All mulches do three things: suppress weeds, retain soil moisture and equalize soil temperature. However, all mulches are not alike. Inorganic mulches last season after season, rain or shine. Organic mulches, on the other hand, eventually decompose but improve the soil. Most mulches come in varied materials, colors and sizes to complement your landscape. Know your mulches and make the best decision for your garden.
Bark mulches, usually lumber by-products, include large bark chips and small bark granules. The bark chips, or nuggets, are usually evenly sized, resist compaction and keep their color for an extended time. Bark chips are slow to decompose and are not easily blown by wind or rain. Bark chips, according to the Cornell Cooperative Extension, often monopolize plant nutrients as they decompose and may harm growing plants. On the other hand, manufacturers sell pulverized bark granules as a soil conditioner. The small size of bark granules decomposes rapidly. Bark granules are not as effective as bark chips for weed suppression.
Wood chips, especially those from recently trimmed trees, contain shredded wood in various sizes as well as leaves and twigs. This diversity in material allows the chips to break down into useful nutrients, making wood chips a good choice for poor soils. The Washington State University Extension states that decomposing wood chips improve the soil structure, do not compact easily and are economical. However, wood chips fade quickly and usually require seasonal refreshing. The varied chip sizes and diverse material composition appear woodsy at their best, but often appear unkempt after windy weather.
Rubber mulches consist of shredded recycled tires. Manufacturers dye the mulch to create a rainbow of available colors, and the mulch is evenly sized. Generally, wind and rain do not move rubber mulches, and gardens appear neat and tidy over long periods. The mulch does not deteriorate quickly and does not improve the soil. Sunlight and microbes, however, eventually break down the rubber. In 2008, the EPA sent out a memorandum listing its concerns concerning rubber mulches; the original tires may contain heavy metals and toxic volatile organic compounds (VOCs). Current studies are still determining if decomposing rubber leaches these toxic chemicals into the soil. Rubber mulches also have a higher initial cost, but do not need annual replacement.
Manufacturers produce stone mulch in fine or coarse sizes. Decomposed granite, a fine stone mulch, is useful along pathways, while larger gravel prevents erosion along slopes or driveways. Landscapers often use stone mulches in xeriscape plans or low-water gardens. Stone mulches do not decompose or shift with wind or rain, and many gardeners create multicolored designs with stone mulch. However, stone mulches do migrate deeper into the soil over time. Walking and driving on stone mulch may also knock stone into lawns, warns the Cornell Cooperative Extension, where rotary mowers may fling the stone.
Straw mulch appears messy and wind-blown, but is a very efficient insulator to protect plants during cold winters; straw mulches may contain weed seeds. Cocoa hulls have a rich texture and are not readily available. Shredded-leaf mulches decompose quickly, offering a quick boost to poor soils.