Thatch in the lawn

What is lawn thatch

Lawn thatch is a layer of organic matter that accumulates between the grass sward and the soil (see photo to the right). Thatch or organic matter is generally decaying plant stems, roots and runners.
A small amount of lawn thatch (about 12mm) is beneficial to the lawn as it creates resilience and improves wear tolerance.

However when the accumulation of thatch becomes too excessive (over 25mm) it creates problems, ultimately resulting in a weak lawn prone to disease, water logging, poor root growth and more.
Therefore it is important to monitor the thatch layer and take remedial action if build up starts to become excessive.

Thatch only becomes a problem when the plant produces more debris than what is being broken down naturally. The rate at which the thatch layer accumulates is determined by certain factors. These include soil conditions, maintenance program, weather conditions, soil pH and the grass species.

The causes of lawn thatch

Previously we mentioned it is important to monitor thatch build up, however if we understand what causes thatch build up we can help prevent it becoming a problem. The main causes of build up are:

• Over fertilising the lawn with nitrogen - Too much nitrogen causes the grass to quicker and in turn produce more organic matter adding to the thatch layer.

• Over watering the lawn - A major contributor to the lawn thatch layer. Thatch is naturally degraded or broken down by micro organisms. The micro organisms thrive in dry soils that contain lots of air space. Watering unnecessarily reduces the available air in the soil, thus reducing the percentage of micro organisms and thatch an natural degradation of the thatch will be reduced.

• Grass species - Certain types of grasses produce more thatch than others. Fescue and Bent which are very desirable grasses are known to produce thatch rapidly.

Why is thatch bad for a lawn

As we have touched upon previously a little thatch is good for a lawn as it creates resilience and insulates the lawn. However when there is excessive thatch it causes major problems on the health of the lawn which include.

• Pest and diseases - An excessive thatch layer is an ideal environment for many lawn and turf pests and disease such as moss, worms and fusarium patch disease.

• Poor surface drainage - Thatch is very water retentive and inhibits drainage, this in turn encourages disease, moss, weeds.

• Shallow rooted weed grasses - Desirable grasses such as Bents and Fescues prefer dry free draining soils and will be replaced with water loving shallow rooted grasses like Annual Meadow Grass.

• Poor drought tolerance - As thatch encourages shallow rooting grasses, the lawn is will naturally have poor drought tolerance during dry periods.

• Reduce the effectiveness of chemicals and fertilisers - Certain lawn care products are more effective if they can penetrate into the soil (root zone). An excessive thatch layer inhibits soil penetration, therefore effectiveness of the products is reduced.

How to control and prevent thatch

Preventing the build up of thatch with the correct cultural practices and lawn care program is better than having to deal with thatch once it has become a problem. A sound management program will help slow down build up and at the same time increase the speed at which thatch degrades. These cultural practices include.

• Correct use of fertilisers - Avoid applying too much nitrogen during the growing season. Too much nitrogen encourages excessive growth and the grass will produce organic matter at an increased rate.

• Avoid over watering the lawn - Don't water the lawn unless it is starting to show signs of drought stress. If you do irrigate the lawn, water heavily and infrequently letting the lawn dry out between each watering. Avoid watering little and often as this keeps the soil moist for longer periods and will have a detrimental affect on the micro organisms which break down thatch.

• Employ a regular aeration program - Aeration plays an important part in the breakdown of thatch in the lawn. The micro organisms (bacteria which breakdown thatch) thrive in dry soils with lots of air space.

• Employ a regular scarification program - Scarifying at key times during the year (spring and autumn) will physically remove lawn thatch and keep the layer at an acceptable level.

• Employ a regular scarifying program - Scarifying removes lateral growth and thatch and keeps the accumulation of thatch in check.

• Top dressing - Top dressing the lawn will help dilute the thatch layer. It is very beneficial to top dress after aeration (particularly hollow tining) as the dressing can be worked into the upper soil profile. This helps increase airspace and remove surface water encouraging the breakdown of thatch.

Removing lawn thatch

If the thatch layer has reached an unacceptable level and is having an adverse effect on the lawn it will need removing. The two most effective methods of removal are scarification and / or aeration using hollow tines.

• Scarification - The most effective and most common method of removing thatch is by scarification. This process involves a powered machine, fitted with a row of vertical tines or blades, which penetrate into the surface of the lawn to remove thatch (see photo to the right). The ideal times for scarifying is during the spring and autumn. However it is important that there is sufficient growth for the lawn to make a complete recovery as scarifying can leave the lawn looking thin and sparse.

• Hollow tining - Another effective way of removing thatch from a lawn is by hollow tine aeration. This is the process of removing small cores of thatch / soil in the upper soil profile. It is removed by the tines which are basically hollow tubes which are pushed or punched into the surface of the lawn. This operation can be carried out using either a hand held hollow tine fork or implement or alternatively a powered aerator fitted with hollow tines. Again spring and autumn are the preferred times for this task.

Both of these methods when used in conjunction with lawn top dressing will be beneficial in helping with the
breakdown of thatch.