Grass Clippings as Compost: There’s a Trick to This

Robert Delwood
8 min readSep 5, 2022

Robert Delwood, A lead API Documentation Writer. But as a gardener in this case.

Photo by Magic K

Introduction

Grass clippings are an excellent compost resource for urban gardeners. But there’s a trick to using it.

One abundant resource homeowners have is lawn grass or lawn clippings. The issue is how best to compost clippings. For starters, there is (almost) no wrong way to composite. Nature wants to break everything down. And it will with or without our help. There are, however, better or faster ways to compost. Clippings are rich in nitrogen and water, both ideal for compost piles and garden mulch. Processing clippings is a trick though. For best results, you have to handle them correctly.

When cutting the yard, there are usually three choices.

  1. Blow the clippings out of the side vent
    This makes the physical effort of cutting the lawn easier since the material is being blown out immediately and the lawn mower doesn’t have to process it anymore. The downside is that the clippings accumulate off to the side. It’ll eventually compost into the grass. In the meantime, it turns brown and many consider it unsightly. The traditional view of this option is the homeowner raking the grass into piles.
  2. Compost directly in place
    Here, the clippings are not vented away from the lawn mower. Instead, the lawn mower blade continues to cut the clippings into smaller pieces. This allows the clippings to compost quickly back into the lawn before it has a chance to turn brown. Lawnmower types include this composting mower. The lawn mower has a special blade designed for composting. This blade has an extended cutting edge. Instead of the normal three to four inches of cutting edge at the end of the blade, the composting blade has a six to seven-inch edge. The extra distance isn’t for cutting the grass but allows the clippings to be cut several times, each as a smaller piece, thus creating its own mulch. Smaller pieces compost quicker and more efficiently. The surface area is increased with smaller pieces because of the extra edges. This is likely the best option for the lawn for homeowners who do not compost.
  3. Collect the clippings
    Most lawn mowers have a bag attachment meant to collect the clippings. This bag prevents the clippings to lay on the ground and possibly look bad for a few days. For many, this is a convenient way to collect clipping to throw away with the trash. For composters, this bag collects green gold.

The Lawn Clippings

Lawn clippings have always been problematic for composting. By itself, it’s not good compost material. If dumped into a single pile improperly, it quickly becomes a smelly, gooey pile, completely unfit for use in gardens. So, it does take effort to use it correctly. There are three options.

  1. In a compost pile
    You can add the clippings to an existing compost pile. It takes a little bit of effort. For this, the clippings must be layered into the pile. Add a layer of clippings no more than two inches deep. On top of that, add an equally sized layer of existing compost material. Alternate the layers until all the clippings are used. The thin layers, the proximity to other, and the rougher material allow adequate water and airflow into the new clippings. That will properly mix the ingredients and cook well. However, not everyone has a well-formed compost pile to handle the amount of new clippings.
  2. Green mulch
    You can add the clippings directly into a garden. Add about two inches of mulch between the garden rows and around each plant. Two inches is enough to get all the effects of being compost without any of the time requirements. This amount will restrict weeds from sprouting, catch water, prevent the dirt from drying out, and will easily compost to return the nutrients back to the plants.
  3. Brown mulch
    Although grass is typically considered a green resource, literally because of the green color, it can also be turned into a brown resource. Brown is actually a much better mulch and compost material. This is what the rest of the discussion is about.

What is a Compost Pile?

Before that, let’s examine the mechanics of a compost pile in general and grass compost pile in specific. As mentioned earlier, nature wants to break everything down. So, it’s just a matter of facilitating that. Typically, there are green and brown materials, and the pile needs both. Green is mostly nitrogen and water. Grass, food, and vegetable material are considered green. Brown is considered carbon. The gold standard of brown is leaves. You can almost make compost with only leaves. You can also use shredded paper office paper or newspaper, although that is a considerable effort.

The mixture and ratio are what’s important. The ratio is considered 30 parts green to 1 part brown. Clearly brown does its part. Those lucky enough to have trees can use leaves. The rest of us may have to make our own. And that can be done with grass.

