Sheet Mulching

I wanted to extend this bed and give it a curve.  I dug in the edging first, then covered the grass with cardboard.

There are two important reasons to use sheet mulching when creating a new garden bed.

• It’s much easier on the gardener.
• You’re practicing “no-dig” gardening, which is a method of gardening which limits the disruption of the soil ecosystem (which in turn leads to healthier plant roots and therefore healthier plants).

The first reason is self-explanatory; because you are avoiding the back breaking effort of digging a shovel into well compacted earth, you are saving yourself both time and sweat.

The second reason is that your native soil hosts a wide variety of native fungi and bacteria which are not only beneficial to plant life, but actually necessary.  The mycelium of fungi work symbiotically with plant roots to help roots more efficiently take up nutrients.  There are bacteria present in the soil that do the same, as well as help control pests.  There is a great deal of research happening nowadays in the area of soil microbiology, and we are continually discovering more and more.  All research points to one fact – the microbiomes of our soil are a critical part of plant growth and even our bodies’ chemical composition.  Basically, you want to protect this microbiome as well as you possibly can.

The main downside to sheet mulching is that it takes time.  If you need your garden bed ready immediately, you will have to dig out and remove existing grass or weeds manually, but if you can think ahead and wait a few months, you’ll be glad you did.

How to do it

Sheet mulching is a simple matter of layering some type of thick compostable barrier down on top of the existing soil (including the grass or low growing plants already growing there) and then placing compost and other plant matter on top of that.  This method smothers the existing grass while at the same time allowing that green grass to compost in place and add nutrients to the soil.

Here are the steps:

1)  Take some cardboard and remove all tape or plastic from it. You can also use multiple layers of newspaper (I’ve found 6 layers to work well), or those thick paper leaf bags you can purchase at big hardware stores.  For large garden beds, appliance boxes help a lot.

2)  Lay the cardboard directly on top of your existing grass (or low weeds). If you have existing plants or weeds growing, you need to chop them down to no more than an inch tall.

3)  Wet the cardboard with your garden hose.

4)  Use a screwdriver to punch holes in the wetted cardboard, about every 8 inches or so. I discovered from experience that this helps later when watering the bed – it allows the water to sink through all of the layers, all the way to the soil at the base.  Most sheet mulchers don’t do this, but I’ve found it helps break down the cardboard much faster.

5)  Dump good quality planting soil and compost on top of the cardboard. You can either buy this by the yard and have it delivered in large quantities (good for a large garden area), or by the bag purchased at the store or nursery. You want the new soil & compost layer to be at least 2 inches thick.

6)  Water this new bed thoroughly. It takes more water than you might think to completely saturate all of the soil down to the cardboard.

7)  Wait a few months, watering occasionally if you don’t receive rain.

Watch for the glorious day when all of the carboard has decomposed and you have what looks like a fresh new garden bed. Note – Keeping the bed constantly moist speeds the process a lot, while allowing it to dry out in between watering prevents the cardboard from decomposing and makes the whole process take longer.

8)  Once the cardboard has decomposed, you are ready to plant your plants!

This is the “after” for the area shown at the top of this page.  This was an extension of an existing bed (I’ll fix the edging later!).

Tips:

♦ Mark out your desired garden bed area with a string, garden hose, or marking paint before beginning the layering process.

♦ Put in whatever edging you want BEFORE sheet mulching the area inside of it. You can either dig an edging trench, install dig-in edging, or place stones as edging.

♦ When you already have an existing bed and just want to add on to it, the process is the same.

♦ If you’ll be growing food in your bed, make sure the cardboard or paper you use to smother the grass doesn’t contain anything toxic. For food beds, unprinted cardboard or paper is best.

♦ You can use this method in raised beds as well. Just place your bed first (metal, wood, or stone), then layer the cardboard, then add whatever dead leaves, small branches and twigs, and whatever natural (unsprayed) plant matter you have around the yard, and place good garden soil on top of that.  Obviously you will need much more soil to fill a raised bed than a standard garden bed, so filling in some of that space with dead leaves and branches helps.

♦ If you want to create a bed that you won’t really be planting in (such as space in between large shrubs where you won’t be planting anything, but don’t want grass growing there either), you can simply top the cardboard with a thick layer of hardwood mulch, which will go further and cost less than good soil. You still need to keep it well watered.

Once your bed is ready, you can go ahead and dig into it, including into the original native soil, to plant your plants.  See my How to Plant Your New Plants page for tips on how to plant for the healthiest results.

Happy Gardening!