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Organic mulches

Pine straw, compost, newspaper, shredded bark, pine bark, aged wood chips, leaf mold, shredded dried leaves

Pine straw

Apply 3–6″ deep. Pine straw stays in place on steep slopes better than other mulches, but it can make soil more acid. Pine straw is great for winter protection on perennials. Avoid using near wood fences or walls due to potential fire hazard.

Pine bark nuggets

Apply 2–3″ deep. 1.5–3″ diameter nuggets are best. Nuggets are long-lasting and termite-resistant but can wash away in heavy rain.

Pine bark mulch

This is best as a soil conditioner, not a mulch. The texture is too fine for mulch.

Cypress mulch

Cypress is long-lasting but repels water when dry, making it difficult to wet soil, especially on mounds or slopes. This is also not a sustainably-harvested material — there are no regulations to manage wild harvesting that depletes wetlands.

Ground corn cobs

Apply 2–4″ deep. Corn cob mulch is excellent for improving soil structure but easily blown by the wind.

Newspaper

Apply 2–4 sheets thick, overlapping at the edges. Newspaper works especially well to control weeds because it completely blocks the sunlight that seeds require for germination. Probably the best use is on paths, covered with another mulch to improve appearance and prevent scattering. Note that While newspaper is effective, it is critical to prevent it from drying out or it will actually wick water away from plants. Roots of desirable plants that travel beneath a path can suffer if moisture is not maintained.

Compost, well-rotted manure, leaf mold

Apply 2–3″ deep. These not only improve soil structure, they also provide nutrients. A 2- to 3-inch layer of compost offers fair weed control; however, most compost provides a good site for weed seeds to grow and can contain weeds itself. Consequently, compost is better used as a soil amendment. However, a layer of compost may be used on overwintering beds of perennials to provide nutrients and help protect plant crowns.

Coarsely shredded leaves

Apply 2–3″ deep. Leaves are an excellent source of humus, but rot rapidly when moisture is abundant (not usually a problem here). They are relatively high in nutrients. Thick-textured leaves, like those of most oaks, are the best choice because they do not mat. Oak and beech leaves are acidic and are especially valuable for azaleas, camellias, and rhododendrons. Maple leaves are basic and should be used in other areas. To reduce blowing of dry leaves, allow them to decompose partially first.

Inorganic mulches

Gravel, stones, brick chips, ground rubber

Advantages

Materials are permanent, fireproof, and can be colored to blend in with features of the home, patio or landscape.

Disadvantages

Materials can be thrown by a lawn mower, are difficult to keep clean, and create a hot landscape environment because they reflect solar radiation. They do not improve soils and make it more difficult to add plants or alter the landscape.

Whole leaves and grass

These can form thick mats that prevent water and oxygen. Thin-textured leaves like maple leaves are especially prone to matting.

Fresh organic mulches derived from vegetation such as grass clippings or straw draw nitrogen from the soil as the material breaks down. This causes a temporary nitrogen deficiency because microbes tend to out-compete plant roots for the available nutrients released. Either allow these fresh mulch to partially break down before using it and/or supplement with nitrogen fertilizer when mulching. Of course, fresh mulch is fine for use on paths where plant roots are not present.

Decomposing woody mulches like sawdust and fresh wood chips break down much more slowly than mulches derived from vegetation and can tie up nutrients for years.

Mulch that smells like vinegar, ammonia or sulfur

Properly composted organic material has a pH of 6.0–7.2. In contrast, organic material stored in a large pile often undergoes anaerobic (low oxygen, high moisture) decomposition and becomes very acidic, with a pH of ~ 3.0. It will smell like vinegar, ammonia or sulfur. Anaerobic decomposition is often a problem with materials high in cellulose such as leaves or hardwood chips, but is rare for pine bark because lignin is the main component. Pine bark is unique because it can be used without composting as either mulch or compost. Only the particle size differs.

The methane, hydrogen sulfide, ammonia, and/or alcohol found in improperly composted mulch or compost are rapidly toxic to plants, with damage usually evident within 24 hours of application. Symptoms include marginal leaf chlorosis or scorch, defoliation, and/or plant death.

Plant Injury From “Sour” Wood Mulch

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