Tony O’Neill, gardener and author of the popular “Composting Masterclass” and “Your First Vegetable Garden,” combines lifelong passion and expert knowledge to simplify the art of gardening. His mission? Helping you cultivate a thriving garden. More on Tony O’Neill
Are grass clippings compostable? Green grass clippings are a good source of nitrogen. Their addition to compost piles, however, requires special care. The combined factors of a flat blade structure and high moisture content can limit airflow and quickly lead to an anaerobic, smelly mess.
Composting grass clippings is perfect for aerobic compost production: They are ideal due to their high nitrogen content. An initial C: N ratio of 33:1 is optimal. Grass clippings are 2.4% nitrogen and 45% carbon, thus a C: N ratio of 18:1 and perfect for compost
Can You Turn Grass Clippings into Compost?
Yes, you can. You will, however, need to consider the following three factors:
- To avoid anaerobic conditions, the moisture content of the material in your compost pile should be less than 60%. On the other hand, if your moisture content drops below 30%, microorganisms’ activity ceases.
- You should aim for a carbon-to-nitrogen (C: N) ratio of between 30:1 to 33:1.
- To avoid anaerobic conditions, you need air to flow. Grass clippings tend to mat. Because the grass blades are flat and wet (when freshly mowed), they stick to each other and prevent natural airflow.
What Is the Moisture Content of Grass Clippings?
When recycling grass clippings, start with element A above – moisture. The moisture content of the completed compost mixture is an important consideration when determining the proper ratios of different materials for composting.
There’s a standard way of reporting moisture content in the composting industry: on a wet basis (or total weight). I go into more detail about how to do this in my Composting Masterclass book.
You take the weight of the freshly cut lawn clippings and compare it to the dry weight to determine the moisture percentage. I’ve done this several times, and the average is 77%. It’s no surprise, then that it starts getting rotten if you leave a pile of grass clippings for a day or two.
Managing Moisture Content in Grass Clipping Composting
We’ve established that we must ensure moisture content below 60% to avoid anaerobic conditions when comping grass. We aim to reduce the 77% moisture content in the clippings while retaining fresh clippings’ nitrogen benefits – so drying it out isn’t an option.
We must add dry carbon matter that will absorb (or distribute) the excess 17 percent water. Even if we use dry leaves, their moisture content remains about 35 percent.
Therefore, for every 10 pounds of grass clippings, we must add about seven pounds of leaves. For now, we’re only considering moisture levels. Next, we’ll consider the effect on the carbon/nitrogen ratio.
Just a note: like grass clippings, leaves have a flat structure and mat quickly. Add relatively finely shredded leaves to increase the surface area available for microorganisms and prevent anaerobic conditions.
Managing Carbon/Nitrogen Ratios When You Compost Grass Clippings
We need to mix the clipping into a shredded leaf mass to capitalize on the high nitrogen levels in grass clipping. Dry leaves have a nitrogen level of 0.75%, and carbon content is 50% carbon (the rest is cellulose and lignin). The C: N ratio for dry leaves is 66:1.
So, to get to a 30:1 ratio, for every 10 pounds of grass clippings, you only need 3.5 pounds of dry leaves to end up with a 30:1 mix. But as we established, we require double that quantity of leaves to manage moisture levels. What is a gardener to do?
Balancing Moisture and C: N Ratios in Grass Clippings
What do you choose when there’s a choice between accepting lower C: N ratios or high moisture content when you compost grass? In our mix above, we added 6.8 pounds of shredded dry leaves to our ten pounds of fresh grass clipping to get moisture below 60 percent.
That caused our C: N ratio to drop to about 37:1. We were aiming for a minimum of 33 parts of carbon matter for every one part of nitrogen. In the composting process, the carbon is reduced to carbon dioxide, with some nitrogen loss. The final compost mix ratio is about 10:1.
So, while the 37:1 ratio will delay the speed of our initial composting process, the nitrogen levels increase as the microorganisms consume the carbon, releasing carbon dioxide. So, it won’t be long before we get to the optimum 33:1 ratio when things speed up.
If we add fewer leaves, focussing on starting with the optimal 30 to 33:1 ratio, our moisture will be more than the crucial 60 percent. It won’t be long before the distinct rot as anaerobic conditions set in.
To salvage the batch of compost, you must add wood shavings – more dry-matter carbon. It’s better to start with a mixture with a total humidity factor of less than 60 percent.
