Mulch for the Home Landscape

When spring arrives, gardeners participate in the annual ritual of mulching their gardens. Used correctly, mulch benefits the garden by holding moisture, cooling the soil, suppressing  weeds, preventing erosion, and protecting trees and plants from damage by mowing equipment. This article will explore kinds of mulch available and the correct ways to use them in the home landscape.

Organic Mulch

There is nothing new about mulch. Trees have been dropping their leaves onto the forest floor, protecting roots and enriching the soil, for millions of years. Organic mulches are derived from living sources such as leaves, pine needles, wood chips, bark, straw, grass clippings, or cocoa bean shells and mimic the mulching activity of the forest. Organic mulches eventually decompose, adding nutrients to and becoming part of the soil.

When deciding what kind of organic mulch to use, consider the source of the material. When I called several local mulch purveyors, I learned that wood mulch can be derived from many sources.  Landscape rubbish such as trunks, branches and leaves from tree removal and pruning can be shredded into mulch. There is no way to know if pesticides were applied to trees or shrubs before they were shredded. Wood waste from construction, including pallets, can be chipped and added to tree mulch. One supplier said they removed any nails before chipping construction debris wood into mulch. It is harder, if not impossible, to derive the source of mulch when purchasing a bagged product. Prior to 2004, wood for constructing decks and plays sets was treated with chromated copper arsenate (CCA) to protect it from insect and fungal damage. While still used commercially, wood treated with these pesticides is no longer used in residential construction. For more information, see the EPA website: EPA Chromated Arsenicals (CCA). Old treated wood is still in existence but is not to be used in mulch. The Soil and Mulch Council is a trade association with voluntary membership. A seal from the Council on a bag of product certifies the product does not contain wood treated with CCA. For more information, see Soil and Mulch Council. The buyer still does not know where or how the mulch was derived or whether other pesticides are present.

Cypress mulch is prized for its ability to resist decay. Available in bags at big box stores and landscape supply retailers, its use has become controversial. Cypress mulch is harvested from wetlands in the lower south. These are fragile areas which do not regenerate quickly, if at all. Some cypress trees are harvested specifically to be turned into mulch, and with the decline of their wetlands, some environmental groups are calling for a boycott of cypress mulch.

Colored mulch is made from wood recycled from construction and demolition sites (C&D) and pallets. Very dry, it absorbs color easily. While it is beneficial to repurpose this wood and the dye itself has not been found to be harmful, there is no way of knowing if the wood itself is contaminated with toxic substances such as CCAs. The University of Massachusetts has discerned that some soil under colored mulch has been contaminated with CCAs or other toxic substances.

Leaves, pine needles, and branches chipped from your own property are hopefully free of pesticides and herbicides and can be used worry free. Rake and chop leaves from the lawn and  spread them as mulch; allow fallen leaves to stay on the beds. As well as suppressing weeds, holding moisture, and enriching the soil, fallen leaves also provide a safe haven for overwintering bees, butterflies, and other insects. As in the forest, new growth will pop up through the mulch in the spring.

How to apply organic mulch

• Remove weeds and grass by hand. Do not use herbicide.
• Edge the bed to keep the mulch in place and create a neat line.

  

•  Spread wood mulch 2-4 inches thick.  Less than 2 inches will not do the job; more than 4 inches will prevent water from penetrating to the roots.
• Do not allow mulch to contact the trunks of trees and stems of shrubs. Keep the root flare of trees exposed. Mulching too close to the trunk can allow disease, insects and rodents an entry point. Over time, moisture in mulch can cause decay in the bark.
• Avoid mulch volcanoes, which are piled against the tree trunks. Volcanoes prevent moisture from penetrating to the roots and instead encourages them to grow up into the mulch, searching for water.
• Water a freshly-mulched area deeply so moisture penetrates to the roots. Shallow watering will encourage roots to grow upward in search of moisture and will actually require more watering.
• Over time, wood mulch will fade and become compacted. Fluff it with a rake to improve its appearance. Top with a thin layer of mulch when the original mulch has decomposed.
• Leaf mulch is self-sustaining and will replenish itself when leaves drop in the fall. If mulching with leaves in a windy area, scatter a few wood chips over the leaves to hold them in place.

Inorganic Mulch

Inorganic mulches include rocks, crushed stone, pea gravel, crushed brick, landscape fabric, plastic sheeting, rubber chips, and rings. Rocks and stones are used in areas where no planting is expected and are themselves considered part of the décor. Stone mulch is not maintenance free as weeds can take hold when seeds land on top and take root. Pea gravel is notorious for becoming weedy. Rocks and stones also have limited ability to hold moisture and do not add nutrients to the soil. They also absorb warmth from the air and can heat up the soil. This can be detrimental to organisms and microbes living in the soil below. Landscape fabric and plastic sheeting are not used alone as mulch. Landscape fabric is used as a base beneath mulch. Because it is semi-permeable, water can seep through. Plastic sheeting is impermeable and no water can get through. Not typically used in landscape beds, it is occasionally used under organic mulch in paths. Neither of these barriers add nutrients to the soil, and both prevent worms, insects and microbes from moving freely, inhibiting their ability to enhance the soil.

