Sheet mulching and lasagna composting with cardboard

Sheet mulching

Sheet mulching is a quick, easy, and inexpensive way to provide protection from weeds for garden beds of any size.

Sheet mulching uses a layer of cardboard, newspaper, or heavy paper placed on the soil as a barrier to smother existing weeds and seeds in the soil, denying them access to sunlight required for photosynthesis and growth. The barrier is then covered with mulch—shredded bark, wood chips, or other compostable materials—rocks, or pebbles on top to keep it in place. Using cardboard tends to be the most popular method. The cardboard layer breaks down after several months (depending on soil type and geographical region), and the mulch will also begin to decompose through contact with the soil. Holes can be cut into the cardboard to install new plants, or the cardboard layer can be applied around existing plants if so desired.

There are several advantages to sheet mulching with cardboard:

• It is an easy and uncomplicated method of composting.
• It can be done a little at a time as materials become available, with local businesses a ready source for all the cardboard you need.
• It can be done on a large or small scale.
• It can be used to improve soil or add to existing beds and borders.
• It is an easy way to expand a garden with a minimum amount of equipment, material, and time.
• It is exceptionally useful to reclaim garden pathways that have been overtaken by weeds.
• As a no-till, no-dig method for planting, it helps maintain carbon sequestration.
• It conserves water and helps prevent soil erosion.
• It helps prevent plant disease by preventing foliage from contact with splashes containing soil-borne pathogens.
• It dramatically cuts back on time required to remove the weeds that would have sprouted absent their smothering from sheet mulching.
• Earthworms love to feast on cardboard; as they do, they will both aerate the soil below and add valuable deposits of nutrient rich castings (“worm poop”).

There are also a couple of possible disadvantages to sheet mulching with cardboard:

• Cardboard creates an interface that limits the flow of water and air into the soil. That could create problems for existing plants whose roots are under the cardboard-covered soil, especially if the cardboard is thick.
• Sheet mulching may not work well in all sites and locations. Soil type, precipitation, and other factors affect the rate of breakdown. Check with your local extension office to determine if this is a recommended composting practice for your region.

When using cardboard for sheet mulching, make sure that the cardboard is of the plain-brown corrugated variety, not shiny, glossy, or printed, which can be harder to compost, and remove any tape or staples before laying the cardboard down. The layer of cardboard should not be too thick, because that will slow down its decomposition.

The cardboard should be well-soaked with water before application, both to allow water to penetrate and to facilitate its decomposition. It may be challenging to determine if the soil below is receiving enough water to maintain a proper level of moisture — it’s a good idea to install a drip irrigation system before laying down the cardboard if you’ll be planting into the cardboard directly. If installing multiple layers of cardboard, it can also be helpful to poke holes in the upper layers to improve air and water flow without creating openings for weed growth.

Lasagna composting

One form of sheet composting is called lasagna composting.1 With lasagna composting, the initial layer of cardboard is topped with consecutive layers of compostable materials, approximately one to six inches each. The material in these layers decomposes over time, slowly releasing nutrients and eventually leaving a fertile bed of humus for planting.

Creating good compost depends on having the right mix of carbon and nitrogen, with carbon-heavy materials often referred to as “browns” and nitrogen-heavy materials as “greens.”2 For lasagna composting, the materials you add should alternate between one layer of nitrogen-rich material, (e.g., composted manure, kitchen vegetable scraps, fresh grass clippings, blood meal) with one layer of carbon-rich material (e.g., sawdust, dried leaves, straw).

Find a list of potential nitrogen and carbon sources below in the "Main ingredients" section. Because cardboard is high in carbon, the first layer of the lasagna should be nitrogen heavy. Repeat the alternating layers of nitrogen and carbon source materials to a height of 18 inches or more, sprinkling in a few handfuls of soil between each layer to provide a supply of the microorganisms essential to the composting process. The top layer should be brown, i.e. carbon, to act as a cover to ward off insects.

Depending on your climate, the new bed should be ready for planting in approximately six months. If the alternating layers are 1-inch, the composting will occur more rapidly, but using thicker layers will take less time to stack. You could also speed up the composting process by blending the carbon and nitrogen materials and piling the mix to the desired height. However, without the layering, it would no longer technically be considered lasagna composting.

11 steps to good lasagna compost

Building any good compost requires planning. Lasagna composting is best started several months before you want to use the planting area. Fall is an excellent time to do so as the material breaks down slowly over the winter and is ready for planting in the spring. But a bed may be started any time materials become available.

