Turn lawn into garden without digging: sheet mulching

Why replace lawn

A lawn is a lot of ground doing one thing. A 2005 study by Milesi and colleagues estimated that turf covers about 163,800 square kilometers of the continental United States, a model estimate, but a large one.

It also drinks. The EPA's WaterSense program reports that U.S. residential outdoor water use is nearly 8 billion gallons a day, mostly for landscape irrigation, and that outdoor use averages more than 30% of household water, up to 60% in dry regions. It notes that up to half of outdoor water can be lost to wind, evaporation, and runoff from inefficient watering.

What sheet mulching is

Sheet mulching smothers grass and weeds under a layer of cardboard or newspaper topped with mulch, with no digging and no chemicals, as Penn State Extension describes it. The grass dies in the dark and breaks down in place, feeding the soil.

"Lasagna gardening" is the layered version: alternating green and brown materials, like a compost pile built right where you'll plant. Oregon State University Extension notes the two terms are often used interchangeably.

Materials

Oregon State advises not making the cardboard layer too thick, and suggests installing drip irrigation before you cover it, if you plan to use any.

• Plain brown corrugated cardboard, not glossy, printed, or waxed. Remove tape and staples, and soak it before laying it down.
• Or newspaper: Penn State suggests about 10 sheets thick, with no glossy inserts; Oregon State suggests 4 to 6 overlapping layers, wetted thoroughly.
• Compost or soil, if you want to plant sooner.
• A top layer of mulch, such as wood chips, shredded leaves, or straw.

Step by step

Penn State Extension's simple method takes three steps, and its details are worth following closely.

Timing

Fall is the best time to build, according to both Oregon State and Penn State. Winter does the work, and the bed is ready to plant in spring.

If you can't wait, Oregon State suggests spreading 2 to 3 inches of compost or soil on top and planting into that. When you do plant, Penn State advises pulling the mulch back so roots go into real soil, not mulch.

The honest cautions

Sheet mulching isn't right everywhere. Oregon State notes that cardboard limits the flow of water and air into the soil, especially when it's thick, though earthworms do feed on it as it breaks down.

Linda Chalker-Scott of Washington State University is more critical. She warns that paper and cardboard can create airless conditions on wet, poorly drained soils, can shed water once they dry out, and can shelter voles and termites. She considers them best for annual beds that are well maintained, not permanent plantings.

For long-term beds and around trees, she recommends arborist wood chips on their own. Her research summary for WSU Extension notes that wood chips tie up nitrogen only at the thin boundary where mulch meets soil, which likely helps suppress weeds, and raise soil nutrients over time. A 4 to 6 inch layer controls weeds; just never dig the chips into the soil.