Mulches for landscaping

As popular as they are in contemporary landscapes, mulches are not a new concept. For as long as trees have grown in forests, leaves and needles have fallen to the ground, matted together, and formed a natural protective layer over the soil.

The English word mulch is probably derived from the German word molsch, meaning soft, beginning to decay. It no doubt referred to early gardeners' use of straw, leaves, and loose earth spread on the ground to protect the roots of newly planted trees and shrubs.

Many different natural and synthetic mulches are available today, but all perform at least three basic functions:

• Reduce soil water losses.
• Suppress weeds.
• Protect against temperature extremes.

In one study comparing various mulch materials with bare soil, soil moisture percentages in mulched plots were approximately twice as high, summer soil temperatures were reduced by 8 to 13 degrees, and the average amount of time required to remove weeds was reduced by two-thirds.

The use of mulches in landscape plantings provides other benefits as well. When water droplets land on bare soil, the impact causes soil particles to fly in all directions, resulting in soil crusting and slow water infiltration. Most mulches break the impact of the droplets, reducing soil erosion and crusting and increasing the penetration of water into the soil.

In addition, mulches improve soil structure in several ways. As organic mulches decompose, they provide organic matter that prompts soil particles to aggregate. Large aggregates increase aeration and improve moisture conditions in the soil. These conditions, in turn, encourage additional root development and biological activity, further enhancing soil structure.

Mulched soils are cultivated less frequently than bare soils. It is now known that soil structure is improved by not cultivating, but this relationship was not always understood.

At one time, it was a common practice to cultivate the surface of soil to form a dry "dust mulch." The theory was that the dust layer would obstruct the upward capillary movement of subsoil water and thus reduce soil water losses.

Research over the past 40 years has shown that dust mulches conserve moisture in soils by inhibiting weed growth. But because they are susceptible to wind erosion, dust mulches actually have a negative effect on soil structure.