Sheet Mulching

By Charlotte Torgovitsky

Organic gardeners all know that the key to healthy, vigorous plants is in the soil. Compost maximizes the bio-diversity of soil organisms, which in turn transforms still more organic materials to accessible nutrients for your plants.

Decomposition takes time, so it’s best to sheet compost an area in fall.

Sheet composting, or sheet mulching, is a passive method of composting, and it is done on the site that you wish to enrich. There is a minimum amount of labor required, and the results take time; but, as in nature, compost happens – and you will be rewarded with beautiful soil and bountiful harvests!

Sheet Mulching. By Kristen Wernick
Sheet Mulching Photo: Kristen Wernick

Sheet composting can be used to create new and fertile gardening sites. Land previously given over to a lawn, or groundcovers, such as, ivy or vinca, or perhaps just assorted weeds, can be reclaimed, with time, by layering materials that will decompose in place.

Soil test

Before beginning CNPS strongly urges you to test your soil for composition, minerals, and legacy issues (past owners’ soil treatments). Soil tests should assist you in making informed decisions about soil amendments.

When to start

Decomposition takes time, so it’s best to sheet compost an area months in advance.  Materials will be decomposed enough to plant in the area the next season; by the next year you will not be able to recognize any of the original materials. Given time the layers of paper and organic materials will have smothered the roots of plants previously growing on the site, and decomposed into wonderfully rich and fertile soil.

Start small

Quantities listed here will convert an area of fifty square feet. To create fertile soils for California native plants, you will need:

  • A 2 to 3 foot stack of newspapers,
  • about 300 sq. ft of corrugated cardboard boxes,
  • about 1/4 to 1/2 cubic yard of finished all-green compost (compost without animal manures)
  • and approximately 5 to 7 cubic yards of bulk organic matter.

This list is meant only to serve as an example; if possible recycle what you have on hand, or free materials that can be had for the asking. Neighbors will often save newspapers if you ask for them; appliance stores are a good source of cardboard. Cotton fabrics can take the place of some of the paper; coffee companies will sometimes give away burlap sacks, and you could also recycle old cotton sheets, towels or clothing.

Lawn clippings, leaves and shredded plant trimmings can be accumulated by your gardeners; straw bales are available cheaply if they become wet, and chipper bark is often dumped free of charge by the tree companies. Be sure to ask the tree company what kind of trees make up the chipper bark; avoid Eucalyptus, California Bay and Walnut. These trees contain chemical compounds that inhibit plant growth; the chipped bark is fine for use on pathways, but not in planting areas. Materials can also be purchased. Corrugated cardboard in large, wide rolls; a loosely woven burlap fabric made for landscape purposes, and finished all-green compost.

Step 1

Water the area well before you plan to start the process. Plants growing on the site will need to be cut to the ground. Don’t remove the cut vegetation; the greens will be food for the decomposers. Roots of most weedy plants can be left in the ground, but woody stumps or pieces should be removed. A dryed-out lawn can be left in place; sheet mulch right over it.

Step 2

This is a great opportunity to amend your soil, and increase its fertility with an array of micronutrients. Acid soils can be sprinkled with lime; alkaline soils will be improved with a little gypsum or sulfur. Bonemeal and rock phosphate will supply phosphorus, which is essential for root formation, early growth and good seed formation. Kelp meal, greensand, or rock dust all add trace minerals.

Step 3

If the area is compacted, or you are dealing with clay, take the time to open it up just a bit before you start to mulch. Simply push a fork or spade into the ground and rock it back and forth to open up holes in the ground. Turning disrupts the soil ecology; so that is a bit of hard work that is best avoided! One of the very best amendments to apply is rock dust; simply sprinkle it liberally over the area.

Step 4

Apply a thin layer of nitrogen rich materials; this can be fresh grass clippings, finished compost, blood or cottonseed meal. This layer will attract the earthworms, burrowing beetles and other invertebrates which will help loosen the soil.

Step 5

Cover the area with about one quarter inch thick layers of newspaper.  Avoid using the glossy pages from the paper, and remove the staples and tape from the cardboard. Overlap the edges by three to four inches, and wet everything down as you work. The purpose of the paper is to create a light blocking layer that will smother the weeds; an added benefit is that the worms also love the darkness and moisture that the paper creates.

Step 6

Add another layer of finished compost or green materials on top of the newspaper, always wetting materials as you build the sheet mulch. All-green compost can be applied in layers up to several inches thick; if you’re recycling grass clippings, be sure to spread them out thinly.

Step 7

Add a layer of cardboard on top of the compost or any other green material. Be sure to remove all the tape, metal staples, or plastic, if you are working with recycled cardboard pieces. Also be sure to overlap the edges liberally, and wet everything down as you work.

Step 8

Now you are ready to add the carbon-rich mulching materials in a layer up to six inches deep. This can be fresh or spoiled straw (avoid hay, which often contains seeds), stable sweepings (as long as there’s not a lot of manure included), chipped yard waste, deciduous leaves (not live oak leaves), or pine needles; or the chipped bark and leaves available from many tree trimmers.

Tip:  Sheet mulching can be built up to a depth of a foot or more; the top layer should be all one type of material (either straw or chipper mulch) for a tidy and uniform look that you can live with for several months.

Step 9

Water the sheet mulched area about once a week if the rainy season does not materialize, or if you are not sheet mulching during the rainy season.

Everyone who gardens can compost; your plants, and our environment will benefit from it. Organic materials are returned to the earth, building rich and fertile topsoil that retains moisture, warmth, and oxygen. Such healthy soil is the underlying support of the entire web of life, and a resource center for healthy plants.

 


Charlotte Torgovitsky is a member CNPS Marin County Chapter & founder of Home Ground Habitat Nursery.  Learn more about Charlotte’s garden on her CNPS Garden Ambassador profile.

2 Comments

    1. Hi Darcy,
      Best of all is from one rainy season to the next before you plant – by then all the cardboard has most likely disintegrated.
      However it is possible to plant thru a newly sheet mulched area – cut a large X in the cardboard layer, and dig into the soil beneath to make room for the plant, then pull the cardboard back around the plant (but not right up to the crown) – and then taper out the other materials so they cover the cardboard, but don’t smother the plant.

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