About Grass Clippings
Leave the grass clippings on the lawn after mowing. If they are not mulched into the turf by a mulching mower, then rake them apart so they do not clump:
Clippings are Fertilizer--Left on the lawn all season clippings contribute 30% of its total fertilizer needs for the year. Because they contain about 4% nitrogen, 1/2 to 1% of phosphorous, 2 to 3% potassium, and smaller amounts of other essential plant nutrients, they basically constitute a 4-2-3 fertilizer growing in your yard. Over a season of weekly cuttings, the decomposing clippings add nutrients to the soil the best way--slowly and steadily. And, it is free.
Clippings Reduce Water Need--When you allow the clippings to fall down among the grass blades, they shade the soil surface. This reduces moisture loss from evaporation. Moist soil is cooler soil.
Mowers and Clippings
Avoid clumping of clippings by using a mulching lawn mower. Its special design and blade cuts the grass blades several times before they fall into the turf. Mulchers also mulch leaves into the turf in the fall, which provides valuable organic matter to the soil. One that has a bag attachment as well enables you to collect clippings when the grass is wet or so long that the clippings will clump for sure.
Collect clippings if you are planning to overseed the lawn and need to have the soil exposed. If, in the fall, you have more than 2 inches of leaves fallen on the surface of the lawn, use the bag to collect the mown leaves along with the cut grass. They make wonderful mulch or compost.
For more related information see Choosing a Lawn Mower.
Clippings are Not Thatch
Grass clippings are not thatch. Nor do they promote the development of thatch. You may see them in the thatch when you look closely, but they are there just temporarily. Clippings contain over 75% water and are broken down by the worms, fungi, and other resident microbes in the soil on which they fall about 2 to 3 weeks. Since you mow more frequently than every three weeks, there is a constant process of decomposition of clippings going on down at the base of the grass plants. It is a very healthy process.