Why mulch? It insulates the soil from temperature extremes, keeping it warmer in winter and cooler in summer. Mulch also minimizes erosion, moisture loss, and weed growth. Perhaps its most important role is preventing frost damage. Not only will mulched plants not be heaved out of the ground by alternate freezing and thawing but also they won’t be tempted to emerge prematurely during a brief winter warm-up.
When to mulch: For perennials, trees, and shrubs, you can leave mulch in place year-round. For vegetables and annuals, wait until the soil warms. For vegetables and bulbs overwintering in the ground, wait till the soil freezes, then remove it in spring.
Usually 2 to 3 inches are enough. If erosion or weeds are a persistent problem or if you’re overwintering tender specimens, spread 4 to 6 inches. In permanent installations, just refresh the top each year to maintain the proper depth.
What to use: Many gardeners prefer ornamental mulches for year-round use or for flower beds. Shredded bark, wood chips, and cocoa or walnut shells are attractive but expensive. For seasonal use in the vegetable patch, hay, straw, chopped leaves, ground corncobs, and even shredded newpaper are inexpensive and effective—and you can turn them into the soil at summer’s end. (Newsprint is effective as long as it doesn’t contain colored ink. Chop it first with a lawn mower or shredder.)
What not to use: Never use a mulch that mats down or cakes together—it will prevent water, air, and nutrients from penetrating the soil. Materials like coffee grounds, peat moss, grass clippings, or sawdust should be mixed with a less-dense mulch before being used.
Match your mulch to the plants. Acid-lovers, including rhododendron and camellias, appreciate an acidic mulch. Try pine needles, oak-leaf mold, shredded oak leaves, or composted sawdust from cypress or oak. On plants that don’t need acid, use neutral materials like buckwheat hulls, corncobs, or straw; if you’re using an acidic mulch, add a little lime.
Dead leaves can be used in several ways. Spread small leaves right on the beds. Let large leaves age a year or so to make nutritious leaf-mold mulch. Or chop large leaves to make them easier to handle. If you don’t have a shredder, run a lawn mower over a pile of leaves or pack them in a garbage can and chop with a string trimmer.
Arborists and utility companies that trim trees around power lines often have surplus wood chips ground from felled trees. Also ask your municipality—some have leaf-composting or brush-chipping sites and offer free mulch to the community.
Aluminized plastic mulch helps repel pests—insects passing by are confused by the light and fly away. Make your own mulch by placing sheets of aluminum foil around plants and anchoring them with stones.
The latest trend is geotextiles—permeable woven or bonded fabrics that blanket the soil and perform all the functions of organic mulches. They are particularly effective in suppressing weeds and are safe to use around permanent plantings like shrubs. Because they degrade if exposed to light and are unattractive, top them with shredded bark or wood chips.
Black plastic mulch is best for vegetable gardens. Its radiant heat warms the soil—up to 3°F higher than unmulched soil—and keeps dirt from splashing on plants, which can cause rot. Lay it on the bed and make slits for transplants or seeds; fold it up at season’s end and reuse the following year. But take note: Since rain can’t pass through, you’ll need to water beneath the plastic with a soaker hose or drip line.
Before mulching roses, soak mulch in a dilute solution of an ounce of bleach per gallon of water for one hour. This will help discourage black spot.
Be aware that some mulches, such as wood chips and sawdust, deplete nitrogen from the soil when they begin decaying. Dig in ammonium nitrate, blood meal, rotted manure, or another nitrogen source before spreading these mulches. And mulching doesn’t mean you can neglect the garden. You will still need to weed, water, fertilize, and monitor pests and diseases.
Don’t pile mulch too close to trunks or stems. It can smother plants, promote rot, and let slugs, mice, and other pests hide near a food source. And don’t use plastic mulch around shrubs and other hardy plants. Because it is not permeable, it cuts off air and water to roots; it also causes soil to heat up excessively in summer.