I admit to being a fanatic about mulching. I think it might be the single most important thing you can do for the health and appearance of your garden. And mulching is way up there on my list of small things we can do to help improve the environment as well.

Mulches are organic and inorganic materials layered on top of the soil. They differ from soil amendments, which are materials mixed into the soil to improve its structure. Mulches help reduce water evaporation, keep soil temperatures constant, prevent weed growth, reduce soil erosion and enhance the appearance of your yard.

Many studies suggest that using mulch can cut landscape water use by 25 percent or more. Since mulch helps retain soil moisture, you will not need to water as frequently. Mulch reduces the soil’s absorption of heat so soil temperatures don’t fluctuate as much, which helps root growth. As organic mulches decompose, they feed the soil and encourage earthworms and beneficial microorganisms.

Bare sun-baked soils promote erosion. Mulches reduce the force of both irrigation and rain water, minimizing both erosion and soil compacting.

And a huge plus in my book is that mulches reduce weed seed germination by excluding light. Over the years, I’ve maintained a three- to-four-inch-thick layer of mulch throughout my entire garden, and I now seldom need to weed.

Commercially available mulches include decorative bark, compost, redwood sawdust and even composted manure and sludge mixtures. Using yard waste as mulch not only improves your garden but also helps with the growing problem of urban waste disposal.

Wood chips, leaves, grass clippings and municipal tree trimmings are all mulch sources. Make sure they are at least partially decomposed or they may temporarily rob the soil of nitrogen. To be on the safe side, add a little slow-release nitrogen fertilizer to the soil before applying the mulch.

Be especially careful about mulching with grass clippings. They should be dried and mixed with other material or they will compact into a slimy mat that prevents water penetration. If you use herbicides on your lawn, fresh grass clippings may harm your other plants.

You can mulch with inorganic products such as pea gravel or colored rocks. But be aware that those light-colored materials will radiate summer heat around your plants, and they do not retain moisture as effectively as organic materials.

 The best time to apply mulch is in spring when the soil begins to warm up. But if you missed that opportunity, it is not too late. Before applying mulch, rough up the soil with a rake to improve aeration, water thoroughly if the soil is dry, and remove weeds.

Three to four inches of a medium-coarse mulch, such as medium bark chips, is sufficient. Use less if your mulch is finer and more if it is coarser.

To prevent crown rot, “donut mulch” around woody plants, spreading the mulch in a donut shape around each plant. You want to cover the root zone but keep the mulch at least two to four inches from the trunk of the plant.

To maintain the desired thickness, add new mulch every couple of years as the existing mulch decomposes. You don’t need to remove the mulch when it is time to fertilize. Water well after fertilizing and the fertilizer will reach plant roots.

Leave some holes in the mulch so bees have some bare ground to build their nests. Bees pollinate more than half of the foods we eat, and their populations are declining. It is in our best interest to create habitat for beneficial ground-nesting bees.

One of my Master Gardener friends says that mulch is like tan over fat. It doesn’t fix the problem but it makes you look better. Mulch conserves water, produces healthier plants, recycles yard waste, prevents erosion, minimizes weeds and makes the garden look neater — all compelling reasons to mulch.

Napa County Master Gardeners are available to answer questions in person, by phone or on their Web site. Call 253-4221 or visit www.mastergardeners.org for information.