Bob Mugaas, University of Minnesota Extension Educator
In the early spring before lawns begin active growth (i.e., foliage is still mostly brown) and the ground is still thawing, lawn grasses can withstand several days of being submerged without suffering serious damage. If floodwaters are cold (<60 degrees F.), as they usually are in early spring, lawn grasses can withstand being submerged for even longer periods of time.
Moving water is usually less harmful to lawn grasses than is ponded, stagnant water. Ponding occurs in areas of poor drainage or results from water being left behind in valleys and depressions when floodwaters recede. Spring flooded lawn areas where the water has risen and then receded rapidly often escape serious permanent injury and death.
Photo 1: Flooding can negatively affect plants and degree of injury is dependant on multiple factors. Wendy VanDyk Evans, Bugwood.org
Once the soil has dried sufficiently, such that it is no longer soggy and slushy underfoot, pick-up and remove debris such as wood, glass, stones, sheet metal, paper products along with other forms of junk deposited by flood waters. It is even good to remove thick layers of leaves or other debris that can smother the grass. Debris can be a safety hazard so exercise caution when picking up and handling this material. Debris left behind can later become a hazard to people operating lawn equipment as well as damaging the equipment itself. It should be noted that the drying process may take two or three weeks, perhaps longer, depending on site conditions. Assessment of potential lawn damage and recovery may not be possible until those areas have dried. Checking for new shoots emerging from the soil is a good way to make an early assessment of damage. Usually, once regrowth has begun, it will continue although it may take several weeks before the lawn has completely filled in and begun to thicken up.
Often a more significant effect of flooding is the deposit of sediment, primarily silt, over lawn surfaces. This can lead to serious soil layering problems and even death of existing grass if deposits are deep enough. Core aerification can be one of the most important and beneficial operations conducted where silt deposits are less than an inch and water has not ponded long enough to cause substantial death of the lawn. When the lawn has begun to actively grow as evidenced by new green grass blades appearing, go over the lawn about 3 times with a core type aerifier. This will help improve overall soil structure, improve soil oxygen levels, help break up soil layering problems caused by the overlay of silt and encourage recovery during the remainder of the growing season. A second round of aerification in early September will be helpful in further promoting active turfgrass growth and recovery through the fall period.
Overseeding can also be done at the time of aerification being sure that good seed to soil contact is achieved. To prepare a smooth seed bed, break up the aerification cores with a lawn rake or power rake (i.e. vertical mower). If desired, lawn seeding can be delayed until mid-August through early-September. Sodding can be done successfully throughout the growing season.
Soil deposits in excess of an inch and just barely covering the turfgrass plants should be carefully scraped or washed from the lawn surface prior to any reestablishment practices being undertaken. This will also help remove any floodwater pollutants left behind that may have a more lasting detrimental effect on the lawn since their concentrations are completely unknown.
If the lawn area is completely buried with inches of silt, then the best renovation strategy may be to accept that the majority of the lawn has already been severely damaged or killed and it will be necessary to reestablish a "new" lawn. Even though the process of silt removal is a lot of work and can be very damaging to existing turfgrass plants, reestablishing a lawn should begin by removing the excess silt as completely as possible. This should be followed by good soil preparation practices whether the lawn is to be seeded or sodded. See Extension factsheet 5775 Seeding and Sodding Home Lawns for more information on seeding or sodding a new lawn.
Where soil removal is not possible, rototill or plow the area thoroughly mixing the soil deposits from the floodwater with the existing soil and dead turfgrass cover. This will help restore more uniform soil conditions, creating a better environment for grass to reestablish. One of the main goals of this operation is to help break up soil layering problems that can be caused by the silt deposits as well as the old sod layer. Seeding or sodding can be done as described in the above mentioned publication.
Another problem that may be encountered with silt deposits is the introduction of potentially new and different weeds to the lawn. Therefore, it may be necessary to use pre- and/or post-emergence herbicides where appropriate during the reestablishment process. Make sure to follow labeled recommendations when using any herbicide to avoid injury to the young grass plants.
While dealing with the lawn may be the very least of one's water problems this spring, those needing to repair their lawn can do so once the soil has sufficiently dried. Local County Extension offices should have the publication FO-3914 entitled Lawn Renovation for additional information on repairing lawns.