How to choose the mulch that’s right for your garden
ORGANIC MULCHES are central to any successful garden. They retain moisture in the soil while helping to control weeds, erosion, and muddy backsplash from rain onto plants. Mulches help maintain an even soil temperature in both winter and summer, thereby encouraging beneficial microorganisms and also reducing the stress of temperature fluctuation on plants. As mulch decays over time, and earthworms incorporate it into the earth, organic matter gets added to the soil, thereby building up what is called “tilth,” or soil structure—something that is especially important for gardeners who must work with organically starved, gravelly, sandy, or clayey soils. Mulches also have an aesthetic benefit: they provide a uniform and unifying color on the ground.
Whenever possible, use mulches that come from your area: long-leaf pine needles in Georgia; white-pine needles in New Hampshire; straw in the Midwest; and processed bark mulch in the Pacific Northwest. They will cost less and look more natural in your landscape. In fact, there is nothing wrong with using shredded leaves from your trees and shrubs, and lawn clippings from your yard. After all, when you mulch you are simply following nature’s cue; if you didn’t rake leaves or cut back perennials in the autumn, you would see that woody and herbaceous plants mulch themselves.
Not all mulches are appropriate for every landscape setting. Coarser, less elegant mulches are fine for berries or fruit trees, or to create pathways in woodlands and between rows in vegetable gardens. In highly visible areas of the garden, however, the choice of a mulch may run to the more delicate, uniform, and costly materials.
BY GORDON HAYWARD PHOTOGRAPHY BY WEBB CHAPPELL
BARK MULCH There are two principal kinds of bark mulch. Ground bark is made by shredding bark stripped from logs. Its reddish brown color varies depending on the species of tree, although dyed versions in various hues of reddish orange are also on the market. You can buy ground bark in three-cubic-foot bags or you can have it delivered by the cubic yard. Pinebark nuggets are one- to three-inch pieces of pure bark. For durability, these nuggets are better than shredded bark, typically lasting several years longer before they decay.
In recent years bald cypress mulch has become popular for its extreme decay resistance. However, much of it comes not from the waste of sawmills, but from entire stands of young cypress trees cut down for the sole purpose of making mulch. Grinding up an introduced species such as eucalyptus to make mulch is one thing; endangering native populations of a species is another. It is worth inquiring about the source of whatever mulch you choose.
WOOD CHIPS Wood chips come from a mix of brush and tree branches, and function much like bark mulch, though they can be coarse and less uniform. With time, the surface of the chips will turn a silvery white. Arborists will often deliver a truckload of wood chips (six to eight cubic yards) to your home for free. Wood chips are useful under shrubs and trees at a distance from the house, to make paths through vegetable gardens or woodlands, and to fill in low wet spots, but in perennial gardens and near the house, refined mulches are more attractive.
SAWDUST Sawdust is a long-lasting mulch that can often be obtained for free. It is traditionally used under blueberries, raspberries, blackberries, and asparagus. If you decide to work sawdust into your soil at the end of the growing season, you will have to add substantial nitrogen fertilizer at the same time, or the sawdust-decomposing microbes will pull nitrogen out of your soil and away from your plants. This is a temporary condition, but it is better to leave the sawdust on the surface and let nature do the incorporating.
PINE NEEDLES Like deciduous leaves, pine needles are another abundant, free mulch. They are especially suited to acid-loving plants such as rhododendrons, azaleas, or hollies, since the decomposing needles can help to acidify the soil. Elsewhere a scattering of lime will offset the added acidity. Pine needles are also a fine mulch for paths through woodlands, especially those in which pines are growing naturally. Pine boughs with the needles still on them make a superb winter mulch for perennial beds, because they allow air to circulate around plants and can easily be removed in the spring. Lay them onto beds after the top two inches of soil is frozen, and leave them on until early spring.
STRAW AND HAY Baled straw consists of the dried stems of barley, oats, or wheat, and is weed free. Hay is made up of dried grasses and almost certainly contains weed seeds, although salt marsh hay, available on the East Coast, is an exception. Because straw has stouter stems than hay, it stacks loosely, which means air and water can pass through it easily. This makes it an excellent winter covering for strawberries. Both straw and hay will tend to blow away in windy sites.
DECIDUOUS LEAVES Because leaves from trees and shrubs are both abundant and free, they are the perfect mulch for those very same trees and shrubs. In the perennial border, the leaves are less likely to blow away or mat down if they are shredded. A leaf shredder, a lawn mower with bagging attachment, or a chipper vacuum can all be used to shred leaves. Shredded leaves can be spread three to four inches thick on the soil around perennials in the fall. In the spring, the leaves will have settled to about one inch in thickness.
