Adding mulch to your landscape provides all kinds of benefits for your garden. Mulch is “The Great Protector” of plants and soil. Think of it as a kind of shelter: it cools, warms, nourishes, provides moisture and keeps out unwanted intruders.

A layer of mulch provides warmth for early spring germination of plants because it helps retain heat in the soil. You can transplant hardened-off seedlings earlier and they will settle into their new home more comfortably. As the growing season progresses, mulch will stabilize the soil temperature. It also helps retain moisture which can be very helpful in cutting down on the amount of water you use in your landscape. Mulch also cuts down on weeds in a garden as it shades the soil from sunlight which may help prevent weeds from germinating. Finally, mulches can prevent soil erosion by wind and rain. In windy areas, gravel or rock mulch may be preferred over lightweight organic mulches. Any mulch that reduces the impact of raindrops will help reduce water erosion, just make sure the soil surface is entirely covered with mulch.

The ideal mulch does not compact readily. It allows water and air into the soil, it is not a fire hazard, and it breaks down slowly. In addition, the ideal mulch is uniform in color, weed-free, attractive and does not blow away.

Organic and Inorganic Mulches
There are two general kinds of mulches to choose from: organic and inorganic. Organic mulches include wood and bark chips, straw, grass clippings and seed hulls. Inorganic or inert mulches include weed-barrier fabrics, gravel and rock.

Selecting a particular type of mulch depends on what you want it to do. If you want to eliminate some plantings altogether to save water, an inorganic mulch like rock or gravel is one choice. There are many colors, sizes, and shapes of rocks and gravel to choose from available from landscaping supply companies who will deliver it to your doorstep. The most difficult parts of this type of mulch are costs and shoveling it into place. Rocks are heavy and are sold “by the pound!” Rocks also soak up heat and this extra heat can affect nearby plants.

Bark chips, an organic mulch, is also useful both to cover ground where you might want to eliminate plantings or to spread around plantings. It usually comes in small, medium, or large chunks and is, of course, much lighter to shovel into place. It can be purchased in bags or delivered loose, by the cubic foot, from landscape supply companies.

Soil nutrients can change due to the use of organic mulch. As organic mulches decompose, some of the soil nitrogen in contact with the mulch is broken down by soil organisms. Consequently, nitrogen deficiency may occur. A sign of nitrogen deficiency is a yellowing, primarily of the lower leaves. When this occurs, add nitrogen fertilizers. For every 100 square feet of mulched area, add 2 pounds of a complete fertilizer, such as 10-6-4, or 1/4 pound of ammonium sulfate. Never use a “weed-and-feed” type of fertilizer in mulched areas.

If the main objective is soil improvement, consider organic mulch that gradually breaks down. If the area is used primarily for annual flowers, it often is more practical to use a temporary organic mulch, such as composted leaves or grass clippings, that can be turned under each fall.

Black plastic (polyethylene) is not always recommended for mulch in landscape areas. Black plastic is impermeable and cuts down on oxygen and water entering the soil. Restricting oxygen and water to the roots significantly reduces plant growth. If a weed barrier is needed, use landscape fabrics instead to allow water and oxygen exchange. However in some instances, black plastic is used to mulch vegetables. For more information on mulching in vegetable beds, see this post:

Grass clippings make excellent organic mulch for the vegetable garden. Apply fresh clippings in thin layers (up to ¼ inch thick) and allow each layer to dry before adding more. The clippings quickly dry and you can add more layers weekly. A few layers will stop weed seed germination. Do not place fresh clippings in thick piles, as they will mat, reducing water and air infiltration, and may cause an unpleasant odor. Do not use clippings from lawns that have been treated with herbicides or other pesticides in the past month. Around lettuce and other leafy vegetable, mulch by carefully hand-placing the grass at the base of the plants.

When and How to Apply
Apply mulches used to enhance appearance and control weeds at any time. If the mulch will be used to protect fall transplants by keeping soil temperatures above freezing longer into the fall (permitting better root growth), apply soon after transplanting. Reapply or refresh organic mulches after they have begun to break down.

If the mulch is meant to reduce frost heave and delay spring growth, apply after the ground has frozen. This type of mulch often is used to protect small bulbs such as crocus and to prevent plants from sprouting too early in the spring.

Apply most mulches to a depth of 3 to 4 inches. Apply straw, dried leaves and similar materials to a depth of 4 to 6 inches.

Newspapers can also be used under wood chips or rocks. The soy-based inks in newspapers are safe. Don’t use glossy magazines and color printing as those inks may contain heavy metals or other chemicals that could contaminate soil. Lay down the newspapers a few sheets thick and top with chips or grass clippings. When changing grassy areas to planting beds, be sure to kill the grass prior to adding the newspaper/mulch layer.

An excellent list of mulch types can be found at this online CSU publication:

Contributed by Deb Ross, Certified Colorado Gardener