The Samuel Roberts Noble Foundation, Inc.

Permanent Raised Bed Gardening: Plastic Mulch Application

Plastic mulches have been available since the early 1960s and their popularity has continued to grow. In addition to commercial growers, countless numbers of backyard gardeners have come to appreciate the many advantages plastic mulch provides.

Some of these advantages include the following:
Earlier crops. Most experts consider this to be the single greatest benefit from using plastic mulch. When combined with raised bed culture, plastic mulch acts to raise soil temperature, thereby promoting faster crop development and earlier yields, up to two weeks in some conditions.
Weed control. I consider this to be the second greatest benefit of plastic mulch. For a mulch to be effective, enough light must be blocked to prevent weed growth. An exception is nutgrass, where the nutlike tubers provide enough energy for the seedling to puncture the mulch and emerge.
Reduced evaporation. Soil water loss is reduced under plastic mulch. While mulching insures greater irrigation efficiency, it also necessitates the need for drip irrigation as all moisture falling on the mulched bed, be it from overhead irrigation or rainfall, is shed from the bed.
Reduced fertilizer leaching. Because water runs off the mulch, plant nutrients are not lost through leaching. While plastic mulch does insure greater nutrient use efficiency, it also limits application of dry fertilizer materials, whether processed or natural (organic), prior to applying mulch. Any additional nutrients the crop receives must be applied via the drip system.
Cleaner product. The edible product from a mulched crop is cleaner and less subject to rot because mud from rainfall or irrigation is not splashed on the plants or fruit.

Plastic film mulch is available in several styles and widths. Black plastic is the most popular because it retards weed growth, warms the soil and is relatively inexpensive. White on black mulch provides a cooler soil temperature and is commonly used for establishing such crops as fall tomatoes or cole (cabbage, broccoli, etc.) crops under hot summer conditions.

Clear mulch provides the greatest soil warming potential of any mulch. A drawback of clear mulch is that it promotes weed growth because the plastic is transparent to sunlight. At the Noble Foundation, clear plastic mulch is used religiously during July and August in fallow beds to control weeds and soil-borne pathogens using heat, in a process known as solarization.

The best qualities of both clear and black plastic are available with IRT (infrared transmitting) mulch. IRT mulch transmits infrared radiation to warm the soil similar to clear mulch, but blocks most visible radiation, as does black mulch. IRT mulches are typically translucent green or brown.

Recently, several types of crop-specific, or "designer," mulches have been developed. For example, tomatoes are partial to red mulch while potatoes favor pale blue or white. Squash and eggplant have responded favorably to blue. Research has shown growing tomatoes on red mulched beds, rather than conventional black mulched ones, can increase harvest of quality fruit by 10 to 15 percent.

It's not the colors of the mulch that enhance yield, but the difference the colors make in the light reflected onto the plants.

Research on the effects colored mulches have on plant growth and yields is ongoing. No doubt other colors will be identified as beneficial to certain fruit and vegetable crops, and new products will be offered as research becomes conclusive.

Various types of woven and perforated mulches that are porous to air, water and nutrients also can be used effectively on permanent raised beds. These "weed mat" or "landscape fabric" types of mulches provide excellent weed control and can be used in combination with sprinkler irrigation systems. Their heavier design endows them with several years of service life. Consequently, they are ideally suited for use with perennial crops such as small fruit.

Expect to pay more for porous mulches. Also, don't expect the same soil warming benefit from porous mulches that can be achieved using thinner film mulches.

Plastic mulch film generally comes on rolls 4- to 5-feet-wide. Less common 3- and 6-feet-wide rolls also are available.

Mulch films are available in various lengths with 2400- and 4000-foot rolls most common. Shorter lengths are available from mail order catalogs. See the appendix for a list of companies carrying plastic mulch.

Mulch films are available in either smooth or embossed surface finish. The embossed film hugs the bed better and is not as prone to loosen during warm weather. Expect to pay a little more for embossed film.

Ranging in thickness from 1 to 1.25 mils (1 mil = 1/1000 inch), mulch film is thinner than the woven fabric mulch sold at home improvement centers. Because it is very thin, don't plan on using the mulch for more than one growing season.

Before you decide on the use of plastic mulch as a component of your raised bed gardening system, consider the increased amount of maintenance required. Growing crops on plastic mulch also is a whole new game when it comes to water and nutrient management. A thorough understanding of the use of tensiometers to schedule irrigation and of fertilizer injection equipment to inject nutrients into the drip system is required to realize the full potential of the mulched growing system.

Most growers would be well advised to master the use of drip irrigation and fertigation (the application of fertilizer through the irrigation system) on unmulched beds before attempting growing crops on plastic mulch. If you are determined to utilize plastic mulch without any previous experience, consult as many references as possible and then use mulch on a limited scale. Seeking the assistance of an experienced grower also would be prudent.

If you intend to use plastic mulch to hasten growth, don't wait until planting to apply it. For spring planting, the mulch should be applied at least seven days before planting to allow time for soil warming.

Beds are ready to receive plastic mulch only when they have been properly prepared. All beds should have pre-plant fertilizer applied, drip irrigation installed, and be crowned, firmed and moist. Never apply mulch to excessively dry beds as the growing medium could settle, allowing mulch to loosen after wetting.

There are two basic methods of applying mulch to permanent beds. One involves wrapping the entire bed profile with plastic and burying the edges of the mulch film in trenches dug along the edge of the bed borders. The other method utilizes wood lath to secure the film to the bed borders.

