Gardening Basics

Mulch Matters

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

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Crushed lava stone acts as mulch for paths around an eye-catching assortment of cacti, succulents and desert-loving grasses.

If trees, shrubs and plants could vote on what covers their feet, it would be mulch every time. Not only do mulches dramatically reduce evaporation of water from the soil surface, but by reducing weeds they help prevent competition for water. As a result, soil moisture stays more constant and roots grow better.

Organic mulches save a bit more water than stone mulches, but just about all mulches do a good job. And almost any soil cover is better than none at all.

Doug Welsh, professor of horticulture at Texas A&M University in College Station, has conducted a number of mulch trials. "On bare soil, two-thirds of the water we applied was lost through evaporation," Welsh says. "When the soil was mulched, only 10 percent of the water was lost from evaporation." Welsh recommends using a mulch that has both large and small pieces so that it will receive water well and also won't easily float or blow away. Use mulch on potted plants too.

So mulch is good. But what kind of mulch? And how much? HGTV Ideas went to the experts for some answers.

Organic or Inorganic?

The Anasazi Indians in the ancient Southwest successfully gardened in arid land, thanks to the gravel and rocks they placed on the soil's surface. Inorganic mulches--rocks, gravel, marble, brick chips--conserve water and shade the soil, but they don't improve it.

Use those mulches in fixed landscape beds that you don't plan on redigging and replant-ing. Small gravels migrate easily, working their way down into the soil. Even fairly large rock or brick nuggets can wash or get kicked out of their landscape beds.

No matter what kind of soil you have, an organic mulch is bound to eventually improve it, even if it never gets turned in. "Organic matter is the magic elixir," says Mike Arnold, associate professor of landscape horticulture at Texas A&M. "It does good things for poor soils of almost any type. If the mulch stays on top, the change will happen more slowly, but earthworms and microorganisms will slowly break it down and mix it in. And of course every time you disturb the bed in any way, such as when you're planting annuals, it happens much faster."

Organic mulch adds fertility to sandy soils and helps hold water and nutrients; it loosens and helps drain heavy clay soils; it adds micronutrients that might be missing from even a good garden loam.

The organic mulches highest in lignin--an organic compound in woody plants--take the longest to break down. Bark has more lignin than wood, so bark mulches last longer than wood mulches. Cypress and pine straw last almost as long as pine bark.

Also, the faster the mulch percolates water and the drier it stays, the longer it lasts. "Pine-bark nuggets will last a long time," Welsh says."There's a lot of air space between the nuggets, and the microorganisms don't have enough moisture to break them down."

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