The good, the bad, the ugly: all about turf-grass thatch
By Jay Deputy
Thatch is a normal and necessary part of all turf grasses. It is an interwoven layer of brown stems and sometimes even roots that build up between the soil surface and the green leafy top part of the grass.
Most of this layer is composed of living plant tissue, but a certain amount of dead organic matter can also accumulate in the thatch. However, contrary to popular opinion, grass clippings do not contribute to thatch buildup if the grass is mowed often enough and at the correct height.
A thin layer of thatch is good, providing a cushion to absorb some compaction of foot traffic and serving as an insulation to keep the soil moist and cooler.
Thatch becomes excessive when it gets to be over an inch thick. Then bad things will usually begin to happen. One of the first noticeable problems is in the appearance and feel of the turf surface. As thatch builds up, mowing height is increased above the soil line and the turf becomes very lumpy and spongy. The mower sinks into the thickened turf and cuts into the brown thatch area. This is called scalping and results in brown patches of exposed thatch throughout the yard.
A common response to this is to raise the mowing height, which reduces scalping for a short time. However, the thatch continues to get thicker and the yard more lumpy and spongy and eventually leads to even more severe scalping.
Excess thatch can restrict water movement into the soil. Dry thatch tends to repel water rather than allow infiltration. If your lawn has dry spots that are difficult to rewet without flooding the area, these are probably caused by dry thatch spots and possibly made worse by underlying compacted soil.
A thatchy condition will increasingly lead to the elevation of the crowns, runners and even a large portion of the roots above the soil surface, which then becomes a part of the thatch. This results in a shallow root zone, which makes the turf very susceptible to water stress. This will also reduce the effectiveness of fertilizer and pesticides.
There's more. A thick thatch is much more susceptible to insect infestation, particularly sod web worm and army worm. Several fungal diseases can become a problem in areas where excessive thatch stays shaded and remains wet for long periods.
What causes thatch buildup? Some species of turf just naturally develop thatch more rapidly than others. Of the warm season grasses used in Hawai'i, hybrid bermuda, seashore paspalum and the zoysia grasses are the most prolific. St. Augustine will build a very deep thatch over a long period of time, while centipede grass seldom becomes overly thatchy.
Since thatch is basically an accumulation of overlying layers of runners and extended blade sheaths, any cultural or maintenance condition that leads to excessive vegetative top growth will contribute to a rapid buildup.
The three most common causes:
The remedy is simple. Maintain a slow healthy rate of growth by fertilizing and watering only as needed, not because you think more is better. This will also require less frequent mowing, which should be determined by how rapidly the grass is growing. Mow often enough so that you remove no more than one-third of the total top growth at each mowing. It is also important to mow at the lower end of the recommended mowing height for the species of grass that you have.
The natural decay process is important in keeping the dead organic material from accumulating too rapidly. A healthy soil micro-organism population will ensure sufficient decay to keep pace with a slow controlled rate of growth. Periodic cultivation by core aeration followed by a light layer of soil or compost top dressing hastens decomposition by providing a more favorable environment for microbial activity.
These are standard maintenance procedures for control of thatch on highly maintained turf. Mechanical coring removes small plugs of thatch and soil, leaving thousands of small holes in the soil that allow penetration of air, water, fertilizers and pesticides.
Top dressing with compost fills the holes with a nutrient-rich medium that results in deeper root penetration and prevents the holes from closing back over. Frequent, light (1/4-inch) top dressings with compost have been repeatedly shown to be the most effective and consistent method to reduce the rate of thatch buildup. Thick applications or using sand are not recommended and will only compound the problem by causing a layering effect that can restrict water and air movement and encourage shallow root systems.
Maintaining the soil pH around 6.5 also will keep the beneficial fungal and bacterial population happy. An adequate earthworm population also is essential to break down organic matter in the thatch layer and to keep the soil loose and aerated.
Next week: What to do once the thatch becomes too thick and unmanageable.
