break up and disappear. Bark lice will not injure your trees, and no control is necessary. 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 them alone and let them clean off the bark of the tree.