How to Sheet Mulch Your New Garden Bed
Sheet mulching is a fantastic method for establishing a new garden bed or revitalizing a marginal growing space.
Permaculturalists love it because it’s a simple and effective no-till strategy for building up thick layers of fertile soil.
At its most basic, a sheet mulch is simply a layer of cardboard or newspaper – there’s your “sheet” – followed by a thick helping of your mulch of choice. But if you’ve got access to some basic materials and time to let them sit, you can build up a tall stack of carbon and nitrogen and watch it slowly compost in place until you’re left with a solid layer of black gold to plant into.
This method was more commonly known to hip organic gardeners of past generations as lasagna mulching, and I think that name does a better job of describing what the goofy piles actually look like after they’re assembled. The “lasagna” connection also helps to explain why everybody out there seems to have their own recipe for building a proper sheet mulch.
I tend to trust Toby Hemenway’s wisdom on all things related to home-scale permaculture, and he assures us that “Sheet mulching is very forgiving. As long as you have enough newspaper or cardboard, plus organic matter of any kind, you’ll end up with great soil.”
My specific recipe for sheet mulch is a variation on Hemenway’s formula, that differs from his style mainly in that I use chickens to prepare a space before mulching. This step is completely optional, but if you have chickens and the means to move them regularly, I highly recommend it.
Why sheet mulch? The main reasons for using the method are to avoid tilling, to suppress weeds, and to build up soil that’s rich in organic matter. This makes it really useful for establishing permanent beds in a new garden site, especially if you don’t need to plant right away – I generally build sheet mulch beds in the fall and winter so they have time to break down before the growing season arrives.
Sheet mulching eats up a lot of materials, so if this is your first crack at it, I would urge you to start small with no more than a couple hundred square feet at most.
Many people will tell you that you need to build the whole pile all at once. While I’d agree that this is probably preferable, I have built sheet mulches on a shoestring budget and added materials over time, and the results were still great. Just don’t lay down any cardboard or newspaper that you aren’t prepared to immediately cover up!
This is my basic ingredients list for one bed that’s a standard 30(ish) inches wide and about 50 feet long:
- cardboard or newspaper: as much as possible
A newspaper layer needs to be quite thick, and since you’ll need a ton of it, it’s really better suited for smaller areas. Cardboard is preferable, and I bet you can score as much as you’d ever want for free from your local recycling center if you ask nicely. Either way, steer clear of anything glossy, and be sure to completely remove any stickers or tape you find.
- two-string straw bales: 3-5
I like straw for its simplicity and consistency, but you might consider substituting or combining other materials that are more readily available. Whatever you choose – maybe old grass clippings, or shredded leaves, for example – be sure that it is high in carbon and fairly low in nitrogen.
- compost: 2-4 cubic yards
Try to get compost from a few different sources if you can. Biodiversity is always valuable. Just avoid the bagged stuff from big-box stores at all costs: these days it’s mostly toxic sewage sludge. I wish I was joking.
- manure and/or topsoil: 1/2 – 1 cubic yard each (optional)
I don’t think these two ingredients are necessary, but they’re worth considering if you have access. A thin layer of manure at the base of your pile will go a long way towards stimulating microbial activity; a finishing layer of soil on top of your pile will give you something other than compost to plant into, which may be beneficial if you’re planning to plant before the pile has had many months to break down.
- your choice of mulch: 1-2 cubic yards (optional)
This will be your final layer on top, the mulch of the mulch. Ideal materials include wood chips, shredded leaves, or perhaps leftover, fluffed-up straw.
How to Sheet Mulch:
1. Prepare the site to be mulched.
Cut down any herbaceous growth where the bed will be, but don’t worry about removing anything. This layer will rot in place underneath your weed barrier with time, and eventually serve as another source of nutrients for the crops you’ll plant on top.
Rather than chopping or mowing anything down, I move our chicken tractor to a space I wish to clear and let them tear it apart for a week or two before proceeding. Their disturbance stimulates soil microbe activity and fertilizes the space without any work required on my part. Sweet!
If you’ve had a soil test done and know what kinds of specific amendments your soil might need – lime to adjust the pH for example – go ahead and add those now. I usually skip this step, but I’m sure it would be beneficial in most instances.
Take a broadfork or garden fork and work the tines down into the existing soil as deep as you can go, then gently rock it back and forth. Do this every few inches across the space you’ll be mulching to aerate and allow moisture to penetrate a little deeper – no need to turn the soil, just open it up a bit. This step is also optional but highly recommended, especially if your site is hardpan clay like mine.
