Thatch

Thatch problems in lawns

The above image shows an accumulation of thatch between the sward and rootzone. A small amout of thatch build up is desirable, but too much thatch can have an adverse affect on the health of the lawn.

Thatch consists of the build up of old grass clippings, decaying leaves and roots.

A small layer of thatch (less than 1cm) is essential for a lawn as it helps create a resilient and hard wearing surface.

However when the thatch accumulates to over 2.5cm it then starts to becomes a problem. When there is too much thatch the following problems occur:

• Drainage is impeded and the surface become soggy and soft during wet conditions due to the thatch layer holding water.
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• When the conditions are dry it is very difficult to get water through the thatch layer and into the root zone where it is needed. Where there is an excessive thatch layer problems with dry patch can occur where the soil becomes hydrophobic (water repellant) and is very difficult to re wet.
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• Root growth is restricted as roots are confined to the thatch layer, this in turn creates shallow rooted grasses like annual meadow grass. These grasses are not very drought resistant and are prone to fungal disease attacks such as fusarium.

Causes of thatch

• Over watering - Applying to much water to your lawn means a lack of air in the soil. The bacteria that breaks the thatch down requires air to thrive therefore a wet soil inhibits bacteria and microbial activity.
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• Over Fertilising - When grass grows it regularly produces new leaves, roots and stems replacing the old ones which die and become part of the thatch layer. These are then degraded by the micro bacteria in the soil. When the grass is fertilised this process is speeded up so that the natural process of thatch breakdown can not keep up with thatch development caused by excessive fertiliser.
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• Not boxing off grass clipping - When the lawn is cut as the clippings contribute to the build up of thatch.

Thatch prevention

It is better to have a program in place to prevent the build up of thatch before it becomes a problem in your lawn, methods of thatch prevention are:

• Don't over fertilise your lawn with nitrogen as previously discussed too much fertiliser increases thatch build up.
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• Don't over water your lawn in periods of drought, too much water can be detrimental to the condition of your lawn. The golden rule regarding irrigation is to only apply just enough water to keep the grass alive and keep the soil profile moist, not saturated.
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• Regular raking, brushing and scarifying will help prevent any lateral growth and help with slowing down thatch build up.
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• Regular aeration will help with drainage and keep the soil profile aerated, this in turn will help encourage soil microbes which aid thatch breakdown.
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• Top dressing will help with thatch build up as this operation dilutes the thatch layer. Top dressing is very useful after aeration, particularly after hollow tining has taken place. By rubbing a suitable sandy top dressing into the channels through the thatch layer you will be improving the drainage. This encourages surface water to move through the thatch into the soil profile.

Thatch removal

Once you have a build up thatch it will need to be removed if you want a healthy lawn. There are a combination of methods you can employ for thatch removal these include:

• Scarifying - This is the most effective and common way of removing thatch. This operation involves using a machine with vertical knife like blades which slash through the turf and rake the thatch out. The ideal time for this operation is spring time when soil conditions are warm enough for consistent growth. A fertiliser can be applied afterwards to help with recovery in the late summer while there is still time for the grass to recover. Overseeding can take place afterwards to help fill in any thin or bare areas. For smaller lawns a rake can be used but it unlikely to as effective as a purpose built machine.
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• Aeration - Regular aeration will help with thatch breakdown. This is because it encourages micro organisms in the soil which break down the thatch. These micro organisms require a well aerated lawn to survive. Hollowtining is very beneficial as it actually removes thatch.