Cover bare soil with anything from straw to rocks to wood chips, and that material is considered mulch. While mulch isn’t particularly old-fashioned, it can be a beneficial addition to any garden, old or new. Mulching discourages weeds, adds nutrients to the soil, and protects it from both wind and rain. Here, then, is what you should know about choosing mulch and what you need to consider before adding it to your garden.
Among historic gardening books, there’s no mention of mulch before the middle of the 20th century, except for dust mulch—the surface layer of loose dirt left after the soil has been worked over with a hoe. Dust mulch was believed to conserve water by slowing evaporation from deep down in the soil, but there’s no proof that it works.
Modern mulching got its biggest boost from Ruth Stout, a popular author and early proponent of organic gardening who is now considered the grande dame of mulch. On the back cover of her 1955 bestseller, How to Have a Green Thumb Without an Aching Back, Stout visually demonstrated one advantage of mulching in her author’s photo: She’s pictured reclining on a thick pile of straw mulch, with no weeds in sight.
While smothering weeds is one dramatic dividend of mulching, there are others. Dust mulch aside, blanketing the ground with a layer of straw, pine needles, or other mulching material slows water evaporation from the soil surface. The fluffy covering also keeps the soil loose and ready to absorb rainfall so that plants will need less watering from you.
Mulches offer less obvious advantages, too. Organic mulches, for instance, are made from materials that were once living and that decompose over time, releasing plant nutrients such as nitrogen, phosphorus, and potassium. Those decomposing materials also create humus, a witches’ brew of complex organic chemical compounds that helps plants absorb nutrients, fight disease, and keep soil loose and moist to promote good root growth. In addition, mulches are insulators that moderate temperature. They keep the soil from becoming too hot in summer or too cold in winter, and retain the ground’s warmth in the autumn, giving roots more time to grow.
Although Stout kept the ground permanently blanketed with hay, you’ll get better results by fine-tuning the placement of mulch to the types of plants you grow. Because annual flowers and vegetables need to put down roots quickly in their one-season growing time, organic mulches should be spread around them in spring once the weather has turned reliably warm. Otherwise, the insulating properties of mulch will delay soil warming and root growth. Leave a little breathing space between the mulch and young plant stems, or they are likely to rot. During the winter, the mulch can remain where it is, provided that you remove or pull it back by early spring to let the sun warm the soil where you intend to plant.
Perennial flowers, on the other hand, have roots that endure year after year, so you can leave mulch tucked around them all year long. In areas where plants need additional protection from winter weather, you can also toss organic mulch right on top of perennials after the soil has frozen about an inch deep. Covering plants before then or leaving them covered after the spring growth begins to peek through the mulch can cause leaves, stems, and roots to rot.
Trees and shrubs also benefit from mulching. After all, in the wild, each year’s leaf drop is, in effect, a mulch. Observe two points when mulching trees: First, don’t pile mulch right up against trunks or woody stems, because rot could result. Second, take precautions against mice, as mulch provides a nice home from which they can gnaw on a tree’s bark. Always keep mulch a few inches back from trunks, and on younger plants, you should also use cylinders of hardware cloth or tree protectors to act as an additional buffer in winter. As with perennials, mulch can remain in place year-round.
Mulches are especially useful in helping new trees, shrubs, and perennial flowers through their first winter, when the freezing and thawing of soil can heave plants right up and out of the ground before they are firmly rooted. The insulating properties of mulch help prevent the wide swings in soil temperature that lead to heaving.
Manic for Organics
Except for pea gravel, which looks just right in rock gardens, I only use mulches based on organic materials because of the plant nutrients they provide. Not all plants have the same nutritional needs, something you should consider when selecting a mulch. Compost, for example is rich in nutrients, so I use it in my vegetable garden beds and around flowers such as delphiniums, monkshoods, and roses that are hungry feeders in need of lots of nutrients. Flowers that thrive in less fertile soils, such as coneflowers, liatris, and yarrow, get mulched with wood chips or autumn leaves, which are relatively poor in nutrients.
Another bonus of organic mulches is you can obtain many of them free. I have a standing request with local arborists for wood chips, which they dump at no charge in a corner of my garden. Which mulch looks best depends upon your taste and style of garden, but I don’t think artificially colored mulches, landscape fabrics, or plastic films look good anywhere. The latter especially don’t offer any nutritional benefits and can even deprive soil of oxygen.
Finely divided organic mulches, such as shredded leaves or various kinds of hulls and shells, are best suited to formal gardens. Rougher materials, including straw and wood chips, are better for informal gardens. In old-fashioned gardens, the best mulches have a fine texture and include compost, buckwheat hulls, leaf mold, and shredded leaves. They provide nutrients and protection but remain unobtrusive, keeping the spotlight shining on your old-house garden, where it belongs.
Lee Reichwrites about mulching in his book Weedless Gardening (Workman Publishing, 2001).