1. Begin by mowing or scalping grass or other vegetation down to the lowest possible level, at the length and width you would like the finished bed to become. Three feet is a good width, as this allows you to reach the center of the bed from either side. You could also work directly in a constructed raised bed.
2. Ensure good drainage by "popping” or loosening the soil underneath the bed with a spading fork or broad fork.
3. Remove any pernicious or persistent weeds such as blackberry, bindweed, morning glory or quackgrass. Lasagna composting may not smother these weeds.
4. Cover the ground with 4 to 6 overlapping layers of cardboard, newspaper, or heavy paper, overlapping edges by at least 8 inches to make sure there are no gaps in coverage (weeds will find any opening, no matter where or how small). Using large sheets of cardboard provides better coverage and fewer areas to overlap. The paper or cardboard barrier smothers the grass and weeds underneath by preventing light from allowing photosynthesis of the plants.
5. Wet the newspaper or cardboard thoroughly and cover with an initial layer of a nitrogen source such as manure.
6. Top the nitrogen layer with an equal-sized layer of leaves, straw, bark, or other carbon material.
7. Water each layer lightly as added, enough to moisten the mixture throughout.
8. Add another same-sized layer of nitrogen: kitchen scraps, green produce scraps, manure or fresh green weeds (minus the seed heads), or a combination.
9. Cover with another layer of carbon material, such as straw, shredded paper, or leaves.
10. Continue to add alternating layers of carbon and nitrogen until reaching the final height (18 inches to 3 feet), with the final layer being carbon.
11. Continue to add alternating layers of carbon and nitrogen, as materials become available. As the material decomposes, more layers may be added, always ending with a carbon layer.

The final step is the “blanket” that discourages flies from laying eggs on exposed nitrogen material such as kitchen scraps. The height of a bed may vary, depending on the amount of material and when the bed will be planted. Generally, the greater the volume of material, the longer decomposition will take. The final layer may be covered with overlapping burlap coffee sacks to keep the materials neat and in place. The burlap will gradually decompose over time but may be removed when planting the bed.

If a pile becomes too wet, cover it with a sheet of black plastic loosely weighted down at the sides. This will help to warm the pile and encourage faster decomposition. This will also prevent nutrients from leaching during heavy rains. If you don’t immediately cover the cardboard/paper barrier with mulch or compostable materials, you may need to anchor the cardboard in place with rocks or garden stakes.

Waiting is the hard part

Lasagna composting is a slow process. There is little or no heat reaction from the microorganisms to speed the process along. A bed may take 6 months or longer to decompose sufficiently to allow for planting.

A bed is “finished” and ready for planting when the layers have decomposed to the point that the original materials are no longer recognizable, and it looks and smells like fresh earth. Or you can get plants started by sifting a 2- to 3-inch layer of compost or garden soil on top of the newly formed bed and planting directly into the lasagna garden.

Main ingredients

The following are commonly used materials for lasagna compost gardening. Almost any garden debris may be used.

Nitrogen sources:

• Used coffee grounds
• Composted manures
• Alfalfa pellets
• Fresh weeds
• Vegetable scraps
• Fresh grass clippings
• Cottonseed meal
• Soybean meal/blood meal

Carbon sources:

• Sawdust
• Leaves
• Cornstalks
• Pine needles
• Peat moss
• Newspaper/cardboard
• Straw/hay
• Wood chips

Build a lasagna-compost raised-bed

Materials

Box

• Two pieces 2×12×12 untreated fir
• One piece 2×12×8 (cut in half)
• One piece 4×4×8 (cut four 11½-inch lengths for corner braces)
• 40 2½-inch galvanized wood screws (10 per corner)

Cloche

• Seven pieces of 10-foot ½-inch PVC pipe
• One piece of 10-foot roll galvanized plumbers' metal stripping to attach PVC to sideboards every 2 feet.
• 28 1-inch roofing nails

Tools

• Hammer
• Drill
• Drill bit to pre-drill holes
• Phillips drill bit for screws
• Tape measure (to evenly space PVC)
• Slide/level/square (to make sure the bed is level and square)

Steps

To get started:

1. Ask the lumber store to cut the wood for you.
2. Screw together corner braces and 4-foot 2×12’s.
3. Add on 12-foot 2×12’s.

Then choose one of the following methods.

Easy method:

1. Lay cardboard, heavy paper or newspaper on the grass where the bed is to go.
2. Fill with leaves, coffee grounds, grass clippings and straw in layers.
3. Top with 4-6 inches of garden soil and plant.

More labor-intensive method:

1. Dig out sod under area to be covered by the bed.
2. Cover the bare soil with wood chips or overlapping cardboard.
3. Turn sod upside down to form the first layer in the bed.
4. Cover with two sheets of newspaper, and layer with coffee grounds, leaves and other material.
5. Add lime and organic fertilizer on top of this level.
6. Top with 4 inches of garden soil or good compost and plant.

• 1The terms “sheet composting” and “lasagna composting” are often used interchangeably, but technically there is a difference: sheet composting can be done with only one layer of mulch, while lasagna composing, as the name implies, involves multiple layers of materials.
• 2The ideal ratio for composting is 30 to 1, carbon to nitrogen (often written as C/N 30:1). While most composting instructions generally call for mixing twice as many browns (carbon) to greens (nitrogen) by volume (e.g., 2 gallons to 1 gallon, two shovels-ful to one shovel-ful) to achieve that ratio, for lasagna composting, the alternating layers of browns and greens are equal in size.