Clumps of grass clippings shouldn't be left on top of the turf. It's best to disperse the clumps on the lawn with a fan rake or blower. Excessive clumping may require raking the clumps up and removing them. If left on the lawn for too long clumps can suffocate the grass beneath it, turning it yellow.
Here are some tips for reducing grass clumping:
Mow When It's Dry
As a rule, you should not really mow the lawn when the grass is wet and never remove more than one-third of the leaf blade when mowing.
Mow the yard only when the grass is dry. Wet grass is much more likely to form clumps than dry grass. Avoid mowing after rain storms or early in the morning when dew remains on the ground. If you absolutely have to mow when the grass is wet and/or has become very long, clumping is likely to happen.
Don't Wait Too Long
Don't let the grass get too long between mowings. The grass gets too long and mowing throws out clumps everywhere or creates a bagging nightmare. You can dump these clumps in the compost pile or let ’em dry for a few days and use as mulch in garden beds. Mow often enough so you’re never removing more than one-third of the blade length at a time (i.e. time to cut when the height gets to 4 inches, not 5 or 6). This may mean mowing twice a week or every four or five days when grass is growing fastest in mid-spring.
People often cut too short, causing more clumping.
Mowing too short also increases moisture and nutrition demands as the grass tries to fight back from near decapitation. And weeds germinate better and get off to a faster start when taller grass blades aren’t in the way. Cut 2½ to 3 inches high – typically the highest setting on mowers. What makes a mown lawn look good is the evenness of the cut – not its height.
Most people are just as happy with a 3-inch-tall evenly cut lawn as a 1-inch-tall evenly cut lawn.
Use Mulching Blades
If you mow often enough, you’ll get tiny clips that quickly disappear into the lawn, especially if you’re using a mulching mower.
Maintain Your Mower
Disconnect the mower deck, if you're dealing with a riding lawn tractor. Scrape the bottom of the deck with a plastic scraper or putty knife. Remove all dried grass and mud stuck to the bottom of the deck. Also, thoroughly clean out the discharge chute on the deck. Layers of dried grass and debris can block up the deck, which causes clumps to form as you mow.
Examine the mower blades. If the blades are damaged or dull, they should be replaced or sharpened. Dull or damaged blades will not cut the grass cleanly, which results in clumping.