The traditional method for transforming grass or weeds into a garden bed involves removing all the plants that are currently there, adding organic matter and nutrients, and then planting. Unfortunately, many grasses and weeds can rapidly regenerate themselves from tiny bits of roots that are inevitably left in the soil. There may also be a collection of weed seeds in the soil waiting for a good opportunity to sprout. Your beautiful new bed is quickly overrun by the same plants you just worked so hard to remove.
Sheet mulching is an alternative method, in which layers of organic matter smother existing plants. This is a popular method in permaculture design. Not only are the old weeds killed, they eventually decompose and provide valuable nutrients to the plants that you do want to grow in that location. The layers of organic matter that you use as sheet mulch also decompose over time, providing a fertile planting bed. Thus, sheet mulching eradicates weeds and prepares the soil of your new planting bed in one fell swoop.
So, how do you sheet much? The following is an outline of the basic steps, which you can adapt to your situation and the materials you have on hand. This method assumes you have a few seasons to wait before you plant.
- Chop or mow down existing vegetation as much as possible, leaving the clipped plants in place.
- Optional: Over chopped plants with a thin layer of manure, which will jumpstart their decomposition.
- Cover the area with a barrier of relatively durable organic material, like cardboard or thick layers of newspaper. Cardboard is better at blocking sprouts from below, but until it decomposes, it can make drainage above the cardboard layer a problem. Make sure you overlap your material at least 8” to 12” so weeds can’t grow up through the cracks. The key is to block out sunlight completely.
- Soak your newspaper or cardboard. This may take several go-overs with the hose.
- Next you can put an 8-12” layer of any organic matter mixed so the carbon-to-nitrogen ratio is between 100:1 and 30:1. Dry leaves are perfect, whether fresh or dry; vegetable scraps are a bit too high in nitrogen by themselves and should be mixed with something higher in carbon like sawdust.
- Apply a layer of compost or top soil at least 4” thick.
- Ideally, plant a cover crop like clover and wait a season or two for all the layers to decompose. It’s convenient to sheet much in the fall and let it decompose over the winter, so it will be ready for spring planting. Until lower layers decompose, anything you plant will only have access to the amount of soil you added in step 6 for establishing roots. With that in mind, if you are impatient and/or had a lot of top soil, you can plant anything that you think will do well with that soil depth.
– Sometimes you don’t have a few seasons to wait before you plant, often you want to do it now! In that case you can still sheet mulch, but weeds might make their way to the surface faster through the inevitable holes you have to make in the cardboard for new plants. Pre-dig any holes for plants that are larger than 1gal size. When you place cardboard over this hole, make an X with your knife in the cardboard so you know where the hole is, and can later push these flaps down, or fold them under. Go ahead and plant the large plants before you do the rest of your sheet mulching steps so you don’t cover the holes with the final layers. Water these plants thoroughly.
– If you have a large patch of lawn or persistent weeds that you want to eradicate (buttercup, horsetail, etc), you will have the most success if you do the sheet mulching in the summer and wait to plant in fall, or even the next spring. This way you are depriving the weeds of both water and light during the driest time of the year, and you are baking them if it’s in a sunny spot.
– Sheet mulching generally cannot eradicate blackberry unless you first remove the crowns by hand. The crowns are like knotty root balls and they are generally near the top 8” of soil, most blackberry shoots that come out of the ground originate from a crown.