In some lawns, organic matter is produced faster than it can decompose and thatch gradually develops over time. Excessive thatch - more than 1/2 inch - may become a problem in highly maintained Minnesota lawns.
What is thatch?
Thatch is a tightly interwoven layer of living and dead tissue existing between the green vegetation (i.e. grass leaves) and soil surface. It is composed primarily of products from stems, leaf sheaths, crowns, and roots that resist decay. Although a minor thatch layer improves the wear tolerance of a lawn, excessive thatch harbors disease organisms and insects making the lawn more susceptible to damage from disease and drought.
Factors that affect thatch
Several factors determine the rate of thatch development in your lawn:
- Choosing vigorously growing grass varieties
- Applying excessive amounts of nitrogen, especially in spring
- Growing species that are known to produce large amounts of tough, fibrous tissue
- Compacted soil conditions leading to shallow root development
These factors decrease the rate at which thatch decomposes:
- Acidic soil conditions
- Poor soil aeration and waterlogged soils which limit microorganism activity
- Pesticides such as insecticides or fungicides that restrict microorganism or earthworm activity
Assessing thatch in a lawn
To determine the amount of thatch accumulation, remove a two-inch deep, pie-shaped wedge from the lawn and measure the amount of thatch between the soil surface and green vegetation. If the layer is one-half inch or less, it usually is not a problem. If, however, the layer exceeds one-half inch, you should implement a program of thatch management.
Thatch control may include both prevention and removal. Preventing excessive thatch should concern all homeowners and turf managers interested in maintaining a high-quality lawn. A thatch removal program should be considered any time thatch has accumulated in a layer more than one-half inch thick.
Preventing thatch accumulation
Prevention is key to minimizing thatch development. Fertilization, aerification, mowing and pesticide use can be adjusted to reduce accumulation.
Fertilizing your lawn
Some fertilization is required to maintain a healthy lawn and foster thatch decomposition. Fertilize only enough to maintain your lawn’s desired color and growth. Fall fertilization (September) is better than spring fertilization because the resulting growth is not as rapid and lush. Have your soil tested by the U of M Soil Testing Lab to determine the proper amount of nitrogen, phosphorus, and potassium to apply.
Aerating your soil
Compacted soils and soils with poor drainage accumulate thatch faster than well-drained soils. Aerification promotes better moisture and air penetration into compacted soils. It helps establish a deeper and healthier root system and also stimulates the microbial activity involved in decomposing the thatch layer.
- The aerifier must have hollow tines or spoons that bring the cores of soil to the surface to be effective long term.
- Fertilize the lawn one week prior to aerification to hasten the time it takes for the grass to fill in the holes left by aerating.
- Run a core aerifier over the lawn several times in different directions to break up compacted soil as much as possible.
- Allow the cores of soil to dry partially before raking them back into the lawn
- You can also allow cores to sit on the surface and crumble apart over time. This "top dresses" the grass with soil containing desired microorganisms that will work at decomposing thatch.
When to aerate
The best time to aerate is between late August and early October, depending on how far north you live. Spring or summer aerification can also be successful, especially when followed with regular, frequent watering. In spring, wait until you’ve mowed the lawn twice before aerifying. Then, unless you are reseeding the lawn, follow up with an application of pre-emergence herbicide after aerifying to help prevent annual weeds from sprouting.
If you mow your lawn regularly so that no more than one-third of the leaf height is removed with each mowing, you don’t need to bag the clippings. Small grass clippings filter down into the grass and decompose rapidly, recycling nutrients back into the lawn. However, the clippings must be uniformly distributed and not deposited in clumps.
While you don’t need to use a mulching mower to let clippings fall back to the lawn, mulching mowers do distribute clippings far more evenly than standard mowers. Using a mulching mower does not, however, lengthen the interval between times that grass should be mowed. Mowing frequency is determined by the growth rate of the grass. If the desired height is three inches, cut the grass when it is no more than four and a half inches tall - regardless of the mower you use.
When to use pesticides
Avoid using pesticides as much as possible. Many pesticides affect the microbial and earthworm populations that are involved in decomposing the thatch layer.
Use pesticides only when a pest problem has been clearly identified and the pesticide is necessary and known to be effective.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
You can remove thatch with a vertical mower (sometimes called a power rake). These machines have steel knife-like or spring-like tines that rotate perpendicular to the ground surface.
- You may need to aerify before vertical mowing when thatch buildup is severe.
- Set vertical mowers so the tines bring a small amount of soil to the surface with the thatch debris.
- Vertical mowers are available at many rental dealers as well as garden centers and suburban hardware stores.
- Some professional lawn care companies also can be hired to aerify or vertical mow home lawns.
- "Power rake" attachments to rotary mowers are not effective in removing thatch and are not generally recommended because they can be quite destructive to existing grass.
Vertical mowing results in a large volume of debris that must be removed from the lawn. Disposal of this debris sometimes presents a problem, though you should be able to take it to a county compost site or add it to your compost bin.
The debris actually makes an excellent soil conditioner when composted, and can be incorporated into garden soil or used as a mulch for gardens, woody plants, and flower beds. It will contribute organic matter and nutrients and improve the structure of heavy soils.
When to vertical mow
The best time for vertical mowing is usually between late August and early October, depending on your location, because grass is growing vigorously then and should recover quickly from any damage. In addition, few weed seeds germinate at that time. A light application of fertilizer (1/2 to 3/4 pounds actual nitrogen per thousand square feet) and regular watering will speed the lawn’s recovery after vertical mowing.
Reviewed in 2018