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Landscaping May Survive Flooding

Don’t rush to rip out your landscaping after a flood because it may at least partially recover.

After floodwaters recede, your yard likely will be covered in thick silt or mud. It also may have the odor of raw sewage, which indicates a lack of oxygen in the soil, and many plants will look dead.

However, some plants have survived after two weeks under floodwaters. They include most native trees, shrubs, perennials and hardy bulbs. Others that have shown good survival are crape myrtles and Chinese holly varieties. Plants that typically do not survive being underwater or show marginal recovery include Japanese holly, Japanese boxwood, Indian hawthorn, nandina, hybrid junipers and hybrid azaleas.

“Salvaging a flooded yard can be economically feasible if you have the time and patience to let the plants recover naturally,” says [insert name and title] of [insert county name].

Before starting to clean your yard, make sure it is safe to enter. For example, beware of downed power lines. Also make sure the yard is dry enough that you won’t cause more damage while cleaning it. Once the yard is dry, remove any trash, debris and uprooted plants.

Most deciduous landscape trees will lose their leaves immediately after a flood, but hardy evergreens many not defoliate. Washing silt off evergreens can be helpful.  One tablespoon of dishwashing liquid per gallon of water is a sprayer works well in most cases.

Wet the plants with plain water, then spray the foliage with the detergent solution. Wait about a minute and rinse the solution off. Work in small areas at a time so the detergent doesn’t stay on the plants too long. Do not pressure wash the plants because that could damage them.

Do not use a high-nitrogen fertilizer on trees and shrubs right after a flood. Flooded trees and shrubs have had a shock and may be experiencing a forced dormancy. Before using any fertilizer, have a soil sample tested to determine if the plants need the added nutrients.

Adding a layer of mulch around trees and shrubs will improve their aesthetic quality. You may be able to find chipped hardwood mulch available free of charge from trees and shrubs that were removed after the flood. Just make sure the chipped wood has been aged properly. Don’t use fresh hardwood mulch because it can tie up soil nitrogen as it ages and breaks down. Also make sure the mulch doesn’t contain any trash or contaminated material.

Bermuda and Bahia grass seem to recover from flooding the best. Hybrids of these grasses also do well. Remove any sediment and debris and mow the lawn, removing only about one-third of the height you normally cut off. Apply about ½ pound of nitrogen per 1,000 square feet of lawn to encourage recovery, then follow normal maintenance practices.

If you have a sprinkler system, you should inspect and flush it. Make sure the power is off before inspecting electrical connections. Replace your irrigation clock if it was flooded. Have a certified professional check your backflow prevention system before letting water from your main drinking water system run through the sprinkler system.

Here are some steps for flushing your sprinkler system:

  • Shut off the water supply and open the drain valve to remove water from the underground pipes.
  • If your system has rotors, remove, shake and rinse them thoroughly.
  • Check any gear-driven rotors mounted above ground to make sure water has drained out of them, then remove and clean them thoroughly.
  • Unscrew the inside of the heads from the casing and rinse both pieces.
  • Open the valves, one at a time, to the full open position and turn the system on manually.
  • Let the water run for at least five minutes.
  • Reinstall the heads and let the system run for about 10 minutes.

If heads are sticking up instead of retracting after you shut the water off, turn the water back on and try stepping on the riser while the head is running. Let it pop back up and step on it again. Do this a few times and shut the system off. If any heads still are sticking up, you should replace them.

To clean drip and micro-irrigation systems, open the end of the line and flush with fresh water. Replace emitters that remain clogged or are damaged.

 For more information, visit www.ag.ndsu.edu/flood or www.extension.org/Floods.

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