Mulches provide many benefits to the landscape, including reducing weeds, retaining soil moisture, moderating soil temperature, and adding a pleasing textural covering to the ground in a formal garden setting. Tree bark is among the longest-lasting organic materials for mulch. Use the appropriately sized bark mulch pieces. Large chunks are coarse and loose-fitting and are difficult to walk on. Shredded or tiny pieces are pleasant to walk on, form a denser mat, but may be so lightweight that heavy rain washes them away.
According to the American National Gardening Association, any product labeled as "bark mulch" must be at least 85 percent bark of that named tree species. The remaining 15 percent can be any variety of organic material, but often it is wood chips from the tree. By contrast, any product merely labeled "mulch" needs to contain only 70 percent of the named material.
Bark mulches tend to last considerably longer than mulches comprised of wood. Bark will last upward of two years in the garden, depending on your climate and rainfall, outlasting any leaf, needle or grass clipping mulches that decompose within months. Larger-sized bark chips will endure longer than those of a smaller size, such as those called "nuggets" or "fines."
Depending on where you live, certain tree bark mulches may be more common, more readily available, and less expensive. In the United States, common tree mulches include pine, melaleuca or cypress. Local harvesting from development or storm recovery may provide other tree species' bark or wood mulches. Mulches made from softwood tree species, such as fir, may contain wood particles with lots of sharp, coarse fibers that can irritate skin.
Dr. Linda Chalker-Scott of Washington State University says that bark tissue is naturally more hydrophobic, or water-shunning, than other materials like wood or needles when used as a mulch. Suberin in the bark is a protective waxy substance that repels moisture. Smaller-sized and compacted bark mulch particles potentially can form a layer that shuns water from easily penetrating the soil. Chalker-Scott also comments that bark mulches may have considerably higher amounts of impurities in them, such as weed seeds, depending on harvesting and packaging techniques at the mill. Lastly, any logs that are exposed to saltwater will contain bark that retains salt and will leach it into the landscape soil, potentially to the detriment of plants.
Acquiring Bark Mulch
Garden centers typically carry a wide array of bark mulches for the gardener. The type or size of bark varies by region based on supply and demand or cost of production or transportation. Typically mulch is sold in plastic bags containing 3 cubic feet of material. Bark mulch also can be purchased in loose, bulk quantities but you often need large amounts for it to be cost-effective or practical. In the telephone directory, you may search under the terms "mulches" or "garden centers" to locate businesses with bark mulch. Also contact your local municipality to learn if any bark or mixed wood chip mulches are available to residents. These mulches result from crews removing old trees or branches dropped during storms.