Q. My father-in-law bags his lawn clippings for the first few mows of the season and doesn't switch over to his mulching mower until the weather warms up, around the beginning of June. He says that the clippings don't decompose in cool weather, and will build up thatch. I, on the other hand, set my mower to mulch right away. So: does time of year and/or temperature matter for using a mulching mower? Both of us have compost bins for leaves and other yard waste, so any clippings we do bag won't go out to the curb. Thanks,
---Mike from Ashburn, VA.
A. You are right and he is wrong. Most American lawns are starved for any kind of natural food , and grass clippings are the most natural food a lawn can receive! By weight, they're an astounding ten percent nitrogen; that's about the same as poultry manure, corn gluten meal , bat guano and other natural high nitrogen fertilizers, and the exact amount of this essential food that lawn grasses crave. Returning your clippings to the lawn gives the grass a gentle, slow-release feeding every time you mow. Removing the clippings literally starves the lawn.
And this thatch nonsense is an old lie that will just not die. As Iowa State University turfgrass Professor Nick Christians has explained many times on the show, thatch is a thick mat of dead roots, stems and rhizomes that builds up at the soil surface, creating a visible layer that can interfere with watering and harbor disease organisms. But most thatch is the result of over-feeding with chemical fertilizers, not returning clippings to the lawn. In fact, clippings are a cure; they raise the level of biological activity at the soil line and help thatch break down.
True mulching mowers—machines with sealed decks and super-sharp blades—pulverize those clippings into a fine powder that's virtually invisible. The smaller the particle size, the greater the biological activity, and the faster the thatch will be composted back into the soil. And cool weather is not an issue—the nitrogen in those clippings heats things up nicely.
So always mulch your clippings back into the lawn. Returning the clippings reduces your lawn's food needs by half; and why would you ever want to take free food away from your lawn?
Q. Should we put our grass clippings into our compost pile? Thanks,
---Betsy in Falls Church, VA
A. No—as we just explained, those clippings provide a gentle feeding to your lawn every time you mow and help break down thatch, and so they should always be returned to the turf. And grass clippings can be problematic in compost piles, often adding way too much Nitrogen to the mix and turning the pile into a stinky mess. And there is another, much more ominous reason to not collect or compost clippings….
Q. Every year I use about seven cubic yards of municipal compost to help make my clay soil more productive. However, this year I am worried that the compost may contain Imprelis residues and do more harm than good. What do you know about the severity of this problem? (I do make my own compost but there is never enough). Many thanks,
---Joan in Princeton, NJ
A. It's not just Imprelis; several recently-developed "persistent" lawn herbicides survive the composting process so fiercely that the resulting "black gold" becomes REAL 'killer compost'—as in compost that kills your garden plants. Imprelis is the one that most people know about because it made the national news a few years back for killing untold thousands of trees and landscape plants that were growing in or near treated lawns.
The truly insidious part of this story is that many homeowners don't even know that their clippings are toxic. Most if not all of these persistent herbicides are "for professional use only", and so the actual lawn owner never sees the warning—buried, in one case, on page seven of a nine page label—that says not to compost the clippings. A homeowner who has a service spray their lawn but who then cuts it themselves could inadvertently use those clippings to make compost that kills their veggie garden. If they put the clippings out for collection, the municipal compost those clippings become could kill thousands of gardens.
So if your lawn is treated with any kind of chemical herbicide, don't collect the clippings or allow anyone else to collect them. And the relatively new issue of the safety of municipal compost? Better facilities have begun testing their finished compost for residues of these herbicides, so ask your source if they are—and if not, why not? (My local compost site simply cut to the chase and now refuses to accept grass clippings.) Just about everyone who produces compost on a large scale is required to test it, so always ask to see any paperwork they have. At the very least it should reveal the compost's pH, nutrient levels and heavy metal content.
And, as Dan Sullivan, who wrote a great article on this scandal for Mother Earth News,* explained in an interview on our show earlier this year (here's a link to that episode ), you can test bulk compost by planting some pea seeds in it. Peas are especially vulnerable to these herbicides, and if the young plants come up healthy, the compost should be safe to use.