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Composting Grass Clippings


Q. My son has been trying to make compost out of three large piles of grass contained by plastic fencing. With all the rain we've had, the piles have become wet, compacted, dense and very heavy. What can be done to make these piles more effective at breaking down? They have been turned, but we recently added a lot of grass—and that plus the rain has made things a compacted mess. I examined one pile today and it's actually like "green manure"; you know, all soft and squishy. That should be really great for the garden...no?
    ---Elizabeth in North Plainfield, New Jersey
A. "No" is correct, Elizabeth. 'Green manure' is a crop that you grow to plow into the ground as living fertilizer. What your son has is just a big green stinky mess. (Actually, THREE big green stinky messes.) This is a common mistake for rookie composters, especially in the summer, when grass clippings are abundant.

While the excessive rain in your region has made the situation worse, trying to compost green grass clippings alone is never a good idea. Those clippings are VERY high in Nitrogen—about 10%. That's pretty much the same level you'd find in really 'HOT' manures, like bat and bird guano. In the simplest sense, these Nitrogen rich components don't become the compost in a pile; instead they provide food for the billions of little microorganisms that fuel the process of turning the other stuff—the so-called 'dry browns' that should make up at least 80% of a pile—into the garden gold our plants so crave.

Now, grass clippings are THEORETICALLY better at this feeding than your average kitchen waste, as the clippings are hot, hot, hot, and with a few exceptions (most notably spent coffee grounds), kitchen garbage is cold, meaning low in Nitrogen. The benefit of adding things like lettuce leaves, apple cores and broccoli stalks to a compost pile or bin is mostly in the soothing of your recycling conscience, not in their ability to create high quality compost.

Now you can use clippings to make great compost, but to do so you have to mix small amounts of well-shredded grass clippings in with large amounts of well-shredded leaves. Do this, and you create the distinct possibility that the living creatures that fuel the composting process will be so perfectly fed and working at such maximum speed and efficiency that you can achieve finished compost in the legendary two to five weeks, depending on how honest you are when you measure the ingredients, the size of the pile (the more mass, the faster it'll cook), and of course, the moisture level. (The best compost piles follow the Goldilocks rule: Not too wet and not too dry. Lots of airflow too. I know, Goldilocks didn't mention airflow. But she should have.)

Anyway, the result of such a noble enterprise is the elusive, much sought-after garden amendment known as "hot compost". Compost that cooks up quickly with the help of a natural source of high Nitrogen is much better food for your plants and provides much more life for your soil. It's also the kind of compost that can prevent disease when used as a mulch under plants like roses, tomatoes and lilacs. And it's the best kind for making compost tea. "Cold compost"—the stuff that results when you just pile a lot of things up, hope for the best and actually get some finished material after a year or so—can be a good plant food and soil improver, but hot compost is MUCH better.

What you have, unfortunately, is not likely to become either kind. I fear that your big piles of slimy wet grass clippings will not improve one bit with the passage of time. Just the opposite in fact.

Ah, but your timing is good to get it right, as we are fast approaching autumn leaf fall. Let lots of leaves collect on the lawn during a dry spell (don't let wet leaves accumulate), go over them with a mower, bag up what should be a perfect mixture of lots of excellently shredded leaves and a small amount of well-shredded grass and then empty this mixture into a big wire cage, a slatted wooden bin, a professionally made composter or something else to hold it all in place nice and neat. The physics should insure that your dry browns and hot wet greens (that sounds kind of risqué, doesn't it?) are mixed together better than humanly possible, which is another secret to super-fast composting. (People who tell you to 'layer' the ingredients in a compost pile failed physics.)

Yes, this will only use a small percentage of the clippings generated by the average lawn, and that's a good thing. Because outside of that autumn leaf drop window, you should NOT be bagging your grass clippings. Instead, "mulch them" back into your lawn with a mulching mower. I use "quotes" because there's no 'mulch' of any kind involved here. A poor name for an excellent instrument of sustainability, mulching mowers pulverize clippings into an almost invisible powder that they then return to your lawn. A powder that's 10% Nitrogen; about as high a natural number as you can get. Returning that Nitrogen to your lawn provides half the food your turf requires in a given year!

It's also the only safe use for clippings from an herbicide treated lawn, as lawn chemicals are designed to have no effect on grass. DON'T use any clippings from an herbicide-treated lawn in a compost pile. Some of the potent chemicals in use today can survive even hot composting and could kill any plants that receive the compost later on. Oh, and stop using that toxic stuff too!!!

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