Keep Roots Cool with Mulch

In addition to keeping the soil cool, mulches also provide benefits by reducing extreme fluctuations in soil temperatures near the soil surface. In hot weather soil temperatures near the surface may show fluctuations of as much as 38 degrees within a 24 hour period while the same soil under a mulch might show only a 12-degree fluctuation. Even without mulch, leafy plants can control temperature fluctuations beneath them simply by shading the soil.

Small or newly planted trees and shrubs, or plants that have been partially defoliated by insects or disease, may suffer root damage because their natural means of reducing soil temperatures beneath them has been hampered. The nature of the mulch itself can influence how well it insulates the soil against heating by the sun. Generally, a mulch may have temperatures as high as that of bare soil. However, it protects the soil, especially if it is a poor conductor of heat. This means that a loose mulch protects against high temperature better than a compacted material.

The color of the mulching materials may also influence soil temperature. Lighter-colored mulches have greater reflectance and keeps soil cooler. There is only one time when the light colored mulch may be a disadvantage. This is in early spring when the mulch may keep the soil from warming, and thereby delay the development of some crops. However, while mulch may delay the development of tomatoes or melons in the garden, a spring mulch is not a great disadvantage to taller trees or shrubs. Black plastic mulch in the early spring can actually help by warming the soil and air around the crops.

In summer it might better be covered with straw or another mulch to avoid excessive heat build-up unless foliage provides shade over it. In addition to keeping soil temperatures more constant, organic mulches also benefit by keeping soil moisture more uniform. This is especially important during the hot and dry periods mid-summer.

One of the important benefits of a loose, porous mulch, in addition to being a poor conductor of heat, is it allows easy penetration of rain or irrigation water. Such a mulch reduces the impact of the rain drops or irrigation on the soil surface, and reduces the tendency of many soils to form a surface crust. This too, allows better water penetration and less run-off. Mulches that provide the best penetration are those that remain loose. Any that tend to form a surface cover that might shed water should be avoided.

http://en.wikipedia.org/wiki/Mulch