How Long Do Grass Clippings Take To Decompose?

Grass clippings are a natural byproduct of mowing the lawn.

Many homeowners are unsure about what to do with their lawn clippings, so they bag them up and throw them away.

However, grass clippings create a nitrogen-rich fertilizer at no extra cost to you, so it is more effective to spread them on your lawn.

But how long do grass clippings take to decompose?

Grass clippings will fully decompose within 3-4 weeks if spread in a thin layer on your lawn. When added to compost, grass clippings will decompose between 1-3 months. If you are spreading grass clipping mulch in thick 3-4″ inch layers, it will take 2-3 months to complete decomposition.

When using grass clippings as lawn fertilizer, it is best to spread them in a thin, even layer to avoid thatching.

Read on to learn more about grass clipping decomposition, including how to properly spread them onto your lawn, when to throw away the grass clippings, and tips on how to speed up the decomposition process.

How Quickly Do Grass Clippings Decompose?

The speed at which grass clippings will decompose depends on the application.

When spread in a thin layer over your lawn, it usually takes 3-4 weeks for the grass clippings to decompose completely.

After 1-2 weeks, the grass clippings will fall to the soil level and begin to break down. 

At this stage, the clippings are no longer visible on the surface of your lawn.

When grass clippings are added to a compost, it will take around 1-3 months for them to decompose fully.

If your lawnmower has a mulching feature, you will be able to create a nitrogen-rich mulch by simply mowing your lawn.

Check out more ways to add nitrogen to your lawn.

When a grass clipping mulch is applied in a 3-4″ inch layer, it may take 2-3 months to decompose.

Is It Safe To Leave Grass Clippings On Your Lawn?

Leaving grass clippings on your lawn is an easy and inexpensive way to fertilize your existing grass.

Grass clippings are full of essential nutrients such as nitrogen, phosphorus, and potassium.

These nutrients are released back into the soil once the grass clippings begin to decompose.

The nutrient breakdown of grass clippings is 4% nitrogen, 1% phosphorus, and 2% potassium.

You may also want to check out other ways to add potassium to your yard.

Your lawn soil should contain around 3% organic matter, while flower and vegetable gardens need 5%. 

Grass clippings are a safe and efficient way to add these nutrients to your lawn and garden.

If these grass clippings are bagged and thrown away, you will have to use a separate fertilizer on your lawn to maintain healthy grass growth.

Using a different fertilizer is expensive and time-consuming, so leaving the grass clippings on your lawn is much easier.

This is known as “grasscycling.” 

It is done by simply allowing the grass clippings to fall on the lawn when you mow.

If you are concerned about even application and thatching, you may bag the grass clippings and spread them on your lawn by hand.

Leaving grass clippings on your lawn also helps to retain soil moisture and discourages weed growth.

Read our post on if you should bag grass clippings for more information.

When Should You Get Rid of Grass Clippings?

If your lawn has any disease which causes brown spots, rust, or fungus, you will need to throw the grass clippings away.

When diseased grass clippings are left on your lawn, they will spread the disease to your healthy grass and cause it to die.

Likewise, these diseases may spread throughout compost, affecting anything fertilized by it.

Throwing grass clippings away or burning them will prevent the disease from spreading and make it easier to eradicate using herbicide or fungicide treatments.

How Do You Make Grass Clippings Decompose Faster?

To encourage faster grass clipping decomposition, be sure to spread them in a thin, even layer, and remove any thatching from your lawn.

Thatching occurs when wet grass clumps become matted together.

Thatch consists of grass roots and stems, and it forms a barrier between the grass and the soil. 

This thatch layer will prevent the grass clippings from making it to the soil layer and cause them to build up rather than decompose.

It only takes a half-inch thick layer of thatch to discourage grass growth.

Plan on regular thatch removal to maintain a healthy lawn.

Warmth and moisture will also cause the grass clippings to decompose faster.

Grass clippings contain between 80-85% water, and they decompose faster than other parts of the plant.

Regularly watering your lawn during the growing season will encourage grass clipping decomposition and provide the necessary nutrients to keep your grass healthy.

Aim to water your lawn twice per week during the growing season to help decompose the grass clippings and maintain a healthy property.

You will need to water your lawn more often during hot weather or drought.

To speed up the decomposition process in compost, turn the compost heap with a pitchfork at least once per week.

