Thatch Control

Table of Contents

• What is Thatch?
• Why is Thatch a Problem?
• What Causes Thatch?
• Thatch Control

What is Thatch?

Thatch is a tightly intermingled layer of dead and living stems and roots that develop between the growing turf plants and the soil surface (Figure 1). It is composed of lignin containing tissues of stems, leaves, nodes and crowns of turf plants. These parts which contain lignin are resistant to decay. Only the upper surface layers of thatch are composed of leaf remnants.

Figure 1. A cross-section of actively growing turf showing a thick layer of thatch (2cm) between the soil and the grass plants.

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Why is Thatch a Problem?

1. Thatch increases insect and disease problems. Many turf insects live and feed in the thatch. A thick thatch layer gives these insects an ideal environment to feed and multiply. Thatch also harbours disease causing organisms. Thatch accumulation increases the incidence of Helminthosporium leaf spot, dollar spot and brown patch.

2. Thatch causes localized dry spots. Localized dry spots are frequently associated with excess thatch. When thatch dries out it repels water and it can be very difficult to wet again.

3. Thatch makes turf prone to scalping. When there is excessive thatch present in turf the crowns (actively growing part) of the turfgrass plants are high above the soil. When the turf is mowed the mower will sink into the thatch. This results in the turf being mowed too closely or scalped. Continuous scalping weakens turf and leaves it prone to weed invasion.

4. Thatch decreased heat and cold tolerance. Again with excessive thatch the crowns are high above the soil surface. The soil acts as a temperature buffer for the turfgrass plant. The soil keeps turf warm in the winter and cool in the summer. When there is excessive thatch the soil is not as able to insulate the turf crowns against temperature extremes.

5. Thatch can reduce the effectiveness of pesticides. There are two methods in which thatch can reduce the effectiveness of pesticides:

• Pesticides can bind to the thatch and they are not able to reach the target insect. This is especially true with soil dwelling insects such as European chafer grubs
• Certain pesticides are degraded more quickly in thatch that in soils

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What Causes Thatch?

Thatch accumulates when the rate of shoot growth exceeds the rate at which the above ground parts of the plant decompose. Practices which promote excessive shoot growth increase thatch production. Factors which affect thatch accumulation are :

1. Vigorous turf species - Turf species differ in their ability to form thatch with Kentucky bluegrass and creeping bentgrass being the worst thatch formers and turf-type perennial ryegrass and tall fescue being non-thatch forming species (Table 1).
2. Soil pH - Thatch formation increases as soil pH decreases. Acid soils reduce microbial activity and hence reduce the breakdown of thatch. Liming acid soils will help reduce thatch build up.
3. Fertilization and irrigation - Fertilization stimulates turfgrass growth. Excessive fertilization will force turf to grown more quickly than the microorganisms can break it down. Excessive irrigation will limit the oxygen in the soil which the microorganisms need to survive. It is important to provide enough fertilizer to keep turf healthy but not so much as to overstimulate it. Deep, frequent irrigation is best to allow soils to dry out and allow air to be replaced by water.
4.  Fungicides - Fungicide use may increase thatch formation. Fungicides not only kill disease causing organisms but they also kill non-target microorganisms which breakdown thatch.

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Thatch Control

The best way to determine how much thatch you have is to cut a square from your turf and examine the thatch layer. In Kentucky bluegrass more than 1.25 cm is considered to be too much. In the case of creeping bentgrass anything more that .75 cm can be a problem. If thatch has accumulated beyond that action should be taken to control it. There are various methods for thatch control:

1. Vertical mowing - Vertical mowing or verticutting is a method of reducing thatch by cutting it up with a blade which runs perpendicular to the ground. Blades should be space about 4.0 cm and set shallowly to just cut through the top most portion of the thatch. Often several passes in several directions are necessary. Verticutting should not be done too late in the fall. Turf needs 30 days of good growing weather to repair itself before winter. The other time when verticutting is recommended is in spring. Again turf need 30 days to fill in before the hot summer weather comes. Crowns that are exposed during the heat of the summer are more prone to high temperature stress. Vertical mowing can be followed by a light topdressing to protect the exposed crowns and a light fertilizer application to stimulate regrowth.
2. Core aeration - Core aeration involves cutting and removing cores of soil/thatch/turf (Figure 2). It physically breaks up the thatch as well as bringing soil which is laden with microorganisms to the surface where they can work to breakdown thatch. Core aeration also improves soil aeration making more air available for soil microorganisms. It is best to core aerate in several directions to get good coverage. Timing for core aeration is the same as for vertical mowing.
3. Topdressing - Addition of topdressing to turf adds microorganisms which can break down thatch. A topdressing program has to be matched with the rate of growth of the turf.
4. Proper fertilization and irrigation - Avoid overstimulating the turf with excessive nitrogen applications and irrigate infrequently and deeply to help maintain a balance between turf growth and thatch decomposition.

Figure 2. Core aeration involves cutting and removing cores of soil/thatch/turf

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