All thatch is bad.
FACT: most lawns have a layer of thatch, even healthy lawns. However, when that layer builds up to more than 1/2", it needs to be reduced.
MYTH: Thatch is caused by lawn clippings.
FACT: Grass clippings are made up mostly of water and nutrients (approximately 90%). Remove only 1/3 of the grass at one time and leave the clippings. The clippings decay quickly and make additional nutrients available.
Thatch in lawns is often misunderstood; both its cause and control. Some lawns have serious thatch problems while others do not. Thatch is a layer of living and dead organic matter that occurs between the green matter and the soil surface. Excessive thatch (over 1/2" thick) creates an environment favorable for pests and disease and an unfavorable growing environment for grass roots plus can interfere with some lawn care practices.
Thatch is a building up dead roots, lawn debris and dead turfgrass crowns. It accumulates as these plant parts buildup faster than they breakdown. Thatch problems are due to a combination of biological, cultural, and environmental factors. Cultural practices can have a big impact on thatch. For example, heavy nitrogen fertilizer applications or over-watering frequently contribute to thatch, because they cause the lawn to grow excessively fast. Avoid over-fertilizing and over-watering Despite popular belief, short clippings dropped on the lawn after mowing are not the cause of thatch buildup. Clippings are very high in water content and breakdown rapidly when returned to lawns after mowing, assuming lawns are mowed on a regular basis (not removing more than one-third of the leaf blade).
As thatch levels accumulate to greater than 1/2", lawn problems may begin, and the thatch needs to be controlled. Thatch may be torn out with a dethatcher or vertical mower, but will most likely return unless the cause is corrected. Mechanical dethatching is also very destructive to the lawn because roots are in thatch instead of soil, so plants tear out easily. Overseeding is usually required afterwards. For this reason, it's best to tear out thatch in early fall for optimum reseeding timing.
Core aerification, followed by topdressing (applying additional soil) are 2 methods that will generally correct the reasons thatch is accumulating. Core aerifying machines will pull up small soil cores to the surface that are left there to act like topdressing. The holes created help solve problems such as compaction or poor drainage. Topdressing is simply adding a thin layer(1/8" - 1/4") of compatible soil over the thatch, which adds microorganisms to help in the breakdown process.
Aerifying equipment can be rented or your local professional lawn care provider can usually provide this service.
DIY: If you think you want to do-it-yourself, consider that this piece of equipment weighs about 200 pounds, requires extensive manhandling to operate and usually the minimum rental time is 4 hours and costs probably more than having a professional do it for you.
Aerifying is an excellent lawn practice with many benefits, as it helps solve soil problems that in turn leads to better root systems and healthier lawns. Aerify in spring or fall (fall is generally the preferred time), making sure adequate moisture exists in the soil. Make two trips over the lawn, the second perpendicular to the first. An average of 15 - 20 aeration holes per square foot is suggested. Cores should remain on the surface and allowed to air dry. These cores act as topdressing that helps degrade thatch. Additional topdressing material could be added after core aerifying if desired.
Lance Walheim, the author of Lawn Care for Dummies, offers these dethatching tips:
The best time to dethatch a cool-season lawn is early fall or early spring; for a warm-season lawn, early summer.
You can use a thatch rake, a sharp-tined rake that rips the thatch out of your lawn-- but using one on a large lawn is a very big job. A better solution is to rent a power dethatcher, which has a 7-horsepower engine and rotary tines on the bottom. This is a tool that will make short work of dethatching.
Dethatching is a hard treatment for a lawn. You therefore want to do it at a time during the year when the lawn is less stressed by environmental conditons and more likely to recover rapidly after the detaching process. The red star in the illustrations below mark the best time to dethatch both warm and cool-season grasses.