Gardening Basics

Mulch Matters

Chunky or fine? Wood or rock? Here's what you may not know about mulch.

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Can Mulch Change Soil pH?

Most people believe that mulches like pine straw and pine bark will turn the soil acid below. Actually, that's true only some of the time. Some soils--such as clay and those with plenty of organic matter--are extremely resistant to change. Many studies have shown little or no effect on soil pH, even with pine straw on an already acid soil. "If you've got a material that's acid or alkaline," says A&M's Mike Arnold, "it will probably have a bit more impact on sandy soil than on a clay soil."

How Much Is Too Much?

You can easily suffocate a plant by mulching too deeply. The same goes for laying any impermeable cover--such as plastic or several layers of cardboard--that doesn't allow the soil to breathe. Says Welsh, "If you keep gas exchange from happening, that's where you run into trouble. You stick your finger under there, and you've got a swamp. It smells sour, and there's anaerobic activity. That's when you get into root problems."

The less porous and more compactable the mulch, the thinner you spread it: for very finely shredded hardwood, no more than two to three inches deep; coarse nuggets, three to five inches; loose straw, up to six inches deep for wintertime protection of sensitive plants. Top off aging or discolored mulch with a minimum of new material.

How Close Should I Mulch?

Don't give wood borers and other insects easy access to your trees, shrubs and herbaceous plants: leave a space of eight inches between mulch and tree trunks or tender stems. Also be sure to keep wood products (and possibly termites) away from your home's foundations.

Is It OK to Use Sawdust?

Sawdust is fine to use as a mulch as long as it's aged and composted first to get rid of heat buildup. Because it ties up nitrogen at the soil surface, don't use sawdust around heavy feeders and plants that have roots close to the surface.

But you also have to know its origins, advises Arnold. "Does it come from treated wood? If so, some of the preservatives are toxic and also have the potential to leach. Also, is there any black walnut or other material harmful to plants?"

How to tell if it's aged enough? Says Arnold: "It will look discolored, more amorphous and more humusy. You might still be able to recognize it as sawdust, but it looks very much degraded." Cypress and redwood sawdust degrade very slowly.

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