‘Compost sandwich’ could feed new garden

QI’ve heard about creating garden beds in sites where no real “soil” exists. If this is possible, how and when is it done?

A: On certain types of surface, planting areas can be created by piling varied compostable materials on it. The process is commonly described as lasagna gardening, sheet composting or making a “compost sandwich.”

First, look for a patch of lawn or a superfluous area in a bare-ground or gravel driveway. The surface should be porous, not sealed off with concrete or asphalt.

Define the boundaries of the bed and lay cardboard over the surface. Top the cardboard with about a five-centimetre layer of newspaper. If possible, spread a loose layer of straw overtop.

Building the bed after this base is complete is a matter of alternating layers of green (high-nitrogen) materials, brown (high-carbon) materials and garden soil.

For the greens, use grass clippings, manure, young weeds, coffee grounds, tea leaves, compost and fine garden trimmings. Some brown materials to use are leaves, straw, shredded newspaper, fine fir bark, wood shavings and aged sawdust. Aim for roughly twice as much brown as green.

Continue adding materials until the pile is at least 60 cm high. It can be higher. The mass will settle over time. If the bed is to be used for vegetables, consider bordering the area with bricks or boards.

The best time to begin building one of these beds is late summer to early autumn for planting in the spring, but the process could be started now. “Lasagna” beds are often made in late winter or early spring for planting almost immediately, but with much more topsoil used in the mix.

Once the heap is thoroughly moistened with the rains, consider covering it with a tarp to hold in nutrients. In the spring, spread topsoil over the area before planting.

A project like this is much like planting squash or pumpkin atop a partly decomposed, soil-topped compost pile in the spring.