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Organic mulch

Mulches of organic materials are highly effective for suppressing small seeded (i.e., <2 mg) annual weeds. Since most garden weeds are annuals and most annual weeds in the Northeast have small seeds, the use of mulches is broadly effective against many species. Mulches are nearly useless for controlling perennial weeds because these have sufficient energy stores in the roots or rhizomes to push shoots up through even very thick layers of mulch. Large seeded weeds (e.g., > 5 mg) may also emerge through substantial layers of mulch.

Weed seeds in mulch

The most commonly used mulch materials are straw, hay, compost, leaves, grass clippings and bark chips. Regardless of the material used, it should be thoroughly checked for weed seeds. Hay, and particularly, late cuttings of grass hay, often contains mature weeds and perennial grasses that you do not want in your garden. Straw is generally free of weed seeds but may have thistle seed heads and grain seeds. Grass clippings often contain dandelion flowers that subsequently turn to seeds, and may also contain crabgrass and annual bluegrass seeds. Tree leaves are generally free from weed seeds but may contain acorns and other tree seeds that subsequently germinate and compete with crops.

When to mulch

Although large seeded and transplanted crops can be mulched almost immediately after planting, it is helpful to delay mulching until the soil is fully warmed and the first flush of weeds has been removed by shallow weeding. Although a thick mulch eliminates most light at the soil surface, even a homogenous appearing mulch layer has partial "windows" through which some light penetrates. If the density of weeds emerging under the mulch is high, more weeds will be positioned to exploit such windows and emerge through the mulch.

How much mulch

The amount of mulch needed depends on the dominant weeds in the garden and the type of mulch. Larger seeded weeds require more mulch than smaller seeded weeds. Hay and straw should be fluffed up rather than applied as slabs from a bale. Although the slabs are very effective at blocking weed growth, they quickly rot and provide a prime seedbed for any crop seed in the mulch and for windblown weeds like dandelion. A 3 to 5 inch layer of straw, hay, or leaves (which subsequently settles to about 2-3 inches) is generally effective against most of the small seeded annual weeds in the Northeast. Because grass clippings and compost are denser, 2-3 inches is generally effective.

Notes on particular mulches

Some mulches pose special problems and advantages. Bark and wood chips cannot be recommended for use in vegetable gardens (except on permanent paths) because their high carbon to nitrogen ratio encourages decomposer microbes to take nitrogen from the soil, thereby starving the crops. Similarly, straw has a high C:N ratio and can temporarily immobilize nitrogen. Whereas the nitrogen taken up by bark chips may be stored for years, the nitrogen taken up by straw is largely released again to the crop later in the season. For heavy feeding crops, hoeing in a nitrogen source just before laying the straw mulch can avoid N deficiency in the crop.

Unlike hay and straw, tree leaves do not tangle into a mat and thus sometimes blow to other parts of the garden where they may smother small crops. Applying the leaves after the crop is up helps hold them in place.

Rye straw commonly releases allelopathic compounds that are toxic to other plants. Since small seeded weeds are more susceptible than large seeded or transplanted crops, this can be advantageous. However, in some circumstances, the toxins may also slow crop growth (e.g., sweet corn during dry years).