Deep Mulch Bed

A deep mulch bed is a growing bed kept under a thick blanket of loose carbon material such as straw, hay, leaves or wood chip. The layer keeps light off the soil so buried weed seed cannot start, slows water evaporating from the ground so the bed stays damp longer, and feeds the soil beneath it as it rots.

Depth is what everything else follows from. About 5 to 10 cm (2 to 4 in) suits a cropped vegetable bed. Thin it to about 2.5 cm (1 in) around a young transplant, building it back as the plant grows; 4 to 5 cm (1.5 to 2 in) over low crops like lettuce and radish; 10 to 15 cm (4 to 6 in) of coarse wood chip where ground is being brought into use rather than cropped. Leave a gap of about 15 cm (6 in) around every trunk and stem, because fresh damp material heats as it rots and will cook bark it touches. It suits a no-dig bed, water runs better under the layer than over it, and the real constraint is a standing supply of bulk carbon.

The mistake that ruins a garden is invisible. Some pasture weedkillers pass through an animal, through composting and through years of storage still active, and more than twenty-five food crops are sensitive, among them beans, tomatoes, potatoes, carrots and strawberries. Hay, straw, manure or grass clippings of unknown history should be tested first: sow a few pea or bean seeds in a pot of the material beside a clean control and watch both for two or three weeks. Contaminated material belongs on grass rather than on a bed. Woody material on the surface takes nothing from the soil, but dig it in and the microbes breaking it down pull nitrogen from the crop. The layer shelters slugs and gives mice and voles cover to gnaw bark from fruit trees, so it is drawn back from trunks before winter. Laid too early in spring it holds the ground cold, it will not stop a perennial weed with a root or rhizome to push from, and being damp it is itself a seedbed for whatever blows in. Clear weeds before laying it, keep the cover even, and top it up as it thins.