What are the Benefits of Using Lawn Clippings in Compost?
Grass clippings offer unique characteristics that will further solve our dilemma above bioavailability.
Bioavailability of Carbon and Nitrogen in Compost Materials
Bioavailability is a term that describes the levels of readily accessible nitrogen and carbon in a composting mix.
Grass clippings are an excellent example of high bioavailability. If you left your clippings on the lawn, it would reduce the need to add nitrogen by as much as 50% – halving the amount of nitrogen you need to add. This is perfect for post-lawn care.
That is because the clippings give your lawn easy access to the nitrogen in the trimmed-off third. This characteristic of grass clippings makes it ideal as a nitrogen-adding composting component.
Bioavailability of Carbon in Shredded Leaves
Leaves, on the other hand, have limited use in traditional composting – limited but still valuable. Because the carbon in leaves is encased in cellulose and lignin, its bioavailability is limited. Usually, it’s fungi that break leaves down into mold.
In traditional hot composting, combining a diverse population of microorganisms does the job. The point is that even though theoretically, our compost lawn clippings / shredded leaves mix is now at 37:1, the reality is that a lot of that carbon is not bioavailable. This helps solve the challenge of a high carbon ratio – the carbon referred to won’t be part of the process until much later.
Can you Use Just Clippings?
Aerobic composting can be compared to a symphony. Just as a single violin can be part of a symphony, its lone tune, however elaborate, does not make a symphony.
So too, as much as grass clippings are 2.4% nitrogen and 45% carbon (a C: N ratio of 18:1), clippings cannot create compost singularly.
When composting grass, grass blades’ structure allows them to clump together easily, limiting airflow. Also, lawn clippings are high in moisture (77% average), increasing their propensity to create an anaerobic mass.
If you put grass clippings in a heap within a day, you will notice a distinct smell of ammonia due to the anaerobic processes and high nitrogen content.
You can create an anaerobic compost from grass clippings by storing clippings in a bag. This is, however, not a preferred method as it is smelly and will take much longer. Breaking organic matter down using anaerobic processes releases hydrogen sulfide and other sulfur-containing compounds.
The organisms in anaerobic environments use nitrogen, phosphorus, and other nutrients to develop, reducing the organic nitrogen to organic acids and ammonia.
Because anaerobic processes are cold (produce no heat), contamination and pathogens in your lawn clippings will not be destroyed and can be transferred to your soil.
Any destruction of pathogens in this environment is usually a result of biological antagonism and is slow. If you want to make compost this way, make the compost this year for use next year.
Do Grass Clippings Smell?
Suppose there’s a putrid smell during the traditional hot composting process. In that case, something is wrong – usually too much moisture or too little oxygen. Compost has an earthy smell, even grass trimmings composting. Discovering the value of autumn leaves was one of my best lessons. They have fantastic potential.
As a leaf mold, they produce soil that retains water and improves fertility. If collected when dry or shredded and dried, leaves store well. Shredding increases their surface area and reduces their matting tendency. Add lawn clippings to dry shredded leaves to produce a mix that clumps into a ball that readily falls apart when touched.
The combination of fresh lawn clippings and dry shredded leaves is an excellent addition to your compost heap – and the product will be odorless.
Are Grass Clippings Fast to Decompose?
Removing no more than a third of the blade length per cutting is preferable when mowing your lawn. Cutting more than a third off provides opportunities for pathogens to damage the health of your lawn. Following this guideline, you should leave your clippings on the lawn as the nitrogen will help it keep it green and healthy. And freshly cut grass is packed with nitrogen, and it’s FREE.
If you cut more than a third off for whatever reason, composting and mixing grass clippings is a good option – if composted using a hot aerobic process. The hot process will kill pathogens in the clippings.
A static pile of grass clippings will become anaerobic, causing foul smells and creating a mass with minimal organic benefit for at least a year. After a year, the pile can be used but would have lost much of its organic benefits.
Mixing Grass Clippings With Carbon Sources.
Mixing the static pile with some dry hay (not straw) will significantly benefit the process as moisture and carbon levels will contribute to better decomposition. Occasionally turning the compost pile and watering the heap will further improve the spread of essential microorganisms.
A compost pile comprising more refined materials (like hay and grass clipping) is susceptible to spontaneous combustion. If you have a static pile like this, I advise building it away from other flammable materials or equipment.
