Recovering your home lawn from damage caused by flooding

In recent months, many South Australians, particularly those with properties along the Murray River, have dealt with near-record-level floods. As the water recedes, people are, among other things, wondering to what extent their lawns have been damaged.

Questions like, “how badly have they been damaged, will they survive, and what do I need to do to get my lawn back” come to mind.

What sort of damage can flooding do to turf?

Flood conditions can damage a lawn in a number of ways, including submersion, erosion, silt deposition and weed contamination.

Submersion.

The most obvious damage is caused by the fact that the lawn will have spent a significant amount of time submerged underwater. Flooding saturates both the lawn and the soil it is growing in, which results in oxygen being depleted. Without oxygen, lawns begin to degrade. They can’t breathe, nor can they absorb nutrients. They will survive for some time by using up carbohydrate reserves that they have stored in their root zone; however, this, of course, is limited. 

Erosion.

Depending on how your lawn was exposed to flood and the speed of the water moving over it, you may find that soil and lawn have been washed away. Topsoil can be removed by fast-moving floodwater, and along with it, the nutrients that exist in the topsoil.

Silt

One thing flood waters almost always leave behind is a layer of soil covering your lawn. This layer will block sunlight and prevent it from photosynthesising. Sunlight is essential in making your lawn grow, and when it isn’t there, it will degrade quickly.

Contamination (weeds)

As floodwaters erode soil from land upstream, they will carry unwanted weed seeds and drop them on your lawn. It is often the case that an invasion of weeds inundates lawns affected by floodwaters. If left untreated, they can quickly dominate lawn areas, especially those that thrive in oxygen-poor soils. Don’t worry – there are solutions for this, so read on!

Will your lawn survive?

The extent of damage sustained by your lawn depends on many factors, which in turn determine the chances of your lawn surviving. Early reports from people with lawns in flooded river communities indicate that warm-season grasses such as couch and kikuyu are recovering well in instances where efforts were made to aid recovery as soon as practicably possible. Factors that influence recovery rates include the following:

Length of time underwater:

The longer your lawn is underwater, the more damage will occur. A US study at Louisiana State University assessed the submersion tolerance of warm-season turfgrasses in submerged turf pots in an initial trial for 93 days and then for 55 days in a second trial.

They then drained the water from the pots and assessed the percentage of shoot material that was alive versus dead. After 93 days of flooding, the research found a high mortality rate for all turf plants – Couch performed the best, with 37.9% of shoots remaining. After 55 days – Couch had 73% of shoots remaining, indicating there was enough shoot development and carbohydrate reserves for the plant to recover. Buffalo grasses had only 33% shoot survival indicating it was not as tolerant under flooded waters. (Ref: Peter Kirby)  

Type of lawn

Some types of lawn fare better than others. If you have a cool-season lawn such as tall fescue, or ryegrass, there is very little chance that it will survive a flooding event like the one we have just experienced.

Warm season grasses, on the other hand, such as couch, kikuyu and buffalo, are much better adapted. Couch and kikuyu have excellent resilience, and to some extent, buffalo does, too, albeit not quite to the same extent. The most common lawns on the Murray flood plains in South Australia are couch and kikuyu, and if that’s you, there is a good chance you can see some good recovery.

The Season

If the flood event occurs and recedes during the warm months, the survival rates are better than if it occurred during the winter. This is because warm-season grasses are active during Spring, Summer and Autumn, which means, where possible, they are ready to start the recovery process as soon as the water has receded and the silt has been cleared.

How quickly you are able to remove the silt and debris

The layer of silt and debris most likely left on your lawn will continue to cause damage until it is removed. The longer it stays on your lawn, the more damage will occur.

Health and maturity of the lawn

Lawns that are unhealthy and or immature prior to the flooding event will have less resilience to submersion and silt deposition. You should consider this when deciding whether or not to regenerate it. 

What should you do?

What you do will largely depend on the above factors and how patient you are. You can completely remove what is there and replace it with new turf or, alternatively, regenerate what is already there.

Reports have been good for regeneration, especially in couch and kikuyu lawns. If you decide to go for regeneration, I’d do the following:

Assess the damage

If your property is on the Murray, try and determine if your lawn is still there. Fast-moving water may have removed it altogether. If it is, decide whether you want to replace or regenerate it.

Remove silt and debris.

If you’re looking to regenerate it, as soon as possible after the water has subsided, remove the layer of soil that is undoubtedly covering it. You may need to resort to a shovel and wheelbarrow for smaller or restricted access areas. If you have the room, consider hiring a mini track loader with a sand bucket to remove the soil layer. They are easy to use and will create the least amount of disturbance.

Ideally, you want to expose the lawn as much as possible, leaving no more than 1cm of silt behind. The more of the lawn that you can see poking through, the better.

Core

Core the entire lawn area, preferably with a motorised, hollow tine lawn corer that removes cores of soil rather than with a solid tine corer. While solid tine corers will create holes in your lawn, they will compact the soil around the tine as it makes its way into the soil.

The idea is to add oxygen back into the soil, which is critical for the lawn to start regenerating. Consider very lightly top dressing with gypsum or white sand, but only enough to fill the core holes. You don’t want to be burying the lawn at this stage.  

Fertilise

Fertilise the lawn area. There is a high chance that much of the nutrient in the soil has been washed away. Use an organic fertiliser rather than a mineral one at this stage, as this will add some much-needed bio-diversity back into the soil. An organic or starter fertiliser will encourage the lawn to recover more sustainably, allowing it to build up its carbohydrate reserves that were most likely depleted while the lawn was underwater. Repeat this process in four weeks.

Apply a soil conditioner

Consider using a seaweed soil conditioner. These products will help strengthen and stimulate the lawn’s roots, helping speed up the recovery process.

Manage the weeds

Expect a flush of weeds in your lawn as the flood waters will likely have deposited a bank of seeds on your lawn area. You may see weeds that you’ve never seen before, which will be frustrating, to say the least. You can prevent weed seeds from germinating by using a pre-emergent herbicide such as Spartan. Be careful how you apply this though, as a heavy application will have a root-pruning effect which is the last thing you want! Stick to the light end of the label rates.

Scarify

Once your lawn has recovered, consider scarifying it. Scarifying disturbs the root system, further stimulating it to become thicker and denser.   

How long will it take to recover?

This is a hard question to answer! It will depend on your lawn type, the season (soil temperature), how long it was underwater, how quickly you were able to remove silt and debris and how mature the turf was when it was flooded.

If you have a mature couch or kikuyu lawn and were able to follow the advice above, I’d expect to see signs of life within a few weeks. The warmer it is, the faster things will happen.

If you’d like to discuss your situation, give us a call on 8298 0555 or send an email to info@paulmunnsinstantlawn.com.au

In creating this blog, I'd like to thank Nathan Tovey from My Home TURF and Peter Kirby for providing us with data regarding flooding on lawns in NSW and Queensland during their recent floods.