Sheet mulching, also known as lasagna gardening, is an organic, layering method that replenishes the soil while keeping the garden neat and attractive. The time frame between fall and the following spring allows several months for the organic matter in the layers to enrich the soil for fruitful spring planting.
Using mostly readily available materials, sheet mulching is an easy timesaving way to install and/or amend soil without having the hard work of digging or tilling.
The day before you start your sheet-mulching project, water the site well unless the ground is already wet with rain. The microbes that will turn your sheet-mulched plot into rich soil need water to do their work. After the water has soaked in overnight, cut down or weed whack any vegetation and remove any stumps/woody pieces. If thistles grow on the site, pull them out rather than weed whacking them, as their seeds are often able to mature and re-sprout into plants. To open up the ground, poke a spading fork across the site to promote better moisture and root penetration.
Choose a site that offers the proper setting as to exposure, drainage and pH for the intended plantings. To successfully feed the soil or smother weeds, it is much more effective to blanket a small area thoroughly than to spread the mulching layers too thin.
Sheet mulching can be as simple as a layer of newspapers topped by 8 to 12 inches of nearly any mulch material.
However, some experienced sheet mulchers suggest that to increase the number of microbes acting on the area that you start by putting down 2 inches of aged compost over the dampened, weed-whacked area. Compost, either mixed into the soil or used as a top dressing, aerates the soil and adds nutrients.
Next, pile on a 1- to 2-inch thick layer of overlapping, light-blocking newspaper (dispose of colored and glossy pages) or cardboard (remove any staples and plastic tape). Wet the newspaper or cardboard layer thoroughly.
Next, add four to 12 inches of loosely piled mulch (yard waste, leaves, finely ground bark or a mixture of these) or loose straw. On top of this layer, add an inch or two of compost and/or composted manure. Top off the layers with 2 inches of straw, leaves, bark, pine needles, or grain hulls. As you add layers, spray on enough water so that the pile remains damp in a "wrung out sponge" state.
Avoid substituting hay for straw as hay can be laden with weed seeds. Avoid using gorilla hair mulch. A study from University of Nevada found that gorilla hair mulch (made from shredded redwood or cedar bark) had the most rapid rate of fire spread and also produced embers which moved beyond the plot and igniting adjacent mulch plots.
For the best outcome, before you start your sheet mulching project, test your soil using a soil test kit available from your local garden center or send a sample of your soil to a professional lab for more comprehensive testing. Depending on the test results or prior knowledge of your soil, consider adding a thin layer of the appropriate soil amendment(s) to your growing sheet mulching pile to improve the physical properties of your soil.
Sheet mulching is a very forgiving process. If you can't find every item, don't worry. As long as you have enough newspaper or cardboard plus organic matter of almost any kind, your efforts will improve your soil’s water retention/infiltration, permeability, drainage, aeration or structure and, simultaneously, provide a better environment for root growth.
Original article by Martha Proctor (12/15/12) for the Marin Independent Journal
Edited for MMG public website by Kathryn Parkinson 8/13/18
Photos courtesy of sacmg.ucanr.edu
Illustrations by UC Master Gardener William Ingersoll, Contra Costa Master Gardener.