ENVIRONMENTAL EFFECTS OF MULCHING VEGETABLES

Few jobs in the vegetable garden are as rewarding as is the task of mulching. Time spent applying mulch to pepper, tomatoes, squash, eggplant and other vegetables will mean extra dividends at harvest time. A good mulch will prevent loss of moisture from the soil, suppress weed growth, reduce fertilizer leaching, and keep vegetables up off the ground. Soil rots which can be caused when vegetables touch the ground can be reduced when mulch is applied under the plant. Hay, straw, sawdust, grass clippings, pine bark and wood chips all make suitable mulch material. The mulch material should be spread 3 to 4 inches deep and 18 to 24 inches wide under the plant.

The cultural practice of mulching has several advantages. Mulches save on the labor of cultivation, since emerging and small weeds perish under their dark barrier. Therefore, they reduce tillage and the use of weed-control chemicals. Water is conserved, because mulches reduce the evaporation of soil moisture by lowering the soil temperature. Waster absorption by a mulched soil is greater than by an unmulched soil; because of its physical properties mulch prevents the formation of soil crusts. Consequently, soil losses from heavy washing and blowing are decreased. In effect, mulches are excellent conservation agents.

The insulating property of mulch averts drastic fluctuations of soil temperature, keeping the soil cooler in summer and warmer in winter. During the summer this improves both root growth and nutrient availability. Winter mulches reduce the risk of root damage. (Protection with winter mulches will be covered further in the following section on protection.) Soils are improved with organic mulches as lower layers decompose and become incorporated into the soil. At the end of the growing season, organic mulches can be tilled into the soil to further increase the organic-matter content. Finally, mulches impart a neat, trim look to gardens, reduce the incidence of mud-splashed flowers and vegetables after heavy rains, and decrease the frequency of vegetable rot caused by soil contact.

Mulches do have some disadvantages. Mulch and moisture around newly emerging seedlings or perennials can provide an ideal environment for diseases, such as damping-off or crown rot. The potential of disease is increased by long periods of rain. Diseases may overwinter in the mulch. Insects and rodents, who find it an attractive habitat, can cause plant damage. These problems can be minimized by avoiding direct contact between the plants and the mulch. Leave 4 inches between the mulch and the trunk of the plant.

The application of a mulch requires labor of varying degrees, depending on the material selected. However, the labor saved from the decreased need of cultivation usually outweighs the initial input.

The type of mulch used affects the growth of plants. Premature applications of organic and black plastic mulches may retard soil warming, and hence deter the growth of plants preferring warmer soils, such as tomatoes, peppers, and eggplants. An exception is clear plastic mulch, which will accelerate the warming of soil and produce earlier yields with warm season crops such as corn or muskmelons. However it supplies much less weed control than black plastic mulches, unless a herbicide is put down prior to the clear plastic. A too thick mulch or one prone to caking can impede the uptake of water and air by the soil, leading to possible plant damage. Mulches with a carbon-to-nitrogen ratio greater than 30:1 can steal nitrogen from the soil during decomposition, causing nitrogen deficiency in the mulched plant. Prior applications of nitrogen fertilizers can prevent this problem. Dry mulches that are combustible may be a fire hazard near buildings or in public places. Woody mulches near buildings can be a vector for termites.

TYPES OF MULCHES OR CHOOSING THE RIGHT MULCH

A practical mulch should be easily obtained, inexpensive, and simple to apply. Availability and cost vary from region to region. Mulching materials may be found in yards, garden centers, lumberyards, sawmills, dairy farms, tree-service firms, breweries, and food-processing plants. A suggested depth is 2 to 4 inches, bearing in mind that too little will give limited weed control and too much will prevent air from reaching roots. Mulches should be applied prior to active weed growth and summer droughts or before the ground freezes if it is a winter mulch. For warm-season crops, such as tomatoes, the mulching should be delayed until blossoms appear. A list of mulching materials follows with specific emphasis on advantages and disadvantages.

