The organic mulches described here are those derived from plant material. This page excludes bark chunks, which are meant to last for a long time in the garden, so are described under Permanent Mulches.
Organic mulches all have in common the fact that they decompose to become part of the soil, improving it in the process. When organic matter decomposes, it becomes a sticky, chemically-complex material called humus. Humus sticks soil particles together, improving the structure of clay soils by forming it into granules.
Earthworms work the mulch into the soil, leaving digested mulch as worm castings in the soil. Some insects also help incorporate the mulch into the soil.
Organic matter might also add nutrients to the soil, depending on the material used. Mulches made of manure and compost are rich in nutrients. Rain and irrigation wash them into the soil, and as they decompose, they release more nutrients to the soil.
Some mulches, such as sawdust, contain carbohydrate but little nitrogen, so decompose slowly unless nitrogen is added. If incorporated into the soil, the bacteria that decompose it can tie up all the nitrogen in the soil until they finish and decompose themselves, an effect called nitrogen draft. Add extra nitrogen fertilizer when digging sawdust or straw into the soil. While used as a mulch, sawdust does not require extra nitrogen because it is in contact only with the surface of the soil. Most commercial sawdust products, such as redwood soil conditioner, contain added nitrogen. Maintain mulches at their original thickness. As the mulch decomposes and thins, add new material.
Most organic mulches are highly absorbent and soak up large amounts of sprinkler water before passing any to the soil beneath. Water them by some method besides sprinklers, such as with soaker hoses, drip irrigation, or furrow irrigation. Remember that the same effect keeps light rainfalls from reaching the soil, and you may need to irrigate mulched areas even it rained recently and unmulched soil is damp.
Some Organic Mulches
Many different materials are available in different parts of the country. Many of these are by-products of different industries, only locally available. For example, cocoa bean hulls available near chocolate factories make an excellent mulch that smell like chocolate. Those listed here are common everywhere or in large areas.
Compost — You can make an excellent mulch at home by composting various kinds of plant refuse such as grass clippings, leaves and plant tops from the vegetable and flower garden. Compost is one of the best materials for improving the soil and adding nutrients. It decomposes and is incorporated into the soil rapidly, making it excellent as a vegetable garden mulch, but must be replenished frequently during the season. Keep a pile of compost near the garden and throw a shovelful on any weeds that emerge.
Manure — Manures make an excellent but unattractive mulch for improving the soil. Some, such as stable litter, contain large quantities of bedding material. Manure can be made more attractive by running it through a shredder. The odor usually dissipates in just a few days. If a manure mulch is maintained on the soil, no further fertilizing should be required. Like compost, manure decomposes rapidly, so needs frequent replenishment.
Chipper Waste — As more communities limit the amount of burning allowed, more garden refuse is put through a chipper. The resulting product is so plentiful it is usually free for the asking. The rate of decomposition depends on the material chipped. If it contains lots of leaves and green twigs, it contains enough nitrogen to heat up and decompose quickly. It it’s mostly wood chips, it may decompose very slowly, acting more like fir bark chunks. Often there’s a mix of rapidly-decomposing leaves and twigs and slowly-decomposing wood chips. Top it off if the depth diminishes in a few weeks. Chipper material is rough-looking and coarse-textured, making it more suitable for large, wooded areas than garden beds.
Lawn Clippings — Green when applied, clippings make an attractive, fine-textured mulch that turns straw-colored in the sun. Spread clippings loosely on the surface of the soil, no more than 2 inches deep, to allow them to dry. Otherwise, they mat down and give off an offensive odor. Build up the depth with subsequent lawn mowings. Dry grass clippings blow in the wind. Do not use grass clippings if the lawn has been treated with an herbicide. Thatch removed from a lawn can also be used as a mulch. Do not use thatch that contains bermudagrass, which can grow from clippings in the thatch.
Leaves — Leaves are available everywhere in the fall. Stiff, curled leaves, such as holly leaves, may be used as the are, but thin, flat leaves like maple leaves should be shredded to keep them from matting. Leaves may be composted or just piled to decompose for a season instead of shredding. The resulting leaf mold is an excellent mulch itself. Leaf mold is also available as a bagged product in garden centers.
Newspaper — Newspapers may be used whole or shredded in vegetable gardens. Layers of newspapers make an effective weedblock. Make them more attractive by spreading another mulch over them. Years ago, colored ink contained lead, but most newspapers today use organic inks that don’t contaminate the soil, so colored pages are as acceptable as black and white.
Ground Bark — Most ground bark products have been composted or nitrogen-stabilized. Fir bark weathers to an attractive silver gray color.
Shredded Bark — This attractive mulch is often used in ornamental beds. Slow to decompose, it needs replenishment less frequently than faster-decomposing mulches.
Straw — Used primarily for winter protection, straw can also be used as a summer mulch in vegetable gardens. It is highly flammable when dry. If used baled, flakes may be laid on the soil to make the mulch. Oat straw is full of seeds. If flakes sprout, turn them over to kill the grass. Mice like to nest in straw during the winter. If mice are a problem, remove the straw before snowfall.
Hay — Hay is a functional but unattractive mulch. Unlike straw, it contains enough nitrogen to decompose quickly, and usually doesn’t contain crop seeds, although it may contain weed seeds. If purchased baled, flakes may be laid on the ground around vegetables.
Ground Corncobs — Because they decompose slowly, ground corncobs are usually used around trees and shrubs. They darken and become more attractive when exposed to weather.
Pine needles — This material makes a light, porous, attractive mulch. Since pine needles contain little calcium, they are especially desirable for acid-loving plants such as azaleas and blueberries.
Sawdust — Sawdust makes an excellent mulch in areas where it is available. Fine sawdust from cabinet shops blows readily and cakes when wet because of its texture. However, coarse sawdust from sawmills, shavings, and other wood byproducts are suitable as mulches.
Redwood Soil Conditioner — This wood product is used primarily as a soil amendment but makes an excellent low-cost mulch. A by-product of the redwood lumber industry, it consists of nitrogen-fortified sawdust that has been composted for 3 or 4 months at the mill.
Peat Moss — Peat moss, although one of the best soil amendments, makes a poor mulch. It blows in even light winds, making it very unstable. After wetting and drying, it forms a cardboard-like mat that repels water and is very difficult to re-wet.