It's about as sexy a subject as orthopedic shoes, though the silver film and chicken feather options offer some panache. But sexy or not, mulch has a big effect on the health, beauty and productivity of the garden.
"There's a huge benefit for weed control and moisture retention," observes Maree Gaetani, public relations director at Gardener's Supply Co. in Burlington, Vt.
Mulch keeps weeds down by smothering them. It keeps moisture in while keeping fruits and vegetables off the soil to prevent mold and rot. And organic mulch benefits soil structure and content.
"It improves tilth [soil looseness] for give better drainage and encourages microbial life," notes Larry Hurley, perennial buyer at Behnke's Nursery in Beltsville.
Recently, there's been an explosion of mulch options, both organic and inorganic. Inorganic mulches, which include plastic sheeting, woven fabric (landscaper's cloth), gravel, marble chips, sand and even recycled tires, are longer lasting than organics. For example, Tread Spread, made by Maryland Environmental Service in Millersville, has the staying power of the Energizer Bunny.
"It takes a minimum of 25 to 50 years to break down," says Nancy Faulkner, Tread Spread product manager. "It can be used in landscaping, and we have some that's perfect under a swing set."
Organic mulches include pine bark, shredded hardwood, planter's paper, straw, leaves, newspaper, sawdust, pine needles, compost, and cover crops such as hairy vetch, which adds nitrogen and tilth but, if not managed correctly, can take over. Scientists with the U.S. Department of Agriculture have even invented a biodegradable film mulch made of the 4 billion pounds of feathers churned out annually by the chicken industry. There are also choices among wood mulches. For example, some are freshly shredded, which robs the soil of nitrogen as it breaks down, while others are composted.
"Our 'licorice mulch' is triple-ground composted hardwood aged over a year," says A.J. Biermann, owner of King Mulch in Rosedale, which is also a state natural wood waste recycling facility. "So 90 percent of leaching is gone. The wood decomposes and provides a pH of 5-6."
King also sells "playground mulch," specially formulated kiln-dried wood mulch that lasts 12 to 15 years and won't stick to children's feet.
Mulches now also come in designer colors.
"We sell colorized shredded hardwood mulch," says Donna Shipp, garden supply manager at Homestead Gardens in Davidsonville. "They come in red, black and brown, and the color lasts one year without fading."
"But you only want to use them for walkways or around deep-rooted plants," notes Frank Gouin, retired University of Maryland horticulturist. "Most of it's not composted and can suck the nitrogen right out of the ground."
Plastic sheeting, once only black, is now available in colors too. But the color here is not so much a fashion statement as a productivity boost. Studies have shown that red plastic mulch increases tomato yields 20 percent compared with black. Green plastic mulches are recommended for cucumbers, melons and peppers. And silver film is great for broccoli and cabbage (cole) family crops.
"The silver is really shiny and that shine repels insects," says Josh Kirschenbaum, product manager at Territorial Seeds in Cottage Grove, Ore. "Also in areas where you get really hot summers, for plants that like a little cooler temps, the sun bounces up off it so it cools soil a little."
And finally, there is Bio-film, made out of cornstarch so it disintegrates completely in three months.
"It's best for lettuces and short-term crops," notes Kirschenbaum. "And it has a [limited] shelf life, so order it next year to use next year."
All mulches have both benefits and drawbacks. It's a matter of "right mulch, right place." Stone mulches work well on "permanent" landscaping, though marble chips and crushed limestone, which are alkaline, will gradually leach into the soil and kill acid-loving plants. Black plastic, which is great for heating the soil in spring, can raise soil temperatures high enough in summer to injure root systems. And if you blanket the vegetable patch in plastic, you need to be sure that water can get to plant roots. (A drip hose beneath the plastic or strategic watering does it.).
Organic mulches work well everywhere, though they need renewing. Because they break down, some alter soil chemistry. Pine needles, for example, add acid to the soil, and are good for acid-loving plants, but not vegetables. Newspaper and straw are both pH neutral, but straw can harbor weed seeds. Ask the supplier before buying. In using newspaper or printed matter, be sure the ink is soy-based. Non-soy inks can be toxic. Hardwood mulch, unless properly composted, may contain high levels of manganese.
7941 Pulaski Highway
Rosedale, MD 21237
Territorial Seed Co.
P.O. Box 158
Cottage Grove, OR 97424-0061
1130 Baltimore Ave., U.S. 1
Beltsville, MD 20705
734 W. Central Ave.
Davidsonville, MD 21035
Maryland Environmental Service
259 Najoles Road
Millersville, MD 21108
Pinetree Garden Seeds
P.O. Box 300
New Gloucester, ME 04260