Mulching for Weed Control in Vegetables

Vegetable growers and gardeners spend considerable time and effort each season on weed control. Good mulching practices, when combined with timely cultivation, crop rotation and optimum crop nutrition, can help keep weeds at bay. Mulches reduce weed growth in several ways:

Ø  They suppress weed seed germination.

Ø  They shade and physically block emerging weeds.

Ø  They give established crops a competitive advantage.

Mulches reduce numbers of annual weeds such as lambsquarters, pigweed and crabgrass by cutting off seed germination stimuli. These weeds germinate from the top 12 inches of soil, in response to daylight, large daily temperature fluctuations, sudden wetting after dry spells, and tillage or cultivation. Any opaque mulch (black plastic or paper, or thick organic mulch) removes the light stimulus. Organic mulches also dampen fluctuations in soil temperature and moisture. Green materials such as grass clippings or mowed cover crops release substances into the soil that inhibit weed germination and emergence. These substances rarely affect transplanted vegetables or large seeds like corn, but they can hinder germination of small-seeded crops like carrots or lettuce.

Weeds that do emerge under a mulch are shaded and physically hindered by the mulch. Generally, annual broadleaf weeds such as pigweed and galinsoga are most readily blocked by mulch. Annual grasses like foxtails and crabgrass are a little tougher and perennial weeds like yellow dock or johnsongrass that emerge from rootstocks or rhizomes are most difficult to suppress.

Mulches can stimulate crop growth by conserving moisture and moderating soil temperature. Vigorous crops suppress weeds by shading them or monopolizing water and nutrients. However, mulch placed over a newly planted seed row may hinder crop emergence. and mulching around young plants too early in spring can retard growth by keeping the soil too cool and wet.

Occasionally, mulches aggravate weed problems. Hay, straw and manure may carry seeds of noxious weeds or of grains or forages that become a nuisance when they "volunteer" in vegetable crops. Also, if the mulch does not provide an adequate weed barrier, weeds that break through will thrive on favorable soil conditions under the mulch.

 Without mulch, many weeds emerge and slow crop growth B. Hay or straw reduce weed seed germination and impede growth, although some vigorous perennials push through C. Weeds cannot penetrate black plastic. but may come up through planting holes and in unmulched alleys.

Plastic mulches

Black plastic mulch can stop most weeds. The widely used 1.0 mil black polyethylene (PE) film blocks essentially 100% of sunlight, and resists tearing by weed shoots pushing from beneath. However, black PE does not add organic matter, and it hinders rain penetration, which may leave the root zone too dry unless drip irrigation is provided. Rain runoff may stimulate weeds in alleys between mulched beds, requiring hoeing or other control. Weeds may also grow through planting holes in the plastic. Heavy duty, rain-permeable, woven plastic mulches ("weed cloth" or "landscape fabric") effectively stop weeds and last several seasons, but cost several times as much as PE films.

Clear PE is occasionally used for heat loving crops or for soil solarization. Because this material transmits sunlight, weeds may grow under the film and eventually tear it. However, during hot weather, clear PE can raise soil surface temperatures high enough to kill emerging weeds.

How to maximize weed control with black plastic mulch

Ø  Lay drip irrigation along crop rows before mulching; irrigate during dry spells.

Ø  Lay plastic on a well-prepared seedbed just before planting.

Ø  A few weeks after planting, sow an annual cover crop in alleys (buckwheat, soybeans. oats and/or vetch), then cultivate shallowly to kill weeds and incorporate seed, or cultivate alleys, then mulch with hay or straw, slightly overlapping edges of plastic film.

Organic Mulches

Organic mulches such as hay or straw do not block weeds as completely as black PE, but they add organic matter and keep the soil moist. They are most effective against annual weeds including pigweed, ragweed, cocklebur, crabgrass and barnyardgrass, and less so against vigorous perennials like yellow dock, horsenettle, quackgrass and bermudagrass. Seven to ten tons hay or straw per acre (one 35-lb square bale per 75-100 square feet) usually give substantial weed control, whereas four to five tons per acre (one bale per 150Ä175 square feet) may not. Market gardeners often use hay or straw, and find that each hour spent mulching can save two to four hours weeding.

Organic mulches suppress weeds most just after spreading, and become less effective as they decompose. Hay or straw applied just after planting a heat-loving crop can slow early-season crop growth, then break down too much by midsummer to control weeds. Many growers cultivate during the first several weeks after planting, then mulch when the crop is well established.

One disadvantage of hay and straw is that the heavy applications needed for weed control may not be feasible on a large scale. Trying to "stretch" the mulch supply by spreading it more thinly will let weeds through, and the mulch will make hoeing or mechanical cultivation difficult. In rainy seasons, more mulch may be needed to suppress weeds, yet heavy mulching can keep the soil too cool and wet for optimum crop growth. Also, several years of heavy mulching with straw or grass hay can upset soil nutrient balance (see Information Sheet No. 10, Mulches and Soil Fertility).

Compost is sometimes used as a mulch, but its soil-building benefits are better realized by incorporating compost into the soil. Loose crumbly compost does not block weeds as effectively as hay. In the VABF mulch study, eight tons hay per acre controlled weeds better than 50 tons compost per acre. Similarly, chipped brush might control weeds better than fine sawdust.

