Permanent Raised Bed Gardening: Plastic Mulch Application

Preparing trenches to bury mulch film

Burying the edges of plastic mulch in pathways using the trench method.

Burying the edges of plastic mulch within the bed using the alternative trench application method.

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.