• Bark. Small pieces of bark are preferred over large chunks. Bark mulches vary, but all are attractive, durable, and suitable for foundation shrub plantings. Contact with wood framing is to be avoided, since bark can be a termite vector. The high carbon-to-nitrogen ratio of bark requires prior application of nitrogen fertilizer.
• Buckwheat hulls. These are light, fluffy, and black. Since buckwheat hulls are prone to caking, they should be applied no deeper than 2 inches. Forceful watering will cause scattering of buckwheat hulls.
• Cocoa shells. These are brown, light, easy to handle, and relatively noncombustible. Cocoa shells have some value as a fertilizer and resist blowing in the wind. Their high potash content harms some plants, so they should not be applied to a depth greater than 2 inches.
• Coffee grounds. Coffee grounds cake badly; a depth of 1 inch is recommended. Coffee grounds contain some nitrogen.
• Compost. This is a good mulch, as it has fertilizer value and soil-like appearance. It is also a good organic amendment for tilling into the soil after the growing season ends.
• Corn cobs. Ground corn cobs are a good mulch. Some find their light color objectionable. Other uses for ground corn cobs, such as in feeds and mash, tend to limit the supply for mulching.
• Grass clippings. These contain nitrogen. Grass clippings cannot be applied thick when green, as they heat rapidly and form a dense mat that restricts the flow of air and water. They should be applied in thin layers and allowed to brown between each application.
• Ground tobacco stems. These make a coarse, good mulch with some nitrogen value. Availability is limited, and they should not be used on plants susceptible to tobacco mosaic virus such as tomatoes, peppers, eggplant or potatoes, since tobacco can be a source of infection.
• Leaves. Leaves are free, readily available in many areas, release some nutrients upon decomposition, and spread easily. However, they have a tendency to form a soggy, impenetrable mat. This problem can be overcome by mixing leaves with fluffy materials, such as hay or straw, or by shredding the leaves.
• Newspaper. This is certainly readily available and economical, but somewhat difficult to apply. The high carbon-to-nitrogen ratio necessitates the prior application of nitrogen fertilizer. A good use for newspaper is as an undermulch; that is, place four or five sheets under a thin layer of an attractive, more expensive mulch.
• Peanut shells. These are attractive and easy to apply. Peanut shells also contain nitrogen and are long lasting. However, peanut shells are carriers of Sclerotium rolfsii also known by the common names of Southern blight and white mold which can be a major problem in the garden. Peanut hulls may also be infested with nematodes and nut sedge seeds and/or tubers. At the present time peanut hulls are not recommended as a mulch due to the problems listed.
• Peat moss. This is attractive, easy to handle, but somewhat expensive. Dry peat moss requires considerable time and water to become moist, so it should be applied only to a 3-inch or less depth and avoided in areas subject to drought. Its acidic pH makes it especially desirable for acid-loving plants.
• Pine needles. These have an esthetic appeal and are not prone to forming a soggy mat as are leaves. They are especially good for acid-loving plants.
• Polyethylene film. This is one of the few mulches that is readily available and economical enough to be used on larger-scale commercial applications. Polyethylene allows passage of gases such as nitrogen, oxygen, and carbon dioxide. Holes or slits facilitate the planting of seeds or plants and water entry. It can last several years if undamaged by machinery. Usually, it is used as black film. Clear film is sometimes used, but it offers limited weed control (unless herbicide is applied before mulching), since light passes through it. Earlier crops can be produced with the clear, and to a lesser degree, black plastic mulch because of the warming of the soil.
• Straw hay. These materials are lightweight and easy to apply, but their appearance restricts their application mostly to vegetable gardens. They are used more frequently as a winter mulch for protection. They are not long lasting and frequently contain weed seeds. Coastal Bermuda is propagated vegetatively and is an excellent hay mulch.
• Sawdust. Aged or partially rotted sawdust makes a satisfactory mulch that lasts a long time. Since it is prone to caking and has a high carbon-to-nitrogen ration, apply it only 2 inches deep after adding nitrogen fertilizer to the soil.
• Spent hops. These are resistant to blowing and have some nutrient value. However, when fresh they have some odor and may heat, thus causing some plant damage.
• Stones or Gravel. This is a very durable mulch with an attractive appearance. It is used often around trees or shrubs. Do not use crushed limestone around acid-loving plants. In an exposed area stones or gravel may reflect heat and damage plants
• Sugarcane Bagasse. This is a good mulch when available in the crushed form. It has a moderately acidic pH, making it useful around acid-loving plants.
• Tire fiber. This is an extremely durable and effective mulch. However, it should not be used around any plants that are being grown for edibles.
• Wood chips. Since these are moderately priced or free, attractive, readily available, and easy to apply, they make an excellent mulch. Their high carbon-to-nitrogen ratio requires an application of nitrogen fertilizer. Wood chips can last about 2 years. As with a bark mulch, one must consider that they can be a vector for termites.