We inspected a lawn this week that was just not responding the way is should. On inspection we found that the lawn had a very heavy layer of thatch.
Now by thatch we mean that layer between the soil and the green part of a lawn. Some people refer to dead or yellow grass blades as thatch. As in, "My lawn has a lot of brown in it. It has a lot of thatch." Well, that is just dead grass blades. Thatch is normally not visible until you start scratching the surface.
Thatch can consist of roots, stems and other organic matter. Some thatch is a good thing. A little bit will help prevent moisture from escaping too readily on a warm day. Somte thatch acts a cushion to prevent the soil from getting too compacted from foot traffic and mower wheels- anything that will push the soil down and drive the air (really we mean oxygen) out of the soil pores.
However, too much thatch can be detrimental to the health of the lawn. Too much thatch acts like a thatch roof and prevents water from penetrating into the soil. The rain and irrigation instead are carried off the lawn and into the storm sewers. Too much thatch prevents fertilizers from reaching the root zone. Too much thatch also increases the likelihood of disease problems. Chinch bug like to hide in the thatch. White grubs feed on grass roots as well as thatch. Anecdotal evidence would suggest that European chafer adults seem to know if a lawn has a lot of thatch and may be more likely to lay their eggs in lawns with excessive thatch.
So, how do you get rid of thatch and keep it away? First off, thatch is not caused by leaving the clippings on the lawn. Secondly, we don't normally recommend usin a dethatcher to remove thatch. While a dethatcher will indeed remove the thatch, this is really only a temporary solution. The thatch may be gone, for now, but without tackling the root causes of thatch, it will be back.
Our recommendation is to core aerate the lawn. Aerate it heavily, i.e. aerate it twice all at once. This will put that many more holes in the lawn. It will remove that many more cores. Each hole will allow oxygen back into the soil. This will encourage the bacteria and other soil fauna to grow and multiply so that they can decompose the excess thatch. Each core that lands on the surface will now "topdress" a small section of the lawn. The soil there will also encourage soil organisms to do their work. Grass roots will quickly grow into the core holes where there will be a better mix of moisture and air. As the grass roots flourish, so the lawn will become healthier.
If you want to speed up the process, the addition of more soil to the lawn surface will mean that the whole lawn will be topdressed instead of just the areas that have the cores. At the same time, sprinkle some grass seed over the lawn. Grass seed that fall into those aeration holes always has a better chance of growing. They are less likely to dry out, less likely to be snatched away by a hungry bird, and more likely to continue growing once they sprout.
In a few weeks, the lawn will start to turn around. This is not an instant cure, but it is the first step to a green, healthy lawn.