Home & Garden

Too much mulch? Choosing and using mulches

gardening by Mary Robson, WSU area extension agent
Plants grow fast in March, and that fast growth unfortunately includes weeds. Late winter chickweeds and little bitter cress go to seed way too fast, and sow themselves all over the garden. How can gardeners manage annual weeds, tidy the garden appearance, recycle plant nutrients, and conserve summer water, all with one garden technique?
   The answer is mulch, a sometimes misunderstood but invaluable garden addition. The word mulch comes from "molsch," a MIddle English term for "soft." The softness can be perceived in two ways--the materials spread on top of soil are generally soft, or softening as they break down, and the soil itself is improved as the mulch gradually disintegrates. Spreading a layer of some organic material on top of the ground is a very old gardening practice.
   Forests create their own mulch, the layer of fallen leaves, twigs, seeds and other natural detritus that gradually accumulates under trees and shrubby plants. Walking on the forest floor, we experience the spongy, soft layer that is a natural mulch. It's possible to maintain some of the fallen leaves under trees and shrubs as a natural mulch. Leaves must be raked and disposed of at the end of the growing season if they are diseased, such as leaves infected with dogwood anthracnose, but otherwise they can be retained normally in the garden.
   Mulch differs from soil amendments that are dug into the ground to improve soil tilth. A mulch is a finishing layer, what you spread after all the other garden tidying and planting is completed. Around trees, shrubs, and perennial plantings, it's important to keep the layer of mulch to only 2 to 3 inches. Adding too much mulch isn't healthy for plant survival. Most plants need oxygen at the roots to grow well, so don't think that piling mulch materials on 6 or 8 inches deep is a good technique. Sometimes gardeners adopt the habit of simply pouring more bark chips on the landscape each year, until the layer gets quite deep. Keep it to 2 to 3 inches.
   Rhododendrons, for instance, have fine, fibrous root systems that are close to the surface of the ground, and they are easily smothered by too deep a mulch. Don't scuffle with a rake under rhododendrons, and don't heap too much mulch under them.
   The other application tip for mulch is to keep the trunks and crowns (growth areas above the roots of perennial plants) uncovered. Don't stack the mulch up against a trunk. The mulch, as it gets wet, can hold moisture against the trunk and may lead to decay or disease. In rural situations, mice and other rodents may move into the mulch for winter warmth and gnaw at the bark, harming the trees. So, allow all plants room to breathe freely and don't pile mulch around trunks.
   In spring, to tidy the garden, pull weeds. Add fertilizer for trees, shrubs, and perennials if your garden requires it. March is the perfect time for fertilizing permanent plants in the landscape, just as plants stir into growth and need nutrients. Don't over-fertilize, and don't fertilize after about July 1, to allow plants to harden off growth for winter. Don't use granular fertilizers without providing water to the plant roots, either as rain or irrigation. Compost and mulches are not fertilizers. They condition the soil, and supply some nutrients in very small quantities. They do not substitute for fertilizer when it is needed.
   If your garden needs soaker hoses, lay them out over the ground after weeding and fertilizing. Then add the mulch. This finishing layer will smother annual weed seeds to keep them from decorating the garden with a froth of new green unwanted shoots. It also holds in soil moisture and prevents water loss. Keep the mulch layer at 2 to 3 inches.
   What materials make good mulches? Many materials, such as wood chips, shredded leaves, home-made composts, commercial composts, and even dried grass clippings work well as mulches. Bark and wood chips are often what people think of first as ground layers. Bark breaks down slowly, looks good in the landcape, and is a good choice for permanent tree and shrub plantings. The bigger the bark size, the more weed control it provides (chunky pieces don't have inviting surfaces for weed seeds to germinate on.) Many people prefer the appearance of smaller dimension bark mulches, which also are long-lasting in the landscape.
   Finer mulch materials, such as compost or shredded leaves, break down and work into the soil relatively rapidly, within one or two years. These are good choices for annual and perennial gardens, for vegetable gardens, and anywhere a crumbly, natural-looking attractive surface is desired. Commercial compost is readily available, and comes in several types.
   Another recycled mulch material is municipal biosolids, composted with sawdust and returned as a soil amendment. It's also possible to purchase steer manure mixed with sawdust. Visit a landscape materials' supplier and compare the various materials if you plan to buy in bulk. They all work well as soil amendments and mulches, so individual choice depends on the appearance of the material and personal preferences. The commercial materials are free of weed seeds and uniform in texture.
   Many mulch materials can be gathered without cost, the most common being fallen leaves. Smaller leaves, such as those from aspen, Japanese maple, or birch, can be re-used as mulch without shredding them first. Larger leaves, such as big-leaf maple, don't work well if laid on the ground whole, because they pack down. Avoid any leaves that are laden with fresh seeds: spreading them out may result in planting a maple forest in the garden where you just pulled out the other weeds. (I speak from painful experience here.)
   Grass clippings work as a mulch, but only if they are laid on a tarp to dry out for two or three days. This process may be just too much work; if so, use the grass clippings as a component in compost, or use a mulching mower and allow the clippings to fall back on the lawn. Homemade compost, either finished or part-decomposed, works well as a mulch and looks natural in the garden.
   Pine needles fall in major quantities in some gardens. They can be used as mulch under shrub and tree beds. They do not acidify the soil significantly, but they do break down very slowly. Weed seeds don't germinate readily in a pine needle mulch. They can get tangled in rakes and be slightly frustrating to use, but they make a handsome mulch in the right situation.
   Sawdust straight from the saw can be used as a mulch. Unless it is aged, however, the sawdust will take nitrogen from the soil, and thus deprive plants of it while decomposing. A way to prevent this is to add about 1 1/2 pounds of actual nitrogen per cubic yard of sawdust, mixing it in before spreading the sawdust. This amount means using 7 pounds of 21-0-0 (ammonium sulfate), or 4.5 pounds of 33-0-0 (ammonium nitrate). A complete fertilizer like a 10-20-20 could also be added, which would also add phosphorous and potassium to the mix. (Use 15 pounds 10-20-20 per cubic yard of raw sawdust.)
   Whatever material is chosen, consider mulching an essential gardening technique. Tucking it around plants provides some of the same maternal pleasure as tucking blankets around sleeping children. Even though we know the plants can fend for themselves, the mulch adds an effective layer representing the gardener's care.

Visit WSU's Agriculture Site on the World-Wide Web at http://www.cahe.wsu.edu.