The dreaded word drought has finally been spoken, although gardeners and farmers have known for at least two years that there was just not enough rain coming down, and the sad spectacle of our shrinking reservoirs was unmistakable. California is not only affected by climate change due to greenhouse-gas buildup, but it is also entering a historic dry cycle. Dendrologists (those scientists who study and count tree rings and such) say “that relatively wet periods like the 20th century have been the exception rather than the rule in California for at least the last 3,500 years and that mega-droughts are likely to recur.” Water conservation is a must, and one lowly form of maintenance, mulching, is not just a strategy but a necessity. One of the most effective ways to reduce evaporation and build a more resilient soil is to apply organic mulches.
Almost anything can be spread on the ground to help conserve the moisture below ground that would be lost through evaporation, but organic mulches have other important benefits. Layering compost, wood chips, or any organic material on the soil surface will not just reduce water loss. It is through the breakdown of the layer of mulch that will also increase the organic matter component of the soil below, providing microorganisms with a source of food. Numerous studies of soils rich in fungi and other microscopic beings shows that they work to keep the soil open and increase both the oxygen and water available to plant roots. Some of the fungi are downright magical, too. Mycorrhizal fungi are capable of forming symbiotic relationships with roots and can increase the amount of water the plant is able to access at least sevenfold (maybe more).
The other main function of mulch is to smother weeds. Eliminating weeds in and of itself is a water-saving move. Remember that plants actually draw the water out of the soil with their roots and release it into the air through the marvelous process of transpiration. As each precious water molecule exits through the stomata (microscopic little “mouths” that breathe for the plant) of the leaves, another molecule is pulled into the plant from the soil below. Think of a straw in a glass of water; you suck from the top and the water in your glass enters at the bottom. Because water molecules just like to hold onto their neighbor molecules (called surface tension, and it works in many wonderful ways), it forms an unbroken column inside the tiny “straws” that form a plant’s water transport system. The process is limited by the amount of water available to the roots. If you maintain only the desirable plants in your garden, you are making more water available to them when you pull or smother the competing thirsty weeds.
The ideal mulch zone around a tree or woody shrub starts a few inches (three inches for shrubs to 12 inches for larger trees) away from the stem and extends out to the drip line of the plant. The drip line, just as its name implies, is the zone directly under the leaves and branches of the tree. Mulch should never be piled against the trunk, where any wounds (like from the mower or string trimmer) are prime targets for infection.
At least three inches of organic material will insulate the soil, reduce evaporation, and contribute to soil health. Mulch layers need to be replenished as they break down. Monitor their depth and add more as they thin and become less effective — and beseech the powers to grant us at least a slight reprieve from the rainless skies.