There was a time in my life when I would dump bags of grass clippings in the garbage each week, not knowing that I was depriving my whole garden of free, abundant, organic garden mulch.
Grass clippings make excellent garden mulch. Dry the clippings first and limit the layer of mulch to less than two inches. Top up with an additional one-inch layer of mulch every 4-6 weeks. Avoid using clippings from grass treated with herbicide, pesticide, or after it has gone to seed.
Before you start spreading the grass clippings straight from your mower into your vegetable and flower beds, let’s take a closer look at how to get the best results from grass clippings as garden mulch.
Do grass clippings make good garden mulch?
Grass clippings make excellent mulch for vegetables, herbs, and flowers, wherever needed in the garden. Mulching your garden with grass clippings helps to maintain the soil temperature and moisture, meaning less weeding and watering that you’ll have to do.
When you mulch with grass clippings, make sure that you keep the layer of mulch to less than two inches to prevent the mulch from matting. When the mulch mats, it blocks moisture and oxygen from getting to the soil.
You also need to keep an area around each plant stem of approximately one inch open and free of mulch so that you can more efficiently water your plant’s root zone and allow oxygen into the base of your plant stem to prevent rot.
Be sure to use the leafy grass blades that are free of seeds, stems, and roots. What I find helpful is allowing about a week of extra growth on my lawn before mowing for mulch. That gives me about an extra inch of grass length which means more mulching material.
If you have leftover grass clippings, you can store the excess in an open pile in an out-of-the-way spot in your yard. This way, you’ll have sufficient mulching material on hand to top up any areas of your garden that need an extra bit of mulch during the weeks until the next time you mow.
One of my favorite gardeners to watch on YouTube, Huw Richards, has a very in-depth video about using grass clippings in the garden, including how to use them to build soil fertility. Check it out and see if you learn something for your garden:
Grass clippings for soil fertility
When you use grass clippings as mulch, they will release a lot of nitrogen, potassium, and phosphorus into the soil, so this is a supplementary organic fertilizer. However, it will take a few months for the grass to break down enough to become available to any plant roots.
If you clear the bed in your vegetable, herb, or flower garden in the fall and intend to leave it fallow through the winter, cover that bed in your garden with a 4-inch thick layer of nutrient-dense grass clippings mulch to help replenish the soil through the winter and keep the weeds away.
Grass clippings for pest control
The grass clippings make a layer across your soil that can help deter the pests that could invade your garden beds.
Slugs tend to stay away from dry grass clippings. Grass clippings that are completely dry irritate and dehydrate their skin. So, if slugs are a problem, make sure the grass clippings are very dry before mulching. Mulching with fresh grass clippings will attract slugs rather than deter them. (Iowa study)
Dry grass mulching can reduce the damage caused by root maggots. Fewer eggs can hatch, and those that do have a way higher larval mortality, according to this Swedish study. It could be that more birds are scratching around in the dried grass and picking out the larvae. I’ll take it!
Refresh grass mulch throughout the season
While other mulching materials like shredded bark and wood chips can last between one and two years without needing to be replaced, grass clipping mulch biodegrades much quicker.
Depending on the season and weather conditions, a standard two-inch layer of grass clipping mulch lasts about six weeks before it becomes noticeably thinner, either from having been mixed into the soil or having blown away.
You’ll notice that if you plant or weed through a layer of mulch, it’ll slowly start to mix into the soil. Visiting birds scratching for bugs will also incorporate some of the grass into the soil, which is actually a good thing. It just means you’ll need to replenish the grass clippings so the mulch is still effective.
I start the season with a 2-inch layer of grass mulch and then add an additional 1-inch layer every few weeks or whenever I mow my grass.
Can I mix grass clippings into the soil?
When you mix grass clippings into the soil, the trouble is that they cease to act as mulch rather than as fertilizer. Since grass clippings are 4% nitrogen, 1% phosphorus, and 2% potassium, using grass clippings as fertilizer equates to 4-1-2 fertilizer. If your goal is fertilizer, then go ahead and mix the grass in. If your goal is to have a mulch layer, leave the grass on top of the garden bed.
What is critical here, though, is that grass clippings require oxygen to decompose. Grass clippings decompose very slowly when mixed with soil because they have less exposure to oxygen. Grass clippings will decompose much faster when lying on top of the ground as mulch because they would have more exposure to oxygen. (study)
Another way you can get your grass clippings to decompose faster is by placing them into the compost bin, where they will have more access to oxygen.
Adding grass clippings to soil can be helpful, but it’s important to do it correctly. Mixing them into the soil can disturb or even harm the root systems of established plants. If you want to add organic material to your garden, do it before planting out the bed.
When you shouldn’t use grass clippings for mulch
Here are a few instances when you should not use grass clippings in your garden.
Don’t use grass with herbicide or pesticide residue
Do not use grass clippings in your garden if your grass has been sprayed with pesticides or herbicides.
This is especially true for the types of herbicides that have been designed for dandelions and other broadleaf weeds. Residue from these herbicides can prevent germination of newly sown seeds and stunt or kill seedlings you plant into your garden.
Don’t use grass that has gone to seed
If your grass has already gone to seed, then by using it as mulch, you are simply transplanting the seeds from your lawn into your garden. Once the seeds germinate and start growing in your garden bed, you’ll have a lot of extra work.
As I mentioned, letting your grass grow longer before cutting gives you more mulching material. However, you should be vigilant, especially during the late spring and summer, to cut your grass before it goes to seed.
Leaves are another excellent option if you have no usable grass clippings. Just like grass, they’re free, readily available, and will add fertility to your soil over time. Here are all the details: Quick tips For Using Leaf Mulch In Your Home Garden.
To learn more about mulching your garden, check out this article about the Best (Free!) Organic Mulches For The Home Garden.