Home & Garden

Why My New Favorite Weapon Against Weeds Is a Cardboard Box

Skip the pulling and especially the spraying, and join me in sheet mulching

A cardboard box under a hydrangea

Photo: Elizabeth Florio

One should never judge a thing by its packaging—unless one happens to be a gardener. Because it turns out the cardboard that passes through American consumers’ households in abundance is a low-fuss, good-for-the-soil, sustainable-many-times-over weed barrier.

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“Sheet mulching” is not a new practice, and professional landscapers like Brandy Hall, owner of Atlanta’s Shades of Green Permaculture, have used it for years to remediate overgrown and undernourished green spaces. But for the average home gardener looking to maintain reasonably tidy flower beds, it’s a revelation. 

In its simplest form, you lay a flattened cardboard box over weed-strewn earth and throw some mulch on top, thus smothering the undergrowth, enriching the soil with carbon, and saving yourself a trip to the curb on trash day. Of course, it can get more involved than that. Many sheet-mulching adherents top the cardboard with compost or planting mix, maybe even a cover crop (Hall recommends daikon radish, winter pea, or millet), and in this way reinvent entire landscapes without a drop of herbicide. 

But if you’re working on a small scale—fighting scattershot weeds within an existing bed of trees and shrubs, for example—all you really need is your mulch of choice and some cardboard. “It’s going to break down,” Hall says, “and you’re just adding organic materials, holding moisture in the soil, creating a nice, fungal-rich environment, and weed suppressing.”

After a year of using this method, I can vouch for its ease and effectiveness. Here are a few things I’ve learned along the way, plus some expert advice from Hall.

Plain brown cardboard is best.

“Many inks these days are soy-based, so they’re biodegradable, but we still use plain corrugated cardboard with minimal ink, like Amazon boxes,” Hall says. Steer clear of glossy boxes, which can contain plastic film, or wax-covered boxes such as those used for produce. Most pizza boxes, though, are fine for the job.

photo: Elizabeth Florio
A pizza box on its way to becoming food for the worms.

You should probably remove the packing tape. 

Skip this task if you must; bits of tape will eventually work their way to the surface of your garden, where you can pick them out. But for a set-it-and-forget-it scenario, I prefer to spend a few minutes de-taping. “It’s a great little party activity,” Hall says with a laugh, “like the modern version of sitting around shucking corn.” 

You may or may not need to prep the ground.

To transform a hardy spread of Bermuda grass into a plant bed, for instance, Hall advises scalping the grass and roots, then making a forty-five-degree-angle cut into the dirt to define your edge and keep grass from creeping under the cardboard. But if you’re simply blotting out a patch of low-lying weeds—like my current garden scourge, Japanese stiltgrass—no such labor is necessary.

Sheet mulching is impermanent and thus inherently forgiving, so you can play with it to see what works. Earlier this spring I mowed an expanse of monkey grass, then threw some cardboard and pine straw on top. The monkey grass initially staged a comeback, forming comical mounds under the straw and making me laugh at my folly. But as the weeks went by, the cardboard prevailed; I recently peeled back a piece to discover the pale, shriveled remnants of a plant that would have taken me hours to dig up.

For maximum hold, layer your boxes and hose them down.

“When you break down the box, it’s going to have cracks where the upper panels fold, which will find any little bit of sunlight,” Hall says. Try shingling or dovetailing the boxes over one another to keep weeds from poking through. It’s also helpful to give the cardboard a quick soak—or pick a time when rain is in the forecast—so it conforms to the ground and stays in place. 

Speaking of which: You probably don’t want to sheet mulch on a steep slope unless you’re willing to introduce an accidental slip-and-slide to your yard.

Choose your topper. 

Planning to sow something in your new bed right away? You’ll need a layer of compost and/or planting soil. If not, it’s okay to spread your mulch—be it wood chips, pine straw, or another organic material—directly on the cardboard, Hall says. 

The wildlife will be okay. 

If, like me, you’re predisposed to irrational environmental guilt such that composting an otherwise wasteful packaging material prompts the fear of separating birds from their breakfast, rest easy. “It breaks down so quickly and there are so many nooks and crannies that you’re not really preventing any soil life from getting in or out,” Hall says. “In my experience you can pull back the cardboard and it’s loaded with earthworms.”

Click here for more sheet mulching tips and a video demonstration.