Mulch is a wonderful way to improve your garden and make gardening easier. So what is mulch, and how do we use it?
A mulch is simply a layer of opaque material placed over the soil surface. Mulch helps produce healthier plants that require less maintenance.
Mulch can be organic or inorganic. Organic mulches include any material of natural origin that decomposes naturally, such as bark chips, grass clippings, straw, leaves, compost, rice hulls, saw dust or grape pomace. Inorganic mulches include plastic sheeting, rocks, rubber chips or non-woven geotextiles.
Mulch provides many benefits. All mulches reduce weeds and help retain soil moisture. They control weeds by preventing them from receiving the sunlight needed for growth. Minimizing weeds benefits plants by eliminating competition for water and nutrients. Young trees may be severely stunted in their early years if they have to compete with weeds.
Organic mulches deliver more benefits than inorganic mulches. When you choose an organic mulch, you are utilizing a material that might otherwise be discarded and that breaks down into organic matter that nourishes the soil. Inorganic mulches, like black plastic sheeting, may seem a good choice at first, but when they start to break down into non-recyclable bits and pieces, you may regret that decision.
Organic mulches help nutrients reach plant roots. They also nurture beneficial bacteria, fungi, insects and worms. These beneficial soil organisms help control many plant pests and outcompete the undesirable bacteria.
Organic mulches also provide insulation, keeping the soil a little warmer in winter and cooler in summer. They reduce erosion, help the soil absorb more rain, and reduce soil-borne diseases caused by splashing rain or irrigation water. By preventing rain from hitting the soil directly, mulches eliminate soil crusting, which can make it harder for seeds to germinate and harder for rain to penetrate in the future. Roots develop better under organic mulches, and the improved water retention encourages roots to extend farther.
As a bonus, many organic mulches are free or low cost.
Apply organic mulch in the late fall, after the first heavy rains saturate the soil; or in the late spring, while the soil still retains moisture but the soil temperature has warmed.
Mow or pull weeds before applying organic mulch. Alternatively, put down a layer of plain cardboard or a four- to six-page-thick layer of newspapers before spreading the mulch. This layer will not only control annual weeds, but also help reduce perennial weeds. Moisten the cardboard or newspaper thoroughly before topping with mulch. They will decompose in place and help feed worms and other soil organisms. This practice is known as sheet mulching, and it has worked wonders in my garden.
The coarser the mulch material, the more thickly it should be applied. A three- to six-inch layer controls most annual weeds. Coarse mulches such as bark chips allow better water and air penetration than fine mulches and typically last longer. Replenish your mulch when it has decreased in volume by half or more.
Pay attention to the source of your mulch. Avoid material contaminated with pesticides (such as grass clippings from your pesticide-happy neighbor) or disease (such as wood chips from sick trees).
Fine mulches like grass clippings and sawdust can mat up and prevent water from penetrating. However, fine mulches are ideal for pathways and other areas where you don’t want anything to grow. Keep all mulches at least three inches away from tree trunks and shrub bark to prevent decay.
Make sure that your mulch is free of weed seeds. Avoid hay and compost that did not reach a sufficiently high temperature to kill weed seeds. (Compost produced by commercial waste-management companies reaches a much higher temperature than most home compost does.) Organic mulches repel many insects but may attract sow bugs, pill bugs and earwigs. Trap earwigs in rolled newspapers, and control sow and pill bugs with diatomaceous earth, or use a natural pesticide for both.
With regular mulching, you will enjoy healthy soil and plants, use less water and (my favorite benefit) weed less.
Napa County Master Gardeners (http://cenapa.ucdavis.edu) answer gardening questions Monday, Wednesday and Friday, 9 a.m. to noon, at the UC Cooperative Extension office, 1710 Soscol Ave., Suite 4, Napa, 253-4221, or toll-free at (877) 279-3065.