Like icing on a cake, a layer of mulch gives a garden a neat, finished look while also performing the important functional role of reducing moisture loss. While mulch for flower beds and landscape plantings may be chosen for its decorative qualities, features such as cost and sustainability typically get more consideration for mulch in a vegetable garden or orchard.
In addition to conserving moisture and suppressing weeds, organic mulches—those made of plant-derived materials—benefit the soil as they decompose, adding organic matter and modest amounts of nutrients. By keeping the soil cool and moist, mulch also creates inviting conditions for earthworms and other beneficial soil-dwellers.
Think about where most of a plant’s roots are located and apply mulch accordingly, advises Robert Kourik, author of Roots Demystified and Understanding Roots. “Mulch trees and shrubs out to the drip line [branch tips] and beyond, while keeping mulch materials away from direct contact with trunks and stems.” In thirsty California, Kourik combines mulch with drip irrigation, placing the water lines on top of the soil and then covering them with mulch both to hide them from view and protect them from degrading in sunlight, all to make the most of every precious drop.
A 2- to 4-inch layer of most materials is enough to get the desired benefits without impeding air and water from reaching the soil. Loose mulches such as straw and hay may be deeper—as much as 6 inches—or can be applied over a weed-suppressing layer of newspaper or cardboard. In spring, cool, damp organic mulch can slow the emergence of bulbs and perennials and keep things too chilly for seeds of annual flowers and vegetables. Pull the mulch back to let the sun warm up the soil, then replace and refresh after plants get growing.
Woody mulches, including sawdust, wood chips, and even straw, can monopolize the soil’s supply of nitrogen while they slowly decompose, leading to nutrient deficiencies and lackluster growth of the mulched plants. Before spreading woody mulches, apply a nitrogen fertilizer such as fish emulsion or alfalfa meal.
The byproducts of farming and food processing often make excellent mulch. Because things like hulls, stems, and other crop residues are viewed as waste materials to be discarded, they may be cheap or even free. Be aware that chemicals applied to the crops may remain on the mulch and cause unintended harm to plants the mulch is meant to benefit. A few examples of these regional mulches include bagasse, a byproduct of sugar cane processing; cocoa, peanut, or pecan shells; ground corncobs or cornstalks; rice hulls or rice straw; salt hay, grasses gathered from marshes in coastal areas; and seaweed.