The wonderful world of mulch

Preparation: All mulches have benefits and drawbacks, so do your homework before buying.

In The Garden

October 17, 1999|By Nancy Taylor Robson | Nancy Taylor Robson,Special to the Sun

In my callow youth, my mother, a devoted gardener, periodically forced me to listen to her expound on the joys of mulch. I was bored beyond belief. Now, I view mulch as garden gold and bore my own children with a litany of its charms. Such is the march of generations.

There are two kinds of mulch: organic and inorganic. Inorganic mulches include plastic, woven fabric (aka landscapers' cloth), gravel, marble chips and sand. Organic mulches include pine bark, straw, leaves, grass clippings, newspaper, sawdust, wood chips, cornstalks or corncobs, pine needles, compost and nut hulls.

Mulch serves several pur-poses. It keeps weeds down by smothering them. It keeps moisture in the soil -- a vital need in years like this. It moderates soil temperatures to help prevent heat stress in plants. It keeps fruits and vegetables off the soil, which helps prevent mold and rot. In addition, organic mulch benefits soil structure and content.

"It changes the structure of the soil dramatically if you use it annually over a period of time," notes Dr. Ben Coffman, an agronomisst at the USDA Agricultural Research Service at Beltsville. "It encourages microbial life and improves tilth." Tilth is the looseness or friability of soil.

All mulches have both benefits and drawbacks. Stone mulches work well on permanent landscaping, though marble chips, which are alkaline, will gradually leach into the soil and kill acid-loving plants like azaleas, rhododendrons, pine trees, camellias and blueberries. Black plastic, which is great for heating up the soil in spring, can raise soil temperatures high enough in summer to injure root systems. Additionally, it acts as a barrier to rain. Landscapers' cloth, even covered with a layer of shredded bark, can allow blown seed to root on top.

Organic mulches work well in perennial beds, on permanent landscaping plantings and in annual beds, though they need periodic renewing. Because they break down, some alter soil chemistry in the decomposition process. Pine needles, for example, add acid to the soil, and are good for acid-loving plants, but not for most vegetables.

The ubiquitous wood mulches we see in plastic bags and steaming in composting piles, come from fir, pine or hardwoods. Hardwood mulch is effective and looks nice on perennial beds, but unless properly composted, may contain high levels of manganese. Additionally, it can use nitrogen in decomposing, which then depletes the soil.

Neither newspaper nor straw affects soil chemistry, and both improve tilth, but straw and other organic mulches can harbor weed seeds. I've been fighting thistles for years, thanks to weed-filled straw (but I still use it). Ask your supplier or experienced customers about weed seeds in their mulch and hope you get an accurate answer. In using news-paper or printed matter, be sure the ink is soy-based. Most newspapers now use soy ink, but many glossy magazines use inks that are toxic to humans and critters.

With mulch, more is not neces- sarily better.

"The depth of the mulch should be no more than 2 to 3 inches," says Denise Garman, a horticultural consultant at the University of Maryland Home and Garden Information Center. "Over-mulching can prevent water penetration, and can cause stem decay and suffocation of shallow-rooted ornamentals."

"Azaleas and rhododendrons, especially, don't want anything close to their trunks," adds Anne Hedgepeth, proprietor of Hedge and Path, a garden-maintenance business in Kent County. "It can cause disease. And shallow-rooted plants send roots out into the mulch," which is one reason weeds are easier to pull from mulched beds.

For several years, the USDA Agricultural Research Center at Beltsville has been studying the benefits of hairy vetch in moisture retention during drought. Vetch, an annual legume (which fixes nitrogen in the soil), is planted in mid- to late September, and is cut down the following May. The object is to get maximum vetch growth to accumulate the maximum amount of nitrogen and mulch. Then, corn is planted through the cut vetch. Even in mid-August this year, an unirrigated field of vetch-mulched corn retained enough moisture to enable a researcher to reach beneath the vetch mat and grab a handful of amazingly loose soil. Researchers have recently used it on tomatoes, too. But, like everything else, vetch has drawbacks.

"It can reseed [and take over] if you don't manage it correctly," warns the USDA's Coffman, who says the trick is to cut it before it makes seed. "Mow it once it's in flower," he advises. "Sometime in early May, depending on the weather."

For hairy vetch seed:

*John T. Harris Inc.

S. Dupont Blvd.

Milford, Del.

302-422-4565

*Johnny's Selected Seeds

1 Foss Hill Road

RR 1 Box 2580

Albion, Maine 04910-9731

207-437-4395

*"Secrets to Great Soil," Elizabeth P. Stell (Storey Communications Inc., 1998)

The right stuff

In choosing which mulch to use, consider:

*The longevity of material

*Permanence of planting

*How decomposition or leaching will change soil chemistry

*Convenience and ease of use

*Aesthetics

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