If you bag clippings, a typical suburban house produces a pile of about three cubic feet, or 3 feet wide, three feet long, and three feet high. More than enough to create a pile. I use a wheelbarrow, and it easily holds that amount. And it’s active. As the bacteria eat and break down the grass, they give off heat. That heat then allows more bacteria to eat, and so on. In about two hours, the center of the pile reaches 140 F. If you doubt that, just stick your hand in there.

A typical grass collection from a single mowing.

The picture above is the volume of clippings I get from one mowing of both yards. It’s almost perfect for me. While I do have a compost pile, this volume of green grass is too much. I need it brown first. Using the wheelbarrow is a problem. There’s too little surface for its volume. As a result, I have to turn it a lot. About eight times a day. But it’ll be ready in three weeks, in time for the next lawn mowing. Notice, too, there’s grass below it. That spilled out about two months prior and I never picked it up or turned it. Because it was thinly layered, about two inches, I didn’t think there was a need to turn it. I was right. Read on.

What are the Problems?

The temperature is the best indicator of the pile working. As good as that sounds, this can be a bad thing. The grass pile left to its own causes two problems.

  1. It compacts under its own weight
    This is caused by heat and decomposition. Again, while this might be considered a good thing when it’s combined with the moisture in the grass, it becomes matted. After the grass mats, it is impossible to separate or restore it back to a fluffy condition.
  2. It turns anaerobic
    As the grass mats, air and oxygen cannot reach the grass. The bacteria turn to an anaerobic process. It continues to break down but without air. This is not good for composting. The most noticeable result is a strong nitrogen smell, which has been described as a urine smell. In an urban setting, the smell may be noticeable, even to neighbors. You will also see white fungus. This can happen in a short amount of time, perhaps as soon as two days.

At that point, matted sections become worthless as compost. They cannot be saved and must be thrown out.

Turning Green into Brown

The trick then is not to let it reach that stage. The key is turning the pile. Just reach into the bottom and pull that material up. Do that several times for each turning. At a minimum, turn it twice a day. More is better. You can get brown grass in a few as two weeks by turning it at least six times a day during that time. Otherwise, it’ll get there in about a month. I keep the pile near the back door and turn it five or even eight times a day, each time when I pass by. My timing is based on the next lawn mowing, so in the summer I plan for three weeks between grass batches.

The other way to speed this up is by having spread out over a great surface area. This allows more exposure to the sun and air. When I can, I use a tarp laid near the garden and compost pile. It allows about six times the surface area. It still needs to be turned but not as frequently.

How ever it’s done the pile becomes noticeably drier; you can test that with your hand. The dried grass will even become prickly, something that you can also test with your hand. And it turns brown. Once the pile has turned brown, you have a lot more leeway in using it. It’s even better as a garden mulch. Or you can mix it with fresh grass to make a slower but less involved compost pile.

Conclusion

The amount of involvement is up to you. Of course, there are a lot of videos about this, and I recommend looking at those. For me, I used to be a lazy composter, and let the pile do all the work. I would harvest it once a year in the spring when preparing the garden beds. When I moved to a more urban location, I was in hurry to create a garden.

Buying all the soil gets to be expensive. I don’t know who came up with the phrase “dirt cheap.” I ended up wanting to make soil. That’s when I got interested in how to create mulch and compost quickly. Compost in general and brown compost in specific isn’t soil. It’s an additive to it, can help add to the soil volume, and over time, it does become soil, integrating into it.

Follow Up

The earlier picture showed my wheelbarrow full of grass clippings, but also some grass on the ground next to it. That grass had fallen out onto the concrete porch during a previous lawnmowing but I was too lazy to remove it. Why, after all? It was out of the way, and was a thin layer, no more than two inches deep, if even that. I figured it would dry on its own. And it did. After about two months, it was a dry pile of brown grass, exactly as I expected.

Well, almost. When I did go to pick it up, it did one thing I hadn’t expected: It composted. I would have bet money against that. I knew it would dry. But the bottom had composted into soil. See the pictures below. There were even earthworms in there. I did say that nature wanted to break everything down. And it does.

I don’t know what to make of this, or how to take advantage of it.

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Robert Delwood

Programmer/writer/programmer-writer. A former NASA engineer, he ensured astronauts had clean underwear. Yet, it was always about API documentation & automation.