Particle mulches are composed of a mass of material spread on the ground, as opposed to a film mulch, which is composed of a plastic or paper sheet. The particle mulch may be composed of compost, manure, straw, sawdust, rock, gravel, or any other material that covers the ground.
Both particle and film mulches conserve water, but they do so in different ways. Film mulches form a physical barrier to water. Particle mulches, being loose, contain dead air space. Air does not circulate readily through the mulch, so when the air is saturated, it remains that way, only passing its water through to the top of the mulch very slowly. To have this effect, the mulch must contain enough pockets of air that moisture can’t pass through readily. Because the pockets in a fine mulch, such as sawdust, are smaller, fine mulches insulate adequately with less depth. Coarse mulches, such as straw, have larger pockets, so need a greater depth to insulate adequately.
Both film and particle mulches block weed growth in the same way — by blocking light so weed seedlings die shortly after germination. To effectively block weed growth, particle mulches must be deep enough to darken the soil it covers. Again, fine mulches are opaque with less depth than coarse mulches.
To insulate the soil and prevent weed growth, dense or fine-grained mulches such as sawdust or sand should be applied 2 to 4 inches deep. Coarse or fluffy materials like straw can be applied much deeper. Straw and similar materials mat down with rainfall and time, to become more dense. Straw should be applied from 6 to 12 inches deep. After matting, it will be ⅓ to ¼ the depth it was applied.
Apply mulch evenly. When using a mulch that becomes soggy when wet, don’t pack it around the stem or trunk of the plant. Pull it back a few inches from the stem or trunk so that air can circulate freely to the base of the plant. Dense mulches shouldn’t be applied more than 5 inches deep. Deeper mulches restrict the flow of oxygen to the soil and carbon dioxide from the soil to the air, suffocating plant roots.
Most garden mulches last only a single gardening season. They must be replenished regularly if they are to be kept longer. They need replenishment because they have decomposed and been incorporated into the soil by earthworms and insects, improving the soil. Temporary mulches are all composed of organic materials, which decomposed rapidly or slowly to benefit the soil.
If weeds begin to grow through the mulch, simply add more mulch where the weed came through. It isn’t necessary to pull the weed; just bury it in mulch.
Sometimes, as in vegetable and annual beds, the mulch is dug into the soil when the bed is emptied, becoming a soil amendment. In other situations, the mulch might be raked up and added to a compost pile, to be returned to the soil later as compost. Or the mulch might be left in place all year round and replenished as needed.
Some mulches don’t decompose, or decompose very slowly, and can be thought of as permanent. These include sand, ornamental rock, river rock, and the larger sizes of bark chunks.