How to maximize weed control with organic mulches

• Select a coarse-textured mulching material.
• Grow your own mulch hay or straw and cut before seed set, or
• Obtain seed-free mulch from a reliable source, or
• Let seedy hay or straw age outdoors for a year before use (most seeds will lose viability).
• Delay mulching until the crop is well established and the soil is warm, usually 3 to 6 weeks.
• Cultivate early on a hot day so weeds dry up and can't re-root, then mulch at the end of the day. Hoe or cultivate shallowly to avoid stimulating weed seed germination.
• Spread hay or straw at one 35-lb bale per 75 to 100 square feet: more if weed pressure is heavy.

Paper Mulches

Paper mulch may not control weeds throughout the season, as its begins to break down a few weeks after application. Kraft paper and newsprint transmit enough light to allow some weeds to grow and eventually break through. Treating paper with waste cooking oil (available free at restaurants) makes it more decay-resistant, but also more transparent. This stimulates spring weeds. However, in summer, soil surface temperature under oiled paper can exceed 120F, which stunts or kills emerging weeds. Black mulch paper, available from farm supply companies, also breaks down quickly, but it blocks light and prevents weed growth while it is still intact.

Some growers lay paper under hay, straw or sawdust to maximize weed control. In several experiments, two layers of newsprint plus hay or straw at five tons per acre suppressed annual weeds as effectively as ten tons of hay or straw alone. Four to six layers of newspaper under hay were successful against quackgrass in one trial. This strategy "stretches" the organic mulch supply, and can reduce risk of nutrient imbalances from heavy mulching.

Reduction in weed growth by different amounts of hay or straw, with or without two layers of newsprint under the organic mulch

How to "stretch" an organic mulch with paper

• Use newsprint end rolls (available at news presses), old newspaper, kraft or other unbleached paper.
• Avoid bleached white paper and glossy colored papers because of possible toxic residues.

• Just before spreading organic mulch: hoe weeds, lay paper, and anchor with soil or stones.
• Use two layers of newsprint or one of kraft paper: more if perennial weeds are abundant.
• Overlap paper edges; cut slits to fit around crop stems to avoid leaving gaps along crop rows.
• Spread one bale hay or straw per 150 square feet or two inches of sawdust, leaves or pine straw.

How to solarize noxious perennial weeds with oxidized paper

Vigorous perennial weeds like bermudagrass. Canada thistle or johnsongrass can grow through as much as a foot of straw, or even puncture plastic. However, they can be defeated without chemicals. One Tidewater grower reclaimed a small area severely infested with bermudagrass as follows:

Ø  Start at the onset of hot weather, with afternoon temperatures of 85 degrees or higher.

Ø  Mow and rototill the area.

Ø  Roll out a single layer of newsprint end rolls and anchor (calm weather helps!).

Ø  Brush or roll on waste cooking oil to make paper transparent.

Ø  Leave in place for several weeks, or until it first starts to break down.

Ø  Mulch heavily, about one bale of hay or straw per 25 to 35 square feet.

Very little bermudagrass had come through by winter, whereas the straw mulch alone had much less effect. Because bermudagrass and other perennials are deep rooted, solarization will weaken but not exterminate them. Follow up with additional tillage and a crop rotation that includes vigorous cover crops and vegetables that form a heavy canopy, such as potatoes or winter squash.

In situ Mulches

Winter cover crops such as rye, hairy vetch and crimson clover can be mowed after they begin to flower in late spring, and left in place as a mulch. Larger farms can do this more easily than spreading hay or straw on multi-acre plantings. Virginia Tech researchers have developed no-till transplanters to set vegetable starts through mowed cover crops. Although the cover crop produces only two to four tons mulch per acre, no-till planting and germination inhibitors from the green residues help reduce weed emergence. Usually, additional measures are needed at midseason to maintain weed control, e.g. cultivation with special heavy-residue cultivators. Several growers are developing an ingenious strip-cropping system for sloping land in which vegetable beds are laid out on the contour, planted with a winter cover crop, and mowed in late spring for no-till vegetable planting. Intervening strips of grass + clover are then mowed to provide additional mulch during summer. More information on cover cropping is given in Information Sheet No. 3, Cover Crops: On-farm, Solar-powered Soil Building, available through VABF.

Some growers lay paper under hay, straw or sawdust to maximize weed control. In several experiments, two layers of newsprint plus hay or straw at five tons per acre suppressed annual weeds as effectively as ten tons of hay or straw alone. Four to six layers of newspaper under hay were successful against quackgrass in one trial. This strategy "stretches" the organic mulch supply, and can reduce risk of nutrient imbalances from heavy mulching.

Mulching alone  will not control weeds, but a good mulching strategy will greatly enhance effectiveness of other measures. In general, if annual weeds become rampant, reduce tillage, cultivate shallowly, and apply seed-free organic mulches. Where perennial weeds present major problems, use a combined strategy of timely tillage, cover cropping, soil solarization, mulching and competitive vegetable crops that form a heavy canopy.

By: Mark Schonbeck

Virginia Assoc. for Biological Farming- USA