Okay, all you lawn nerds out there, this one’s for you – an entire article devoted to the mysteries of thatch. What, exactly, is thatch? Is it always a problem? Do grass clippings cause thatch?
Thatch is a distinct layer of spongy brown organic matter consisting of living and dead roots, leaves and stems that accumulates between the actively growing grass and the soil surface beneath.
A certain amount of thatch is quite normal and even beneficial. Like mulch, it shades the surface, reducing summer soil temperature and evaporation. It competes with crabgrass. Thatch provides food for beneficial microbes and supplies organic matter to the soil. And it filters stormwater to reduce groundwater contamination.
While excessive thatch is unlikely to kill the grass directly, it can lead to turf health issues that surely can kill your lawn. More than one inch of thatch will block the movement of air, water and nutrients into the soil. This impervious layer causes the grass to root into the thatch layer to get nourishment, which reduces the drought resistance of the turf. Thatch can harbor disease-causing fungi and insects.
Thatch production is all about balance. When the production of new grass shoots is equal to the microbial decomposition of old roots, leaves and shoots, excessive thatch build up won’t be a problem. But, if grass is growing too quickly or the microbes aren’t functioning properly, this balance is lost.
Excessive nitrogen fertilizer makes the grass greener, but it makes the lawn grow faster, too. Also, chemical fertilizers, fungicides and insecticides are detrimental to the beneficial organisms in turf. The result can be too much grass growth combined with reduced microbial action which will allow thatch to build up. Consider an organic lawn care program. Organic fertilizers do not push growth. Rather, they are slow release and nourish the soil in order to provide a healthy growing medium for the grass roots.
Watering should be infrequent and deep. Frequent watering (which is the way irrigation systems are often set) keeps the surface moist, which is where thatch accumulates. Grass roots and shoots will begin to grow in this moist thatch layer instead of in the soil. Grass growing in the nutrient-poor thatch layer is more prone to die when the thatch does dry out. This then adds more dead plant material which accumulates in the thatch layer. Over-irrigating also creates waterlogged soil which damages the aerobic microorganisms.
Many new homes have the topsoil stripped off and subsoil layers exposed. Subsoil is lacking in organic matter, microbes and earthworms. Sod is laid directly on this subsoil, which sets the grass up for thatch issues since the new growth may remain in the sod layer instead of the subsoil. If this is your situation, hollow-core aeration, which removes soil plugs from the turf, will help prevent or correct this shallow rooting. If you are about to lay sod, incorporate organic matter, topsoil and fertilizer into the subsoil beforehand.
Earthworms are important for soil aeration and thatch reduction. Certain insecticides and fungicides are highly toxic to earthworms. Avoid these products or use them rarely.
Everyone has heard that grass clippings are a primary cause of thatch but that’s really not the case. Short clippings will break down quickly and return balanced nutrients back to the turf. Note: short clippings. Infrequent mowing of tall grass will contribute to thatch accumulation. Mow at a height and frequency so that no more than one third of the grass height is removed in a single mowing. Of course, if you already have excessive thatch, then clippings will make it worse.
How do you know if your lawn has a thatch problem? Cut out a small, triangular-shaped plug of turf several inches deep and take a look. If the spongy layer above the soil is more than three-fourths to one inch thick when you compress it, you should consider having your lawn dethatched.
Mechanical dethatching should be done in late summer or fall, when the weather is cooler. Do not dethatch when the soil is wet and do not try to remove the entire thatch layer in one treatment. Aeration will also help to stimulate bacterial decomposition by improving the availability or air, water and nutrients.
After dethatching, it can be useful to lay down some grass seed. Grasses that produce a large amount of side shoots, like Kentucky bluegrass, also tend to produce thatch readily. What is the best seed for our area? To quote Gregg Munshaw from the UK College of Agriculture, “Kentucky is in a transition zone between warm and cool season grasses; we can grow all types of grasses equally poorly.” He recommends turf type tall fescue for our area.