Pile grass clippings so that they do not overheat and ignite. Better still is to use them immediately with a good carbon source and make lawn clipping compost.
The difference between straw and hay is that the former is mainly constituted of cellulose and the latter of bioavailable carbon.
How Long Does It Take For Grass Clippings Compost to Finish?
Are grass clippings fast to break down? Grass clippings have a higher level of bioavailable nitrogen than, for instance, horse manure. (2.4% vs. 1.6%). Incorporated in sufficient quantities with shredded leaves and other green and brown matter, creating a C: N ratio of 33 to 30:1 will create a vibrant composting environment.
Under the above conditions, and with close monitoring and management (temperature, air, moisture availability, and mixing) and depending on the size of the total compost pile, you could create black gold within a week or two.
Is There a Way to Prevent Grass Clippings From Turning Smelly?
The smells created by grass clipping heaps or bags result from the mix not getting enough oxygen (enough is more than 5%). The organisms responsible for anaerobic processes reduce organic matter to waste – acids, ammonia and hydrogen sulfide, methane, and sulfurs – smelly stuff.
The above happens in anaerobic environments. Including air in the process (more than 6%) changes the decomposition process and the team of microorganisms involved – guys less odiferous in their approach.
The two best ways to make grass clipping compost are by incorporating it in an aerobic compost pile or your trench composting process. Both are good, and both are odorless.
What Are the Advantages of Using Grass Clippings?
While it is commonly believed that composting is about producing organic matter, the truth is that it’s more about farming with microorganisms. What benefits your soil, providing ways for your plants to access nitrogen, potash, phosphate, and other vital nutrients, are microorganisms.
Grass clippings are a perfect microorganisms farming material – and can be used directly on your lawn. In the latter instance, if you leave the clipping on your lawn, the nitrogen in this highly bioavailable material will improve your lawn’s health without causing thatch problems (grass thatch is a root problem that can be caused by added nitrogen, not grass clippings.
This is one of the best lawn care tips I can give you.
The readily available nitrogen will boost your composting efforts if added to your compost heap and mixed in with more refined materials (like chopped hay or shredded leaves).
What Are the Disadvantages of Using Grass Clippings?
One of the challenges composters faces is the inclination of grass clippings to clump and become anaerobic – exacerbated by its high moisture content. Your compost heap is a symphony (I’m stuck on the analogy) of carbons, nitrogen, textures, structures, bioavailability, moisture, oxygen, microorganisms, and temperatures.
The fineness of grass clippings compost(assuming they are clippings and not cuttings) makes it difficult to spread throughout the mass. Pre-mixing the clippings with shredded leaves can help manage this disadvantage of grass clippings.
Removing the grass from your lawn can rob the nitrogen from the soil over time.
How Often Should I Turn My Grass Clipping Pile?
There are two reasons for turning a compost pile. Firstly you want to control temperatures and humidity (the two are factors of each other)
Secondly, you want to spread the activity of the microorganisms to areas that have not yet been processed.
Decomposition is mainly done by two categories of organisms running a relay. Temperature ranges for each group are relatively specific. Once the mesophilic organisms reach their peak, the thermophilic organisms are ready to take the baton. Still, thermophilic organisms can create temperatures so high that it endangers most microbial life.
Can You Use A Hotbin To Compost Lawn Grass Clippings?
The composter must monitor the climbing heat-up process to know what happens and when. Intervention is needed when temperatures in the pile’s center (where most of the activity happens) reach 135 to 140 degrees Fahrenheit. It’s then when composters should come in to break up the party.
Moving what is in the center to the outer regions and bringing cold parts of the pile to the center ensures that microorganisms are equally distributed and organic material is equally decomposed.
After each reshuffling, the mesophilic organisms start the process again, raising temperatures until they’re superseded by thermophilic organisms that take the batch to the next peak. Each turning will lower the temperature peak until the batch is fully composted.
Although composting grass clippings requires additional management, specifically preventing clumping and reducing moisture levels, its role in composting is valuable. Combining grass clipping from your lawn with shredded leaves at a ratio of one part leaves with 1.5 parts grass clippings is a good starter for a vibrant compost pile.
Composting (or microorganisms farming) is the foundation of having a healthy garden. It provides disease and pest control, improves soil texture and water management, and provides ample food for your plant. If you found this article interesting, watch out for our Composting Master class book.
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