Will Grass Clippings Kill Weeds?

Effective weed control is one of the key components of a healthy lawn.

Dense grass growth will help prevent unwanted weeds, but there are times when you may need to use other methods of weed control.

To avoid using potentially harmful chemicals, many homeowners use organic materials instead of commercial herbicides to stop the growth of weeds.

One of the most readily available organic materials is grass clippings from your own lawn.

But will grass clippings kill weeds?

Key Takeaway:

Grass clippings will not kill all lawn weeds, but they may be used as mulch to smother small annual weeds and prevent weed seeds from sprouting. Mulched grass clippings also help retain soil moisture, insulate plant roots, and release nutrients as they decompose.

To suppress invasive weed growth on your lawn, apply no more than 1-2″ inches of grass clippings.

Mulch deeper than this on your lawn will lead to excess thatch build-up and prevent moisture from reaching the soil.

For garden beds, a thicker layer of mulched grass clippings about 3-5″ inches deep may be used depending on the plants growing there.

Keep reading to learn how to use grass clippings to prevent invasive weeds and fertilize your lawn.

will grass clippings kill weeds

Ways To Use Grass Clippings

You create grass clippings every time you mow your lawn.

Before you discard this free organic material, put it to good use as a lawn fertilizer or garden mulch.

You may also add grass clippings to your backyard compost pile to increase the nitrogen content.

Leave Grass Clippings on the Lawn

Leaving the grass clippings on the lawn after mowing is one of the easiest ways to fertilize your lawn and create a weed barrier.

As the grass clippings decompose, they release essential nutrients such as nitrogen, phosphorus, and potassium.

To avoid harmful thatch build-up, do not leave grass clippings over 1″ inch long on your lawn.

It is best to cut no more than 1/3 the length of your grass each time you mow.

Mow your lawn frequently and use a mulching mower to keep grass clippings short.

A thatch layer is also more likely to form when the grass is wet, so always mow your lawn when it is dry.

Annual aeration of your lawn is not only beneficial to the soil but also prevents excess thatch build-up.

Use Grass Clippings for Garden Mulch

Another way to get the most out of your lawn clippings is to use them as mulch for your vegetable garden or flower bed.

Grass clippings make an excellent garden mulch to protect plants from extreme hot or cold temperatures and keep annual weed seeds from sprouting.

A layer of grass clippings prevents dry soil by reducing moisture evaporation and provides a source of nutrients for the healthy growth of garden plants.

Beneficial microorganisms also speed up mulch decomposition by feeding on the released nutrients.

Add Grass Clippings to the Compost Pile

Grass clippings are an excellent nitrogen source, so adding them to your backyard compost pile makes sense.

Take care not to add too many grass clippings to your compost heap, as a thick layer of grass will become moist and develop a foul odor.

Keep your homemade compost heap manageable with an even distribution of grass clippings and dry leaves or straw.

How To Use Grass Clippings to Prevent Weeds

Chemical herbicides may damage flowering plants and are unsafe to use in vegetable gardens.

Fresh grass clippings are a great organic choice as an effective weed barrier, and they also add essential nutrients to the soil to make your garden healthier.

Grass clippings may release up to 25% of their nutrients into the soil, creating an inexpensive and chemical-free garden fertilizer.

With proper mulching techniques, grass clippings will also prevent soil erosion.

Further Reading: 11 ways to get rid of weeds forever

Let the Grass Clippings Dry

After mowing your lawn, dry clippings for at least one day before raking and bagging them.

When grass clippings are wet, they produce heat as they dry.

This extra heat from wet grass clippings may cause damage to your garden plants by burning the leaves, stems, or roots.

Remove Existing Weeds

Before applying the mulched grass clippings to your lawn or garden, remove any existing weeds.

Removing deep-rooted, perennial weeds is crucial to ensure they do not grow back.

Lawn clippings will not kill existing broadleaf weeds, and they may be able to break through no matter how thick the mulch layer is.

The mulch will prevent the growth of weeds by depriving dormant weed seeds of the sunlight and oxygen they need to sprout.

Apply Mulch in Thin Layers

Only apply the mulch in 1/4″-inch layers at a time, gradually building up to 3-5″ inches in garden beds.

Keep the layers of mulch only 1-2″ inches deep on lawns to prevent harmful thatch from developing and ensure water reaches the soil.

This process may be repeated up to 2-3 times per month until the desired mulch thickness is achieved.

These thin layers allow the grass clippings to decompose properly before they become moldy.

Fresh clippings are likely to become moist if applied in thick layers.

When these clippings decompose without oxygen, they will create a foul odor.

It takes around 2-3 weeks for grass clippings to fully decompose on a lawn under normal weather conditions.

Do not layer the mulch on top of desirable garden plants, as a lack of oxygen or sunlight may damage them.

In the late fall or early spring, aerate your garden bed and mix the grass clippings into the soil up to 8″ inches deep to add nitrogen.

When To Avoid Using Grass Clippings as Mulch

There are a few instances where it is not advised to use grass clippings as a mulch for your lawn or garden bed.

You may remove the clippings manually with a rake or use a bagging attachment on your lawn mower.

When a lawn has any of the following issues, it is recommended to bag the clippings.

Fungus or Lawn Disease

If your lawn has a fungus or other disease, it is best to bag your clippings to avoid contaminating your entire lawn or garden.

Common signs of lawn disease include:

  • Irregular brown or yellow patches
  • Dark spots on grass blades
  • Visible fungal spores/mushrooms
  • Powdery mildew

Do not mulch your grass clippings until your diseased yard has been treated and is healthy again.

Long Grass Clippings

Grass clippings longer than 1″ inch will also need to be bagged because they will not decompose properly when mulched.

Avoid long clippings by frequently mowing or using a mulching mower.

Chemically Treated Lawn

If you have applied chemical herbicides to your lawn, using your grass clippings as mulch is not advisable.

Herbicides used for broadleaf weeds will also kill garden plants.

Residual chemicals on the lawn clippings will easily transfer to your flower or vegetable garden.

Wait 60 days before using the clippings as mulch after treating your lawn with broadleaf herbicides.

Weedy Lawn

When you mow a weedy lawn, there is a chance for weed seeds in the clippings.

Using these clippings as mulch will disperse these weed seeds over your lawn or garden.

You will likely have more weeds, creating an even bigger problem.

Mowing a weedy lawn more frequently will help prevent weeds from developing seed heads.

Before using the grass clippings as mulch, it is best to wait until your lawn is free of weeds.

Further Reading: Bagging grass clippings and avoiding spreading weeds