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, Tex., 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."
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 replanting. 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 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 micro-organisms will slowly break it down and mix it in."
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 is a lot of air space between the nuggets, and the micro-organisms don't have enough moisture to break them down."
Dark or light?
Mulch cools the soil by either ab-sorbing heat from the sun and not transferring it to the soil (dark organic mulch, such as bark and wood) or reflecting the heat (light mulch, such as rocks or light-colored woods) so that it's not passed down to the soil below. In most cases that's good, because overheated roots must work harder.
In very hot or sunny areas, however, the heat radiated from the mulch can do a number on sensitive plants. Roger Kjelgren, associate professor of urban horticulture at Utah State University in Logan, has found that on a summer day dark-bark mulches can reach temperatures of 140 F to 150 F hotter than asphalt. Since heat rises, that means plants can shut down and stop growing.
"It depends on the extent of the mulch area," Kjelgren says. "In a large area of mulch, putting an oakleaf hydrangea out there would probably fry it. Over time, as the plants that do well establish sufficient cover, the mulch becomes shaded."
So if you're planning to use bark mulch in a sunny bed, make sure the plants are heat tolerant. This is especially important if the mulched area is large and the plants are young and small. Most inorganic mulches don't get as hot because they transfer some heat to the soil below. The exception lava rocks heat up almost as much as bark mulch.
Remember, too, that as winter approaches, a sunny day could interfere with the plant's adjustment to cold. Dark mulch can absorb the heat during the day and release it at night.
How should it smell?
If you're buying mulch from large piles that may not have been turned in a while, make sure you give it a sniff test first. Good mulch has a clean smell like fresh-cut wood or soil. A foul odor ammonia, rotten eggs means the mulch has gone sour, a toxic condition that results from too much moisture and too little oxygen. Sour mulch can seriously damage and even kill plants within 24 hours after application.
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 show 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," Arnold says, "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.
"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," Welsh says.
The less porous and more compactable the mulch, the thinner you spread it: for finely shredded hardwood, no more than 2 inches to 3 inches deep; coarse nuggets, 3 inches to 5 inches; loose straw, up to 6 inches deep. 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 8 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 reduce 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?"