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Extension > Yard and Garden News > Damage Assessment and Repair Options for Late September flooded lawns.

Damage Assessment and Repair Options for Late September flooded lawns.

Bob Mugaas, UMN Extension Educator - Horticulture U of MN Extension and Brian Horgan, PhD. Associate Professor, Extension Turfgrass Specialist U of MN Department of Horticultural Science

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Photo 1: Receding flood waters from a public park. Bob Mugaas.

Flooding is not uncommon during spring and summer here in MN. Yet residents of southern MN are experiencing significant flooding this fall. Justifiably, repairing lawns and landscapes is probably not the highest priority. However, should attention turn to wanting to do 'something', here are a few tips regarding the assessment of potential lawn damage and options for repairing them, even yet this fall.

Have flood waters actually done any damage to my lawn? In general, where flood waters have risen quickly to cover the lawn area but also receded quickly (within 2 or 3 days), there has probably been little permanent injury to the lawn. With shorter days and cooler temperatures in fall, lawn grasses are usually able to remain green and alive through brief periods of being submerged. Picture 1. As flood waters recede and the soils dry, soil oxygen levels improve aiding plant growth. In this situation, little to no repair is usually needed.

In many instances, water flowing over river and creek banks gets trapped on your property and is not able to flow back to the river. Picture 2. In this situation, stagnant, flooded turfgrass conditions may persist for several days or longer. Water loss is now a function of evaporation and soil infiltration capacity rather than flowing back into the stream or river. Damage assessment can be done once the water has disappeared from the lawn.

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Photo 2: Ponded areas of water left behind receding flood waters. Bob Mugaas.

Where grass plants appear brown and rotten, it is safe to assume that the grass plants have died and this area will now need to be reseeded or sodded. On the other hand, if the water evaporated or infiltrated the soil within 2-3 days, grass plants may have survived. These temporarily flooded areas may not be as dense as they were prior to being inundated with water and some reseeding can be beneficial to restoring a healthy dense cover of turfgrass (see notes on overseeding later in this article).

In most cases, water continually moving across a lawn surface is less problematic for the grass plants than non-moving, stagnant water. The differences in these two water movement characteristics will often dictate the amount of sediment deposited on that lawn surface. Sediment deposited is often associated with slowing water flow or ponding.

The likelihood of permanent injury to lawn grasses will depend on the depth of that sediment deposit. If deposits are less than 1/2 inch, usually there are no serious problems for home lawn grasses maintained at higher mowing heights and the grass will still be visible above that sediment layer and continue to grow normally. Picture 3. However, even light sediment deposits can form a distinctly different layer of soil that can ultimately create future soil water and nutrient infiltration problems. To help prevent this occurrence, core aerification (two to three passes) over the lawn will break through the sediment layer thus avoiding future soil problems.

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Photo 3: Surviving turfgrass following short term submergence. Bob Mugaas.

Core aerification on home lawns can be done this fall once the areas have dried out sufficiently so that the area is no longer soft and muddy when walking across it. Additional core aerification can also be done in the spring just as the lawn is beginning to green up and again around Labor Day next year. This additional aerification in 2011 will help ensure healthier soil conditions that in turn support a healthier lawn.

Where resources may be more limited and aerification isn't possible, brisk raking will help break up that sediment deposit. Again, this can be done once the soil has dried and is no longer soft and muddy underfoot.

Where sediment has been deposited at levels deeper than about 1 inch and the grass is barely visible through the sediment, grass plants will likely die. In these cases, it is usually better to carefully remove some of that material and overseed to restore the lawn area. A thorough core aerification will be beneficial following sediment removal.

The late September to early October time period is a difficult time to successfully overseed and establish a new lawn. The lack of establishment and maturity achieved by the young grass plants often results in very poor winter survival thus necessitating another seeding in the spring. Therefore, we recommend waiting until the ground is cold but not frozen to sow seed (usually early to mid-November). This process results in seed that does not germinate this fall but begins to actively grow in the spring. This is known as dormant seeding and can give lawn seeds a head start on germination and growth next spring. While results of this practice can be variable, when done correctly and Mother Nature cooperates with sufficient snow cover, successful lawn establishment can be accomplished. The good news about considering dormant seeding is that attention can be given to dealing with home and property losses without feeling like something also needs to be done right now to fix a lawn.

A rather unintended consequence of fall flooding is the introduction of new weeds into lawns and landscapes. By late September, there are many annual and perennial weedy plants actively dispersing their seeds. Flood waters can be a significant means of spreading many of those seeds into places where those plants have never been present. While there is nothing to do right now, be watchful for new and different weeds showing up in lawns, gardens and landscape areas next spring and summer. Early removal or treatment with an herbicide is good practice and limits the amount of herbicide needed to achieve control.

While lawns are not a first priority when dealing with other home and property losses due to flooding, at some point down the road most folks will want to restore their lawns, gardens and landscapes thus restoring a sense of 'normal' to their lives. Hopefully, this information will be of help when that time comes.

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