Gardening Basics

Mulch Matters

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

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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.

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