Lawn Challenge Course - Lesson 5

Here is Lesson 5 - "Thatch in Lawns! in our Lawn Challenge Series from the University of Illinois.

Thatch and How to Manage It

Thatch is a dense layer of living and dead organic matter on the soil surface.

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 inch thick) creates a favorable environment for pests and disease, an unfavorable growing environment for grass roots, and can interfere with some lawn care practices.

The primary component of thatch is turfgrass stems and roots. 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 overwatering frequently contribute to thatch, because they cause the lawn to grow excessively fast. Avoid overfertilizing and overwatering. 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)

In northern Illinois, environmental factors typically are another primary case of thatch. Conditions favoring thatch include heavy, wet soils; alkaline, or high pH soils; and soil compaction. All are common in northern Illinois.

As thatch levels accumulate to greater than 1/2 inch, 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 late August for optimum reseeding timing.

Core aerating helps degrade thatch and also helps solve some of the causes of thatch.

Core aerification, followed by topdressing are two 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 to 1/4 inch) of compatible soil over the thatch, which adds microorganisms to help in breakdown.

Aerifying equipment may be rented or services are available to do it for hire. Aerifing 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, making sure adequate moisture exists in the soil. Make two trips over the lawn, the second perpendicular to the first. An average of 15 to 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.

Lawn Repair and Renovation

When repairing or renovating lawns, specialized equipment can increase effectiveness and make the job much easier. Match equipment to the task that needs to be done for best results.

Aerating machines that actually pull out soil cores are suggested for use on lawns.

As mentioned already, core aerification is an important and highly recommended practice for many lawns. This process is useful to help reduce soil compaction and thatch, improve surface drainage, and improve conditions prior to overseeding. Core aerifiers insert hollow tines into the lawn and pull out plugs of soil. Size of cores removed will depend on the machine used, soil moisture, and type of soil. Core spacing also varies with the specific machine being used. Machines can be rented or aerifying services are available for hire.

Spikers are similar to core aerifiers in that they make holes in the soil. However, they use solid tines, and thus, do not remove cores.

Vertical mowers have rotating blades arranged vertically that can cut into turf and soil. These machines can be used to remove thatch (dethatching). Turfgrass rooting in the thatch is typically torn out, so reseeding is suggested afterwards. Vertical mowers can also roughen the soil prior to overseeding areas.

Slit-seeders are useful for lawn renovation projects. Slit-seeders combine vertical mowing with seeding. As the machine goes across the lawn, it opens the soil and deposits seed directly into the soil opening. Most slit-seeders have a roller that helps firm the soil after seeding. Seed is metered at a predetermined rate; it's suggested to apply half the desired seeding rate in one direction and the other half on a second pass perpendicular to the first.


Silt-seeding equipment is useful for lawn renovation.

Since the seed is placed in direct contact with the soil, seeding success is usually high when using slit-seeders. In addition, existing grass and debris does not need to be completely removed prior to the overseeding process. Timing should be the same as for conventional lawn seeding, which ideally would be late August into early September. Many rental agencies carry slit-seeders or many lawn and landscape services can do it for hire.

Determining the cause of the lawn decline is the first step in the lawn renovation process. Many lawn problems originate from poor soil conditions. Heavy clay, compacted soils, and poorly drained soils may be the reason a lawn is doing poorly. These situations can be corrected during renovation. On the other hand, many lawn problems tend to be due to pests, weather conditions, or poor lawn care practices. Perhaps improved mowing, fertilizing, and watering may be all that's required to achieve acceptable lawn quality.
Typical Lawn Care Mistakes and Problems They Can Create
Lawn Care Mistake Potential Problem Favored
Mowing Too Short Crabgrass, Diseases
Frequent, Light Watering Crabgrass, Disease
Overwatering Thatch, Disease
Overfertilizing Disease, Thatch
Underfertilizing Disease, Weeds

Once the problem has been identified, the renovation process may begin. Think of renovation as fitting one of three levels: overseeding with little additional work; significant work, but allowing existing grass to remain; or completely removing the existing lawn and starting over.

The decision of which level to choose depends on how bad the lawn looks and what caused the problem. For example, if the lawn is just a little thin, overseeding with a quality lawn seed in late August or early September may be the answer. Use of a slit-seeder is an ideal way to overseed lawns. Seed may also be broadcast over thin lawn areas, but there needs to be good soil to seed contact. Dethatchers or vertical mowers can also be used to tear out excess debris prior to overseeding. In addition, slit-seeding could also be done directly through grass and/or weeds killed with the nonselective herbicide glyphosate. All of these types of overseeding procedures do not require additional soil modification.

When soil problems exist under a lawn, there are ways to address them without tearing up the lawn. As mentioned earlier, core aerifying is suggested for problems such as thatch and soil compaction. Aerifying, overseeding, and slit-seeding (breaks up cores) may be an ideal level of renovation for many lawns.

Unfortunately, some lawn problems, such as soil problems of severe compaction, high clay levels, or poor drainage, may require starting over. Remove existing grass or rototill it. High populations of perennial weed species may require use of a nonselective herbicide, such as glyphosate. Thoroughly work the soil to a depth of six inches. Add amendments such as compost, rotted manure, organic topsoil, and peat. Follow proper selection and establishment procedures (refer to Seeding and Sodding Lawns - Lesson 3) to get the new lawn off to a good start.