Sheet Mulching: How to Smother Weeds, Build Soil & Conserve Water the Easy Way
Sheet mulching is a fancy phrase for building a massive compost pile across the surface of a field or garden. Rather than pile up your manure, leaves, crop residue, and other organic materials in a squat pile, you spread it all out where the compost is needed, skipping the effort to build the pile, turn it, water it, and otherwise coerce it to break down into rich, brown earth—a process that goes on for months.
But sheet mulching does even more than that.
Beneath the layers of compost materials, sheet mulches typically include a layer of cardboard to keep grass and weeds from growing through—a great way to smother unwanted vegetation or convert a sod lawn into a garden. Before long, both weeds and cardboard decompose and feed the soil with organic matter, while you pat yourself on the back for finding such a clever way to recycle and relieve yourself of the constant chore of weeding.
Sheet mulching also traps moisture in the soil. Every farmer and gardener knows that mulching is a must to cut down on irrigation, but the cardboard used in sheet mulching is much more effective at trapping moisture than typical wood chips or straw. When I used the sheet mulching technique in the parched landscape of California, I found the moisture in the soil lasted at least five to 10 times longer than a thick layer of mulch on its own.
Another reason to try sheet mulching: earthworms love cardboard. They literally eat it and excrete “black gold” from the other end.
The actual process of sheet-mulching is simple:
1. Get the right materials. You’ll need cardboard, mulch and/or organic matter, and manure (but that’s optional). At a minimum, you just need enough cardboard to cover the earth and enough mulch to cover the cardboard. From the there the sky is the limit: You can pile up as many layers of manure and organic matter as you want. Woodchips, straw, leaves, crop wastes, and animal bedding are all examples of organic matter—basically anything you would put in a compost pile, other than kitchen scraps, which you probably don’t want strewn about your yard.
Recycling centers are a great source of cardboard. Unless you are sheet mulching a tiny area, try to grab the biggest sheets of cardboard you can find, such as boxes from bicycles and kitchen appliances, to make things easier. Avoid cardboard with glossy color print, as the ink may not be biodegradable. Some people use a thick layer of newspaper instead of cardboard, which is efficient only for sheet mulching small spaces.
2. Lay down the cardboard. Unless the soil where you’re planning to sheet mulch is already very moist, set up a sprinkler and soak it before laying down the cardboard. Remove all the tape from the cardboard (it’s not biodegradable) and lay the cardboard over the soil/weeds working from one end of the space to the other.
It’s critical to overlap the pieces of cardboard by at least six inches to prevent the more tenacious weeds from weaseling through the gaps to the surface, where they will quickly become re-established in the rich soil you’ve made for them. Once the cardboard is in place, wet it down until it becomes heavy and limp so it doesn’t shift around or blow away in the wind as you add layers of mulch and/or compost materials on top.
3. Add organic matter. If you’re just adding mulch, spread what you have in an even layer at least 2 inches thick on top of the cardboard and call it done. If you’re also using manure, spread it in two-inch layers alternated with a one-inch layer of organic material between each one, finishing with a final layer of mulch on top. Another option is to simply cover the cardboard with finished compost and then cover the compost with a protective layer of mulch.
Late spring when the weeds are getting out of control, but haven’t yet spread their seeds, is a great time to smother them with sheet mulch. But it can be done at any time of year. There are just a couple caveats to keep in mind:
If your goal is to plant perennials—whether fruit trees, edible shrubs, vines, or flowers—you can lay down the sheet mulch first, and come back to plant whenever you are ready, brushing aside the mulch and cutting small holes in the cardboard where you’ll dig the hole for each plant. If your perennials are already planted, work around them.
But if you want to plant annual crops, you need to let the sheet mulch decompose in place for a full year before loosening the soil for planting. If you build a really deep layer of sheet mulch—say, 12 inches or so (which will decompose to probably one-fourth that depth after a year)—you can plant your annuals the following year without even tilling the soil.