LAWN CLIPPINGS Because they are rich in nitrogen, lawn clippings are best left on your lawn, where they can decompose and feed the soil. However, they can also be used as mulch. Lay down no more than an inch and a half of fresh lawn clippings; this will quickly reduce in volume by half. Be careful not to lay down a thicker layer, because the clippings will start to ferment. This won’t hurt your plants, but it will certainly smell bad. Grass clippings are especially good for onions, which have shallow roots that are easily damaged by weeding. If your lawn has been recently treated with herbicides, however, don’t use the clippings as mulch.
COCOA-BEAN AND BUCKWHEAT HULLS
These two kinds of mulch are distinctive by-products of food processing. Their fine, even texture and attractive brown color have made them popular mulches. Also, cocoa-bean hulls add the pleasing aroma of chocolate to the garden, albeit temporarily. Both mulches are available in bags, though if you live near a processing plant you can buy them much less expensively in bulk. Given their comparatively high cost, you will probably want to limit your use of these two mulches to small rose or herb gardens, and formal perennial beds.
What about plastic?
Sheets of polyethylene film are a popular mulch in the vegetable garden. Whereas organic mulches cool the soil, plastic film heats it up. Laird directly on the soil, black plastic can raise the temperature of the top three to four inches of soil by as much as 5’F, and clear plastic can raise the temperature by 10”. This means you can thaw soil in early spring for, say, a planting of peas, or you can grow warm-season vegetables, such as melons, tomatoes, cucumbers, peppers, and eggplants, through slits made in the plastic.
TIMING OF APPLICATION Because organic mulches have an insulating effect, where warm soil is important the ground should not be mulched until the soil has had time to warm up in the spring. Of course, permanently mulched beds can have additional mulch added to them at any time.
DEPTH Beware of applying too much mulch. Two inches is usually sufficient. If you use less, the mulch will neither hold moisture nor suppress weeds; use more and you run the risk of preventing oxygen from reaching surface roots while also encouraging waterlogged soil. One exception to the rule is to mulch more thickly if you are planting in the fall; if you do this, pull all but two inches of the mulch away in the spring. Keep mulch three to six inches away from the trunks of trees, the stems of shrubs, and the bases of perennials, vegetables, and annuals. Air circulation around the base of plants helps prevent rotting and discourages boring insects. Mulch placed near the base of plants provides winter cover for mice and voles, which are likely to girdle woody plants and trees.
QUANTITY To determine the number of cubic feet of mulch you will need, multiply the width of the plot by its length to get the total square footage you will be covering. Because you will be spreading the mulch about two inches deep, multiply this number by .17. (Two inches is equal to one-sixth of a foot, or .17.) For example, if you want to mulch a bed that is 10 feet by 20 feet, or 200 square feet, multiply 200 by .17; thus you’ll need to buy 34 cubic feet of mulch, or around 11 three-cubic-foot bags. When mulch is sold in bulk, it is usually measured in cubic yards. (One cubic yard is equal to 27 cubic feet.)
MAINTENANCE Once mulch begins to decompose, weeds will begin to appear on its surface. Because organic mulches are generally loose, weeds are easily pulled from them. There is no need to work decomposed mulch into beds; earthworms will do that for you.–G.H.
Black plastic is an effective weed barrier. Clear plastic is not. Recently introduced IRT (infrared transmitting mulches) combine the benefits of greater warmth and weed control. Plastic film mulch should not be left in place for longer than four months, lest the underlying soil become deprived of oxygen. The thicker grades of plastic film (six to eight mil) can be reused for several years, while the thinner ones (one to two mil) are usually discarded after a single year’s use.
Woven landscaping fabric is another form of plastic mulch. It warms the soil and holds down weeds, but unlike sheet plastic, woven landscaping fabric allows water and air to penetrate the soil. Landscaping fabric is more expensive than plastic film, but it is also sturdier and lasts longer.
When used in perennial beds and around shrubs and trees, landscape fabric is usually covered with a layer of bark mulch or other organic mulch. Although the result looks attractive and appears to be maintenance free, using landscaping fabric this way can create more problems than it solves. Unless you weed vigilantly, weeds will take hold in the top layer of mulch, and their roots will reach down through the woven landscape fabric and into the soil below. In just one or two seasons, you will be faced with the laborious task of pulling up the cloth, the mulch, and the thick tangle of roots.–G.H.