To remove water from a flooded garden it is best to divert the water and if necessary, pump out excess water. However, this will not reverse damage caused by 12-24 hours of flooded soil. Oxygen-deprived roots quickly die.
The FDA recommends that any produce present during a flood should not be consumed. Produce is susceptible to bacterial contamination and contaminants on the produce are impossible to remove. For additional information and procedures for specific produce see Post Flood Vegetable Gardens Can Be Risky, Garden Produce and Flooded Fields. Also, see Produce Safety and Flooded Fields.
When flooding contamination occurs it is best to haul away plant materials. It is recommended to wait until the following planting season to rototill and allow the land to dry and settle; 90 days is the recommended wait period. An extra precaution is to plant a non-edible crop for a season. Soil testing is also a good idea (see below).
Flood water may leave behind a layer of sediments on your garden and lawn, or it may have eroded portions of your landscape. Once the water has receded, pick up debris that could be hazards to people and pets. Remove leaf litter and other material that may smother the grass. Then let the soil drain and dry. It may take several days or weeks for the lawn to be dry enough to work. If top soil has eroded or new soil has been deposited, you will have to decide whether to add (in the case of erosion), remove flood-deposited soil, or work with the lawn you now have. Adding or removing soil can be expensive.The most practical choice may be to work with your new circumstance. See Recovering a Flooded Landscape for more information.
Colorado State Extension has tips regarding cleaning up turf and lawn areas. If you live in other states, be sure to check with your Extension office regarding appropriate times to over seed grass. You may also want to consider steps to reduce future flood damage while you are recovering your yard.
It is a good idea to have soil tested every three or four years under normal situations, but especially so if your lawn and garden have been flooded to determine what amendments may be needed. If you think your lawn has been contaminated by a nearby known source, you can test for specific contaminants but this could be very expensive. Depending on your situation you can do a routine soil test or water analysis, test for organic chemicals in soil or water, or do a bacteria analysis. Contact your local Extension office for information on how to test soil and for the location of soil testing laboratories.