How Long Does It Take for Grass Clippings to Decompose?

Bagging up grass clippings just to put in the trash is valuable time in your weekend lost. So you think… hey. Grass clippings are plant matter. Couldn’t they just decompose on the lawn? It might even be good for it.

And it is. There are very few risks and your lawn will get a nice extra dose of nitrogen and organic matter. But just how long will it take to decompose? Are you going to build up thatch? Is your lawn going to disappear under layers of cut grass?

Left on the lawn, grass clippings only take a few weeks to decompose (and may disappear from sight before then), but the time will depend on the type of grass, the size of the cutting, and the weather. To speed up the process, cut the grass clippings into smaller pieces with a mulching blade on your mower, or mow more frequently to take off less of the grass blade.

What Can You Do with Grass Clippings?

Grass clippings are an amazing (and free!) source of nitrogen, carbon, and organic matter. In fact, grass clippings can return up to 25% of nutrients back into the soil!

Instead of sending grass clippings to the garbage dump, you can:

  • Leave grass clippings on lawn (AKA grasscycle)
  • Add them to your compost pile to add nitrogen (when fresh) or carbon (when dried)
  • Put them in your municipal compost bin
  • Use grass clippings as mulch to prevent weeds and keep the soil moist between waterings
  • Feed either fresh or dry grass to any livestock you may have (even chickens!)
  • Add as bedding (dry) or food (fresh) to vermicompost bins
  • Give grass clippings away to community gardens or on Craigslist, Kijiji, etc. There’s always someone who can make use of them!

Why You Should Leave Grass Clippings on Your Lawn

Leaving grass clippings on your lawn isn’t lazy (well, there’s more benefits than that), but actually improves your lawn as grass clippings:

  • Return nitrogen to your soil. Green grass clippings are an excellent (and free) source of nitrogen. The microbiology will break down the grass into nutrients that your grass can then absorb. More nitrogen means lusher grass without having to use synthetic fertilisers.
  • Add organic matter to the soil. Organic matter provides a lot of boons for your soil, from increasing the absorption of moisture to keep your lawn watered longer (especially important in drought and hot weather), aerating your soil, and feeding the beneficial microbiology, insects and worms that break down organic matter into nutrients that grass can absorb. Ideally, your lawn soil will have 3% organic matter. (Garden beds and vegetables need around 5%.)
  • Provide shade for the soil and weed-preventing mulch as grass clippings decompose. Your grass will still get plenty of sun, but shaded soil will stay moist for longer (meaning less watering) and will help prevent weeds from popping up (as weeds need sunlight to germinate).
  • Save you time and labour by not having to bag up grass clippings. You also save money on mowers since you won’t need a bagging attachment.

Watering after mowing or mowing before a rainstorm will push grass cuttings down towards the soil, hiding grass clippings away sooner.

Are There Any Risks Decomposing Grass on Your Lawn?

While decomposing grass clippings are very beneficial to your garden, there are a few risks that you should be aware of so you can easily mitigate them.

Grass clumps (either from mowing wet grass or cutting more than 1” at a time) can block out the sunlight enough to damage or kill or the grass underneath. Avoid mowing wet grass whenever you can as mowing wet grass can damage the grass and mower, and results in a bad-looking lawn. Also, try to keep on top of mowing so you only need to take off an inch or less.

It’s hard to avoid mowing when it’s been raining every day for weeks, which encourages the grass to grow fast. If you do need to mow, then break up the grass clumps to a thin covering that’s easily pushed down by further rain.

If grass clippings are too long and bulky, then bag up the excess for your compost pile.

Remove grass clippings if your lawn is suffering from diseases like leaf, rust or dollar spot. By removing the grass clippings, you’ll help reduce the severity of the disease severity, or at least not spread it further. This is one time when it’s a good idea to put grass in the trash rather than compost it!

Don’t leave grass clippings in gutters and by drains. You don’t want them being swept into storm drains or into local water sources as it won’t be good for the water. Keep grass clippings on the lawn, not in the water.

If your mower isn’t safe to operate without the bagging attachment, leave it in place! Safety first! You can then either spread the grass clippings onto the lawn with a rake or dump them into your compost bin.

There is one thing that people think is a risk, but actually isn’t:

Grass clippings DON’T cause thatch. Thatch is when there’s a layer of brown, spongy matting at the base of grass that cuts off the soil’s access to water and air. Grass clippings decompose too quickly to contribute to that. Thatch is mainly caused by slow decomposing plant matter like roots, rhizomes, and crowns.

If the thought of leaving your grass clippings on the lawn makes you really uncomfortable, then start a compost pile in the corner of your yard. Let the grass clippings and leaves compost over the season, and once finished, apply a ¼” layer of compost over your lawn. More work on your end, but you still get the nutritional benefits.

It only takes a few weeks for decomposing grass to return nutrients to your lawn. So unless you need to mow in the rain, take off too much of the grass blade, or have disease problems, leave your grass clippings where they fall and your lawn will love you for it.


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