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Do You Need To Kill Grass Before Laying Mulch? (Complete Guide)

do you need to kill grass before laying mulch down

Mulching in one form or another has been common for centuries. After all, mulch is simply dead plant matter. Gardeners have long understood the benefits of the nutrients produced by this plant matter, as well as its ability to retain moisture for growing plants.

The most typical modern use of mulch is as a way to create a beautiful space while curtailing invasive weeds and lessening maintenance time. But can you simply dump mulch on existing grass and weeds and walk away, expecting it to smother whatever plantlife is living in the soil?

After several decades of organic gardening experience in various parts of the country, I can say fairly definitively, no. In fact, many of today’s yard grasses are able to survive adverse conditions for some time and then bounce back quickly. In some instances, mulch will simply help to hold in moisture, making grass grow more quickly and more thickly than before.

Does that mean you must use an herbicide before mulching? Not necessarily. There are actually several safer alternatives, depending on your end goal.

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Preparing for Mulch

laying mulch over grass

The first question you need to answer is what your long-term goal is for the area you plan to mulch. Are there existing plants that will be surrounded by the mulch? Will you be adding plants, such as annual or perennial flowers, in the mulch itself? Is the mulch part of an overall zero-scaped lawn, with rocks or gravel the only other ground cover?

It’s a good idea to cut a trench or place a barrier of some kind between mulched areas and grass areas to keep grass roots from passing into mulched areas. Edging can also keep piled mulch from being washed onto or away from the edges of sidewalks and driveways.

Killing Grass With Herbicides

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You have the option of using a general herbicide if you are not mulching around other plants or planning to add any greenery in the near future. As an organic gardener, I’ve never used it if I could help it. And if you’ve been paying attention to some TV commercials, you know some herbicides can be downright dangerous.

If you do decide to use a general herbicide, wear protective clothing and avoid breathing it in. It may take a bit of time for the spray to take full effect. Read the label carefully for proper use and wait times.

Landscape Cloth or Plastic

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This list of ground prep options is listed from worst to best. Next to using an herbicide, putting down landscape cloth may be the worst thing you could do for your soil. The material cuts moisture and nutrients off from the soil, causing it to slowly die. And water from rain or a sprinkler system may actually run off the cloth or plastic, causing potential erosion issues, especially around the edges.

If you’re zero-scaping or laying stepping stones, landscape fabric or plastic may work great. But if there’s any plant life in your plan, you may want to go with another option. And you’ll want to think about ways to keep your mulch from being washed away if you have heavy rains in your area.

Cardboard

Cardboard is easy to obtain, many times for free from local retailers. It breaks down fairly quickly and can be cut through to add plants. Large pieces can be laid down like overlapping shingles to completely block grass and weeds from receiving needed sunlight.

Here are a few negative notes to consider about cardboard. Most modern cardboard boxes are corrugated, with a certain amount of chemical adhesives used in the manufacturing process. This cut down a bit on its organic qualities.

Also, you’ll want to watch out for cardboard with wax or other less permeable coatings on it. In spite of these negatives, cardboard is probably the easiest and cheapest mulch liner out there when it’s done right.

Newspaper

Of all the mulch-liner options, newspaper has been my personal favorite and is touted by other web-advisors as being largely organic and soil friendly. The biggest problem is finding enough of it. Your local library may have a recycling bin, or you may have a neighbor or two who still receive an actual newspaper. With most information available online these days, there’s just not as many newspapers around as there used to be.

As a mulch liner, newspaper will help keep moisture in while keeping grass and weeds under control. But it may be a challenge for larger areas. It’s best to put it down a little at a time and then get the mulch on it quickly, before it can blow away.

Wait for It

It’s important to give the mulch layers time to smother underlying grass and weeds before you begin to open holes in it for other plants. Probably a minimum of 6-8 weeks should be allowed before disturbing new mulch.

Cardboard can be easily punctured after about 6 months if it’s been regularly dampened, though you don’t have to wait that long to plant. You might consider building the soil up a bit when planting in cardboard-lined mulch, just to allow a bit more air and light.

The Truth About Weeds

dandelion weeds in green grass

Mulch can be lovely to look at and easy to maintain, but don’t expect it or any other ground cover to completely eliminate unwanted plant growth. Over time, weeds with noxious root systems will work their way up through just about anything. It’s just part of their survival mechanism. You’ll want to dig out and possibly spray as much of these roots as possible before beginning your mulching process.

As your mulch liner and the mulch itself deteriorates, there are more opportunities for grass and weed seeds to find their way up through the cover and begin to grow. It will always be necessary to check mulched areas regularly and pull any unwanted plants as they appear.

After a couple of years of faithful maintenance, you’ll find there are fewer weeds and residual grass growth to contend with from the original ground cover. Remember this is nature you’re trying to tame. The fight will never be completely over.

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