Tree rings and pathways made of rubber are now available in many garden catalogs. I have not found any research determining whether rubber material damages the soil or plants, but like plastic sheeting and landscape fabric, rubber heats up the soil, potentially damaging wildlife below and adds no nutritional value to the soil. Rubber also looks artificial and out of place in a natural setting.

Living Mulch

Using plants to cover the soil is a beautiful alternative to other forms of mulch. In place of invasive ground covers such as liriope, ivy, vinca and pachysandra, consider covering the soil with native plants. Allow the native plants to spread by roots, runners and seeds until they cover the ground. With a living mulch, expect a dynamic and changing landscape rather than a static bed where every plant stays where you put it. As the plants expand and grow, they will crowd out weeds, prevent erosion, and prevent run-off with their deep roots. As a bonus, native plants provide food for bees, butterflies and other beneficial insects and offer overwintering habitat for them. Using one type of native plant as a ground cover is an option, but a diversity of plants will create a living landscape, appealing to a variety of insects, birds and other living creatures. Unlike a traditional flower bed, the plants comprising a living mulch are allowed to mingle and fill in all of the allotted space. Over time, the garden becomes self-sustaining. The chore will mostly be thinning out plants that become too exuberant or too large for their space. With the ground covered with plants, there is no need to purchase and spread wood mulch every year.

Tips for using native plants as mulch

When choosing a combination of native plants, base your selection on which plants grow well in the same conditions – moist or dry, sun or shade. Rather than just squeezing in plants close together, be aware of their habits so they will nestle appropriately as they mature.  Unless you want a monoculture, avoid plants that are rampant spreaders and cannot share a space. Low-growing ground covers, whether used alone or in combination to make a tapestry are always appropriate. Another option is to plant in layers, incorporating some taller plants that look good in all seasons with shorter plants taking the stage later, and ground covers weaving through. Try for a sequence of bloom, with new plants arising as others diminish. Under an oak tree in my garden, creamy violet (Viola striata) is the ground cover base. Pussy toes (Antennaria plantaginifolia) grows in a sunnier, rocky area near the driveway, and woodland stonecrop (Sedum ternatum) mingles along the edges.  White wood aster (Eurybia divericata) provides a low, green ground cover behind the violets in early spring. It matures to two feet in summer and sports white blooms August through September.  Spikes of blue-stemmed goldenrod (Solidago caesia) bloom in the fall.  In a sunny area, achieve the same effect by mimicking a meadow or grassland. Sun-loving milkweed (Asclepias), black eyed susan (Rudbeckia hirta), and goldenrod (Solidago) can be punctuated with the spikes of a grass such as little bluestem (Schisachyricum scoparium). In a moist area, vary the plantings with bee balm (Monarda), Joe Pye weed (Eutrochium), and blue mistflower (Conoclinium coelestinum). In a new planting, mulch the area with wood chips or leaves until the plants fill in. In a woodland area, allow fallen leaves to stay on the ground, providing a natural mulch.  Do not cut back stems of living mulch in the fall. Plants left in place after they stop growing provide winter interest and offer seeds to birds and overwintering sites to insects. Wait until the soil is warm in the spring to do a final cut back to protect any creatures over-wintering in stems. If possible, cut the stems into smaller pieces and leave them in place as more mulch.

Refer to these sites to learn which native ground covers are suitable for your area:

Plant Virginia Natives

MGNV Native Ground Covers

Piedmont Master Gardeners The Garden Shed: Cover the Soil With Native Plants

Summary:

• When used properly, mulch benefits the plants by holding moisture and discouraging weeds. It enriches the soil as it decomposes.
• Mulch with leaves from your own property as much as possible to mimic the ecological activity of the forest.
• Consider the source when using wood mulch.
• Avoid colored mulch which has been found to leach toxins and do not use rubber pellets or rings which look out of place in a natural setting and provide no benefit to the soil.
• Stones are decorative mulch and should only be used in areas that cannot or will not be planted. Stone mulch offers little benefit to plant heath and no benefit to the soil. Pea gravel becomes very weedy.
• A living mulch of native plants crowds out weeds, hold moisture with their deep roots, prevents run off all while adding both beauty and ecological value to the landscape. Native plants will spread, eventually covering the soil and reducing cost and labor for the gardener.

 

Featured Photo:  Deborah Harriman

SOURCES:

Colored Bark Mulch, UMass Amherst

Mulch – A Survey of Available Options

Mulching Trees and Shrubs, U Maryland Extension

Should I Boycott Cypress Mulch? Michigan State University Extension

Springtime Mulching, Virginia Cooperative Extension