Grass clippings are a free source of nitrogen that can reduce by one-third to one-half the fertilizer needed. Clippings also prolong the effects of any fertilization by returning nitrogen in an organic, slow-release form that promotes steady grass growth.

Studies show that the wet weight of lawn clippings ranges from 3.73 tons per hectare annually (in a low fertility regime) 15 to 27.17 tons per hectare annually. This is equivalent to between 200 and 2,700 garbage bags of clippings from one hectare of lawn (about 20 average sized home lawns). The burden this places on landfills is enormous?EUR??,,????'??+for example, before Michigan passed a law banning clippings in 1994, yard waste accounted for 20 to 25 percent of the trash disposed in landfills.

A study done by the Michigan State University Dep. of Crop and Soil Science comparing plots with bagged clippings to grasscycled
plots reported:

• Grasscycled plots greened up earlier in the spring, grew faster, and stayed greener in the fall.
• Grasscycled plots had fewer broadleaf weeds than bagged plots.
• Bagging clippings nearly doubled the total mowing time compared to grasscycling.
• Bagging generated between 6 and 11 tons wet weight (1,000 to 3,800 cubic feet) of clippings per acre per year.
• Neither the bagged nor grasscycled areas developed any thatch build-up.

If more than one-third of the grass plant is removed in mowing, heavier clippings with their higher proportion of coarse stem material take longer to break down. In some cases, this can be overcome by running the mower over the area again to chop clippings more finely.

Nutrient Content of Clippings

The fertilizer value of clippings is an important reason for leaving them on the lawn. One report found that a year?EUR??,,????'???s clippings contained 2.4 kg (5.3 lb.) of nitrogen, 0.8 kg (1.8 lb) of phosphorous, and 2.2 kg (4.8 lb) of potassium per 93 m2 (1,000 ft2) of lawn.

The nutrient ratio in clippings is about 5:1:3 (N:P:K), which is similar to that of high quality turf fertilizers.

The nitrogen is in organic form, thus is released slowly with no potential to leach off site.

Where clippings were removed, the annual nitrogen fertilization rate had to be increased by about 1 kg per 100 m2 (2 lbs/1000 ft2) to maintain the desired color and density of turf in Michigan.

A Guelph study reported that up to 40 percent of applied nitrogen is removed with clippings. Based on this data, clippings removed from one hectare of lawn area may contain the equivalent of 17 to 56 bags of high quality turf fertilizer.

Clippings also add organic matter to the soil. This has been shown to improve the infiltration rate of water in comparison to sites where clippings were removed.

Grasscycling vs. Thatch

Clippings do not contribute to thatch accumulation in a healthy lawn. With their high water content (87 to 88 percent in Fredericton tests), they shrink rapidly and filter down through the turfgrass to the surface. Soil organisms (fungi and bacteria) decompose them quickly. Thatch can build up, however, in conditions where the activity of soil microorganisms and earthworms has been suppressed. This can be caused by low soil pH, applications of fungicide, insecticides or excessive fertilizer, compaction or poor drainage.

When is grasscycling not appropriate?

If more than one-third of the grass plant is removed in mowing, it takes longer for the heavier clippings with their higher proportion of coarse stem material, to break down. In some cases, this can be overcome by running the mower over the area again to chop them more finely. In other cases, especially in wet weather, the excessive clippings may need to be removed.

Yard waste places quite a burden on landfills. Before Michigan passed a law in 1994 banning clippings from landfills, grass and other yard waste made up 25 percent of the landfill?EUR??,,????'???s contents.

Managing Thatch

Thatch is the dark brown mat of undecomposed plant tissue that lies above the soil. It is made up of stems, rhizomes, stolons, leaves and roots. These products are high in lignin, which is tough for soil microorganisms to break down.

A moderate thickness of thatch (up to 2 cm or about 3/4-inch) is a necessary and beneficial part of a lawn:

• Creates a cushion effect, binding individual grass plants together and helping to resist wear from foot traffic.
• Mulches the soil, which reduces the potential for erosion and water evaporation
• Provides an important source of natural food for soil microorganisms.

When the thatch layer is deeper than 2 cm (3/4 inch) it becomes a problem that can lead to declining turf health, shallow root systems, and reduced performance of pesticides.

Causes of Excessive Thatch

Excess thatch is a man-made problem. It develops when the dead organic matter from the actively growing turf accumulates faster than it decomposes.

Any cultural or environmental factor that stimulates excessive shoot growth or impairs the decomposition process increases the rate that thatch accumulates.

Since earthworms and soil microbial activity decompose thatch, any practice that depresses this activity will result in thatch accumulation.

Thatch problems are not seen in turf where healthy earthworm and microbial populations exist in the soil. While we cannot see the soil microorganisms, indicators that they are healthy would be:

• healthy turf growth
• good levels of organic matter (two percent or more) in the soil
• significant earthworm populations.

Factors that can suppress the earthworm and microbial activity needed to decompose thatch include:

• high fertilizer rates that stimulate rapid turf growth
• low soil pH
• applications of fungicides, some insecticides, and to a lesser extent, herbicides
• compaction
• poor drainage
• excessive irrigation.