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Gardening in Gwinnett: Assessing flood damage to landscape

The recent record rainfall and the resulting floods have potentially harmed plants in the home landscape. Trees, shrubs and turfgrass areas have been in soils saturated with water and many have been under water, in some cases for days, which may cause serious damage.

The water in the flooded soil displaces air from the pores of the soil and can "drown" the plant. Different species of plants vary in their tolerance to flooded conditions. What should you do if your home landscape has been flooded? How do you assess the damage to the trees and your lawn? The first step is to make sure the site is safe to enter. Has the water receded and is the ground safe to walk on?

In regard to the lawn, Bermuda is the turfgrass most tolerant to being submerged under water; Zoysia and Tall Fescue are somewhat tolerant while Centipede grass is not tolerant. The extent of the damage depends on how long the lawn has been flooded. To determine if turf damage has occurred, look for green leaves, green to white runners on top of the soil and white roots below which is a sign the plant is healthy and will most likely survive. Brown to black stems and roots indicate the turfgrass plant is probably dead. Lightly rake the brown turf areas to determine if some grass has survived.

If your lawn has suffered damage, you can take several steps to save it. Improve the drainage by cleaning out storm drains and possibly digging temporary surface drainage ditches to allow any trapped water to drain away. Remove any accumulations of soil and organic debris on the lawn.

If 50 percent or more of the lawn has been damaged, then you will most likely have to plant new turfgrass in the dead areas. If the area damaged is less than 50 percent, most likely the surviving turfgrass will be able to grow and fill in the dead areas, although some sodding or seeding may be necessary. If your lawn is a warm season grass, like Bermuda, Centipede or Zoysia, now is not a good time to sod it. Wait until April or May to do so. Meanwhile, overseed the lawn with a cool season grass, such as Tall Fescue or Rye, to provide grass cover for the cold weather months.

There are a range of flood tolerant levels among different species of trees, but most suffer if under water for any length of time. Many factors affect the impact of flooding on tree growth such as the time of year the flooding occurred, the amount of time the tree was under water, the depth of the water, its life stage, and the overall structure and health of the tree.

In addition to causing a decrease in soil oxygen availability, the water can deposit silt and sand on the surface of the root zone. Even as little as three inches of water can potentially smother and suffocate the roots. Also, strong water currents can scour the base of the tree and erode away the soil, exposing the roots. The trees are more susceptible to insect and disease problems. Trees weakened by being under water or even just in soils that have been heavily saturated with water have a greater potential of toppling over in windy conditions due to weakened roots and poor root to soil contact.

Over the long run, the health and structural stability of the trees impacted by the flood will be questionable. Because of the growth patterns of some trees, the effects of flood damage, like the drought in recent years, will not appear until a year or more into the future.

Timothy Daly is an Agricultural and Natural Resource Agent with the Gwinnett County Extension Service. He can be reached at 678-377-4010 or tdaly@uga.edu.