The trench method of mulch application is ideally suited for use with recycled auto tire beds and plastic mesh beds. A 40-inch-wide bed requires a 6-foot-wide roll of mulch to wrap the entire bed and provide enough excess to adequately bury edges in the pathways.

If the pathways are too water-soaked to accommodate digging trenches, consider burying the edges of the mulch in trenches dug along the interior edge of the bed borders. A 4-foot roll of mulch is ideally suited to this technique when applied to 40-inch beds.

When preparing trenches, be sure they are of sufficient width and depth to securely hold the edges of the mulch when soil is replaced. Use a garden hoe to prepare 6-inch-wide by 3-inch-deep trenches in the pathways next to the bed. If the soil in the pathways is tight or compacted, a tiller is used to loosen it prior to trenching.

When preparing trenches along the interior edge of the borders, use a square-point shovel to carve out 4-inch-wide by 6-inch deep trenches.

While effective, the process of digging trenches to bury mulch film requires considerable time and energy. In an effort to make mulch application more user friendly, we developed a technique that uses short pieces of wood lath to attach the film to the bed.

To apply mulch film to an auto tire bed using the lath technique, start by installing a single 1-inch hex head sheet metal screw into each pocket, placing the screws in the middle and 1 inch below the top of the pocket. Do not fully insert the screws at this time, just get them started.

You will also need to prepare 1-inch-wide by 6-inch-long lath made from 1/4-inch plywood. Plywood is preferred over other types of wood lath because it resists splitting when screws are used. Prepare one piece of lath for every pocket. A single 4-foot by 8-foot sheet of plywood will supply 268 pieces of lath.

Unroll the mulch film using one of the following methods:

Traveling roll – Place the roll of mulch at one end of the bed. Place a weight, such as a concrete block, on the free end of the roll to hold the film to the ground. Insert a pipe through the roll. Pick up the roll by the pipe "handles" and walk to the other end of the bed, unrolling the film as you go. This method requires two people.

Stationary roll – This method involves placing the pipe handles on portable stands at one end of the bed. Walk the loose end of the film to the opposite end, unrolling the film as you go. This method requires only one person (although two is preferred) and is easier on the back.

Attach the free end of the film to the end of the bed using the lath pieces. Working with one piece of lath at a time, wrap the film around the lath, pull the top of the rubber pocket away from the bed and insert the lath behind the pocket. Releasing the pocket top should lock the lath in place. Secure the lath to the pocket using the preinstalled hex head screws.

Proceed to the other end of the bed. Cut the film from the roll, leaving an extra foot of film to work with. Repeat the fastening process on this end. As you roll the film around the lath, pull the film towards you to remove any slack. Don't overstretch the film because tearing could occur.

Using the same pocket and lath technique, attach film to both sides of the bed, stretching the film crossways to remove any slack prior to attaching.

To remove the film, back the hex head screws out of the pockets until the lath pieces are released. Collect the lath pieces and store them in a dry location to prevent warping. Dispose of the mulch film in a responsible manner. Leave the hex head screws in the pockets where they will be ready for service the next time film is applied.

Applying plastic mulch to a corrugated sheet metal bed using the lath technique is similar to an auto tire bed with a few differences.

Begin by unrolling the mulch film using one of the two methods previosly described.

Attach the free end of the film to the end of the bed using the lath pieces. Working with one piece of lath at a time, wrap the film around the lath, pull the film over the top of the bed and attach the lath to the side of the 2-inch by 4-inch frame using 1-inch hex head screws. Space the lath pieces 1 foot apart.

Proceed to the other end of the bed. Cut the film from the roll, leaving an extra foot of film to work with. Repeat the fastening process on this end. As you roll the film around the lath, pull the film towards you to remove any slack. Don't overstretch the film because tearing could occur.

Using the same lath technique, attach film to both sides of the beds, stretching the film crossways to remove any slack prior to attaching. Attach the film to the portion of the wood frame located between the sucker rod stakes.

A helpful tip when applying mulch on a windy day: if at all possible, walk with the wind while unrolling the film, keeping the film as low as possible. The first time the mulch goes airborne, you'll appreciate the advice.

Depending on how you've configured the drip irrigation, you possibly will need to cut a hole in the mulch to accept the drip supply line feeding one end of the beds.

After removing the plastic mulch, collect the hex head screws or simply reinsert them into the wood frame. Remember to store the wood lath in a dry place when it's not in use.

A few closing comments on the use of mulch film. While black plastic mulch is recommended for use on spring planted crops, problems can arise if used in conjunction with summer plantings. In the southern U.S., the extreme heat generated with the use of black plastic mulch exerts tremendous stress on young seedlings.

During summer months, some growers substitute white or silver-colored mulch for black. These lighter colored films are more efficient in reflecting sunlight, thus preventing excessive heat buildup in the beds. They are commonly used for establishing such crops as fall tomatoes or cole (cabbage, broccoli, etc.) crops under hot summer conditions.

Organic materials offer an effective alternative to plastic for summer mulching. At the Noble Foundation, we mulch all of our summer plantings with compost. Summer use of organic mulch offers all the advantages common to plastic mulch, plus it constitutes a source of slow-release fertilizer. When incorporated into the soil, organic mulch increases the water and nutrient holding capacity of the soil and improves soil tilth.

different colors of mulch film
  • Raised Bed Gardening images