Use a thermometer to check the temperature of your compost pile. If the temperature falls below 110° degrees Fahrenheit (43° C), the pile will need to be turned.

Any time the compost is allowed to cool off or dry out, it will slow down the decomposition process.

Cover your compost pile with a tarp whenever it rains to keep excess moisture out.

Can Grass Clippings Be Used in Compost?

Grass clippings provide an excellent source of nitrogen and organic matter for your compost.

As a general rule, your compost should have a 30:1 carbon to nitrogen ratio.

This ratio is calculated by weight, so this would mean you will add one pound of grass clippings for every thirty pounds of dry leaves or other brown material such as branches or newspapers.

Adding too many grass clippings to your compost will cause it to release toxic ammonia gas because of the nitrogen content of the grass.

To prevent the ammonia gas from building up, add a small number of grass clippings to your compost at one time, and add more leaves as necessary.

If your compost has an odor of ammonia grass, it is best to balance it out before using it as a fertilizer.

Too much ammonia will have a detrimental effect on your plants, causing them to become yellow or die out.

It is advisable only to add dry grass to the compost pile. 

This avoids adding too much green material to the compost and disrupting the decomposition process.

What If the Grass Has Been Treated with an Herbicide?

If you have previously treated your grass with herbicides or fungicides, it is best to wait at least one week before adding the clippings to your compost.

This waiting period allows the chemicals used on the grass to break down.

As a general rule, leave the grass clippings on the lawn for at least 2-3 mowing sessions before adding them to your compost pile.

Herbicides on grass clippings will break down more quickly in compost than they will in the soil.

What To Avoid Adding to Compost

Never add human, dog, or cat feces to your compost, as there is a risk of spreading harmful bacteria and diseases.

It is also wise to avoid adding food scraps such as meat, bones, grease, eggs, or dairy products. 

Food scraps are more likely to attract insects and rodents.

Do not add any diseased grass clippings to your compost.

If temperatures in your compost pile do not reach at least 140° degrees Fahrenheit (60° C), the disease on the grass clippings will not be destroyed.

Using compost containing diseased grass clippings runs the risk of spreading the disease to any areas you fertilize.

Are Grass Clippings Safe To Use In A Garden?

Adding grass clippings in a 3-4″ inch layer in your garden will not only provide vital nutrients for your garden soil but will also protect your plants from extreme weather conditions.

A layer of grass clippings will also prevent weeds and help the soil retain moisture.

The thick layer of grass clippings will take around 2-3 months to fully decompose and release all of their nutrients into the soil.

You will need to add a new layer of grass clipping mulch approximately 3-4 times per year to maintain healthy growth in your flower or vegetable garden.

Commonly Asked Questions

Will grass clippings compost on their own?

Grass clippings should not be composted on their own due to their high nitrogen content.

When compost with grass clippings, it needs to be balanced with a mixture of carbon-releasing materials, such as dried leaves.

Too many grass clippings in a compost mixture will cause toxic fumes from the release of ammonia gas.

In a balanced compost mixture, grass clippings will add much-needed nitrogen and other nutrients when mixed with yard waste such as branches or dry leaves.

Make sure the grass clippings are completely dry before adding them to your compost, so they decompose faster.

Fresh grass clippings may contain too much moisture and will slow down the decomposition process.

Composting with grass is an excellent way to eliminate an excess of grass clippings leftover from mowing your lawn.

Do grass clippings help bare spots?

After planting grass seed, adding a layer of grass clippings to any bare spots on your lawn will encourage new grass growth.

The grass clippings will add a layer of protection from harsh weather elements and keep moisture from evaporating.

How often should you cut the grass?

It is best to establish a weekly mowing routine during the growing season.

If your lawn gets a lot of rain, you may have to mow more frequently to keep up with the grass growth.

You may also need to mow your lawn more frequently at the peak of the growing season, which typically occurs during the spring and early summer.

Certain types of grasses grow faster than others, so plan accordingly.

Always follow the one-third rule by only cutting one-third of the length of the grass blade. 

This prevents cutting the lawn too short and causing the grass to die out.

Always set your mower blades to the proper height for the type of grass you have.

The recommended grass height is between 2.5-3.5″ inches for cool-season grasses. Warm-season grasses are usually kept shorter at 1.5-2.5″ inches.

Check out our complete guide with tables in our article on how often to cut grass.