How to Keep Weeds out of Mulch

You bought a ton of mulch, lovingly placing it around your plants, stepping back to admire your hard work when you finished. The next Saturday morning, you look and can’t find the petunias. Dang weeds.

Weeds are sneaky, and they love mulch. Why wouldn’t they? Mulch holds moisture and provides nutrients to the plants. If you want to control weeds, you need to be on a never-ending hunt to pluck and pull every odd green speck you see in your garden. If you don’t, a day or two later, these green aliens will be everywhere, making you ask yourself why bothered to mulch.

Don’t Buy Cheap Mulch In a study from Washington State University, lead researcher Linda Chalker-Scott, Ph.D., said: “Mulches lacking pedigrees can be carriers of weed seeds and other undesirable plant parts. While controlled research on this problem is lacking, anecdotal evidence suggests that improperly treated crop residues and composts, as well as bark mulches, are often carriers of weed seed.”

So, to start, make sure you buy the good stuff: Bags from a top brand or in bulk from a reputable dealer. Get small-particle, thick mulch. A study from Murray State University looked at three types of mulches: wheat straw, shredded miscanthus (perennial stalk-like grass), and dry miscanthus. The researchers looked at the effects of “different organic mulches on weed presence, soil characteristics, and growth” on zinnias. The winner for weed control? The shredded grass. Why? Because it did the best job keeping the sun from getting to the soil.

Put Down Enough Mulch

Dr. Chalker-Scott’s study also determined that it could take up to a 4-inch layer to effectively control weeds if you’re using coarse mulch. The type of mulch matters in choosing your depth.

Don’t skimp on mulch. You need a thick layer to stop the sun from reaching the ground. Without the sun, the weeds won’t sprout. With quality small-particle mulch, a minimum of 2 inches deep should do the trick. But don’t go overboard. The plants need to be able to breathe. Three inches is a good rule-of-thumb maximum. Now for the proverbial “except” clause:

These depth recommendations work, except when you have the wrong size mulch. The depth of your mulch—and its power to control weeds—is highly dependent on the mulch itself. If you’ve got mulch with large wood-like pieces in it, chances are good that the sun will find a way to the soil. The mulch you want is made of tiny pieces that clump together well, preventing the sun from getting through. You need to buy the good stuff and layer it to the right depth or hire a landscaper to do the job.

It’s important to say that we might be pushing our luck with the phrase “weed-proof,” even though it is what we all want. Look for organic mulch with pieces that are less than a half-inch in size, which is small particles—you want mulch to block the sun like the shredded grass in the Murray State study. Pieces that are wood chunks around 1 ½ inches in length won’t work well.

But don’t just put your fresh lawn clippings down as mulch—there’s not enough room here to explain how many ways that can backfire. Just don’t.

Bottom-Layer Considerations

You can mulch right on top of your soil, which is what most people do. But recommendations go back and forth on whether putting newspaper or cardboard down and wetting it before you mulch is a good weed-control idea or not. This is an environmentally friendly solution that is frequently recommended. There are also mountains of ads that suggest landscape fabric (I vote no—I found it a significant nuisance when adding plants later). Landscape fabric is also a lot more expensive than newspaper.

Wet newspaper isn’t perfect, however, and complaints about it giving insects and small rodents places to nest are valid. Dr. Chalker-Scott says these materials can be effective for annual beds if properly maintained. She says this works with soils that are regularly worked and irrigated (watered!). However, if your mulch is thick enough, your weed maintenance should be minimal (but do pay attention to the little green aliens; remember the petunias).

A final word of caution: If you’re thinking about using herbicide on the weeds that sprout—a squirt here, a squirt there—remember that many of these products will kill your plants, too. And the sprays do leach out, ending up where you don’t want it. Always be careful with these products. They can kill bushes and trees, too, if you apply them often and heavy enough or oversaturate the area around the base of a tree. Herbicides are labor savers—and they work—but they’re not without drawbacks. Most of my herbicide use is limited to the gravel driveway.