Finally, water the heck out of the whole space. The soil needs ample water for microbes to do their thing, and once you’re done sheet mulching, moisture will have a really tough time penetrating all the way down to the ground.
2. Lay down the weed barrier.
Now that the site is ready, it’s time to lay down that cardboard and/or newspaper you’ve been hoarding away like a packrat until this moment. The goal here is to keep weeds out of your new growing space, so be sure to overlap your pieces by several inches to prevent the most pernicious of ‘em from working their way up through.
You will definitely want to keep a hose handy throughout this step to wet down the pieces as you go.
3. Add a layer of manure and/or compost.
The first layer to go on top of your weed barrier should be something fairly high in nitrogen – preferably manure (bedding), compost, or else freshly cut green grass clippings. You’ll want it to be about an inch thick at most.
4. Pile on a thick stack of carbon-rich material.
Your next layer will consist of materials that are high in carbon but not woody. This will form the bulk of your sheet mulch, and you’ll want to have enough to build it 6-12” tall.
As I mentioned above, I like straw bales for their ease of use: two fleeks laid side-by-side give me a bed that’s about 30” wide, and two fleeks stacked up give me a consistently thick pile – no need to “fluff up” the straw. But if you have other comparable materials on hand, definitely go with those. The cheapest option is almost always best in this instance.
When deciding how much material to use in each layer, keep in mind the golden ratio of composting—30 parts carbon for every 1 part nitrogen—but think of it more as a frame of reference than a hard-and-fast rule here. It’s crucial to keep your carbon content substantially higher than nitrogen, but you’ll drive yourself insane if you spend too much time calculating precise ratios of what will, ultimately, be a pile of dirt.
Everything turns to compost eventually – that’s one of the more magical things about being a living creature on this planet. A healthy ratio of carbon to nitrogen will help avoid smelly and slimy anaerobic conditions, while also keeping the total breakdown time to within a year or less.
Once you’re done piling it up, give it a long and thorough watering. This layer will hold an impressive amount of water, and that is ideal for facilitating the breakdown process.
5. Add another layer of compost (and topsoil, optionally).
After the carbon, you will want to add more nitrogen-rich materials. If this is going to be your final “meaty” layer, I would opt for compost over other possibilities, since you will most likely plant directly into it eventually. If you’re stacking the pile extra high and planning a second layer of carbon, then this layer could be manure, fresh grass clippings, etc. Pile on a solid inch or two, at least.
If you’re planning to plant sooner (within a month or two) rather than later (six months or more), you might find it useful to add a layer of topsoil to finish off the pile and provide extra dirt to plant into. Hold off on that if you’re planning to add more layers.
6. Keep building higher (optional).
You could decide to be done after your second nitrogen layer, like me, or you might choose to keep building the pile up higher – I’ve heard of sheet mulches stacked as tall as three feet high. In that case, repeat steps 4 and 5 to your heart’s content, remaining ever mindful of that golden 30:1 ratio. (My recipe doesn’t account for this, so you’ll probably need to at least double the amounts of materials that I suggest up top.)
7. Finish with a long-term mulch.
This layer is not a requirement, but it’s a good idea not to leave bare soil exposed, so you will probably want to cover your new lasagna bed with something like shredded leaves, wood chips, or fluffed-up straw. You’ll eventually push this layer back when it’s time to plant.
Let the sheet mulch rest for at least a month or two, but four or more is preferable to give it time to break down. You might want to wait a full year before planting, depending on what kinds of materials you used and what your carbon to nitrogen ratio is. Sheet mulching is a “slow solution,” which is another reason permaculturalists dig it, so give it time! You wouldn’t take that lasagna out of the oven after only five minutes of baking, would you? Would you??
When it comes time to plant, depending on how much your sheet mulch has broken down, you can either dig a hole through your weed barrier and plant into the ground, or else plant directly into the pile itself. Direct-seeding isn’t ideal for a sheet mulch, so you will want to transplant seedlings.
Note that sheet mulching is not a permanent solution, but it is a great multi-year fix: production will be high in the first few years, and then begin to taper off as the mulch is depleted. After several years of use, there may be mineral imbalances below your mulch that could reduce productivity. At this point, you have a few different options:
- Sheet mulch again, likely with diminishing returns
- Switch up the management plan with simple compost and mineral applications, forking for aeration, traditional mulches, etc.
- Relocate the garden and convert the area into perennial plantings of trees and shrubs, or else recreational space
The choice is yours, but I am confident that you will know what's best for your garden when the time arrives. In the meantime, may you enjoy bountiful harvests from your backyard garden, sheet mulched or otherwise!