What to Do With Grass Clippings

This photo may not be worth a thousand words, but it started a nice conversation. It so happened that, during one of my ongoing documentations of the export of nutrients from Princeton's yards, the owner pulled in. I figured he'd think, "Oh, just another Princetonian photographing my beautiful grass clippings," but to my surprise he came over and expressed interest in knowing what was so interesting about two blobs of discarded green.

It seemed inauspicious to begin a conversation by saying there's a (unenforced) borough ordinance against putting grass clippings on the street, but a mutual interest in composting quickly emerged. I offered news that the county extension master gardeners recommend leaving grass clippings on the lawn, so that all the clippings' nitrogen returns to the soil rather than getting washed down the street into Carnegie Lake. The dreaded thatch buildup of yore, which once spurred homeowners to bag up grass clippings, apparently dissolved into a myth.

Grass clippings' high nitrogen content endows them with the power to do great good or considerable harm. Massing them in piles tilts them towards harm. They pack tightly, shutting out oxygen, thereby making perfect habitat for anaerobic bacteria to feast on the rich organic matter. Break open a pile of grass clippings that have been sitting for awhile, and you will learn the hard way that the anaerobic decomposition process produces vapors profoundly repellent to humans. Aerobic bacteria, by contrast, do not produce nasty odors. Therefore, the best thing to do with grass clippings, if one is determined not to leave them on the lawn, is to give them access to air by spreading them in a thin layer either on a compost pile or as a thin mulch under shrubs.

Particularly relevant this time of year, autumn leaves, chopped up as one's mowing the lawn, can also be left to settle down into the ground between the grass blades.