Sheet mulching — aka lasagna composting — builds soil, saves time

Sheet mulching, also referred to as lasagna composting, uses a layer of cardboard or newspaper placed on the soil with mulch on top. The cardboard helps to smother existing weeds and seeds in the soil. The cardboard layer breaks down after several months (depending on soil type and geographical region) and the mulch will begin to decompose through contact with the soil.

There are several advantages to sheet composting:

• It is an easy and uncomplicated method of composting.
• It can be done a little at a time as materials become available.
• 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.

A disadvantage of sheet mulching is that the cardboard creates an interface that limits the flow of water and air into the soil.

Sheet mulching may not work well in all sites and locations. Soil type, precipitation, and other factors affect the rate of breakdown. Best to check with your local extension office if this is a recommended composting practice for your region.

10 steps to good compost

To build any good compost you need to plan ahead. Sheet composting is best started several months before you want to use the planting area. Fall is an excellent time to sheet compost 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. The basic technique involves placing alternate layers of carbon materials and nitrogen materials directly on the soil. Layers should be fairly equal to allow for even decomposition. Aim for 1-inch layers, although you can go deeper.

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 wide 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.
3. Remove any pernicious or persistent weeds such as blackberry, bindweed, morning glory or quackgrass. Sheet composting may not smother these weeds.
4. Cover the ground with 4 to 6 overlapping layers of newspaper or cardboard. This carbon material 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 a 1-inch layer of a nitrogen source such as manure.
6. Top the nitrogen with 1 inch of leaves, straw, bark or other carbon material.
7. Add a 1-inch layer of nitrogen: kitchen scraps, green produce scraps, manure or fresh green weeds (minus the seed heads), or a combination.
8. Cover with another layer of carbon material — straw, shredded paper, leaves, etc.
9. Continue to add alternating layers of carbon and nitrogen until reaching the final height (18 inches to 3 feet).
10. 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. This 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 speaking, 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.

Waiting is the hard part

Sheet composting is a slow process. There is little or no heat reaction from the microorganisms to speed the process along. A sheet compost 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 in the lasagna garden.

Main ingredients

The following are commonly used materials for sheet mulch/lasagna 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 raised bed sheet mulch garden

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

1. Ask the lumber store to cut wood for you. Screw together corner braces and 4-foot 2×12’s. Add on 12-foot 2×12’s.
2. Easy method: Lay newspaper on the grass where the bed is to go. Fill with leaves, coffee grounds, grass clippings and straw in layers. Top with 4-6 inches of garden soil and plant.
3. More labor intensive method: Dig out sod under area to be covered by the bed. Cover the bare soil with wood chips or overlapping newspaper. Turn sod upside down to form the first layer in the bed. Cover with two sheets of newspaper, and layer with coffee grounds, leaves and other material. Add lime and organic fertilizer on top of this level. Top with 4 inches of garden soil or good compost and plant.