Thatch is a layer of both dead and living shoots, stems and roots that accumulate just above the soil surface. Thatch is not really made up of grass clippings – grass clippings are mostly water and decompose easily.
Thatch is impossible to measure without taking a soil sample or cutting out a small spot of turf to expose the cross section of soil. Thatch levels will vary across the lawn so it is good to check a few spots. You can check thatch with a pocket knife by just slicing vertically into the soil. Slice on four sides and pull out the plug so that you can look at the cross section as shown in the photo.
A little thatch can be good because it acts as an insulator cooling the ground in hot weather. When thatch gets over ½ inch thick, however, your lawn becomes more susceptible to insects, diseases and weeds. Thick thatch keeps water and nutrients from reaching the root system, which will cause it to turn brown faster in the hot weather and make it more susceptible to insects (grubs, billbugs, chinch bugs), diseases (brown patch, dollar spot, red thread, pink patch etc.) and weeds. Studies show that grass that is weakened by insects or diseases has more weeds than one that aren't.
Thatch accumulates when your grass produces shallow roots that die off and are not decomposed quickly. Fertilizing with quick release synthetic fertilizers can make thatch worse by stimulating shallow root growth and then also killing off beneficial soil microbial life that would otherwise help to decompose thatch. Some things that can make thatch worse include:
-Watering often and for a short time period
-Having a sod or predominately Kentucky Bluegrass lawn
-Using Quick Release Synthetic fertilizers
-What should you do if you have a lot of thatch?
If you have a lot of thatch (over ½ inch), the first decision you need to make is whether or not you want to try to work with the lawn you have, or if you would prefer to completely start over (renovate).
If you are going to work with what you have, there are two things you should do. First, recognize that your lawn is going to be susceptible to pests and you'll need to take adequate steps (using the safest organic materials) to prevent those pests. Secondly, you should begin a program to reduce thatch buildup and begin its decomposition. To protect a Thatchy lawn from problems and begin thatch reduction we recommend:
-Soil Test and work to bring the soil into balance
-Core Aeration in the fall with a microbial decomposition enhancer (Bio Dethatch)
-Liquid Aeration in the spring
-Apply beneficial nematodes in the spring and fall to minimize billbug and grub damage (Spring Insect Control and Fall Grub Control)
-Summer applications of garlic to minimize chinch bugs and suppress diseases
Are these steps all absolutely necessary? No, you can do some or none, but these will increase your odds of protecting your lawn and reducing the thatch problem.