Dark or Light? Mulch cools the soil by either absorbing 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 don't have to work so hard.
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 to 150 degrees F--hotter than asphalt. Since heat rises, that means plants can shut down and stop growing. The plants most prone to suffer? Those with large leaves and any that prefer partial shade or that can't tolerate heat.
"It depends on the extent of the mulch area," says Kjelgren. "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, which may interfere with the plant's adjusting to cold winter temperatures.
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. The odor of vinegar, ammonia, rotten eggs or silage means that 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 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.
