How to Cover Raised Beds With Plastic

Plastic covers can protect garden plants from cold temperatures and insects, but the best method for covering raised garden beds with plastic depends on the element from which you hope to protect the plants.

Soil solarization is a non-chemical method for controlling soil-borne pests such as bacteria, fungi, nematodes and weeds by raising the soil temperature high enough to kill the pest organisms. Soil heating is achieved by covering the soil in non-planted beds with clear plastic sheeting for a period of four to six weeks during the hottest part of summer. The plastic allows solar radiation to penetrate and then traps heat in the soil. In ideal conditions, the top 6 inches of soil can reach temperatures up to 140 degrees Fahrenheit.

Rake

Watering device

Tape measure

Shovel or trowel

Clear plastic sheeting

Prepare the raised beds' soil by raking it smooth and level. Water the beds so that the soil is moist to a depth of 12 inches.

Dig a trench 4 to 6 inches deep on all sides of each bed, using a shovel or trowel.

Lay clear plastic sheeting along one side of each bed, with the edge of the sheeting in the trench. Fill the trench soil to cover the edge of the plastic sheeting for each bed.

Move to the opposite side of each bed, and pull the plastic sheeting across the bed, getting it as tight against the soil as possible. Holding the plastic taut, and bury this edge of the plastic in the trench on its side of the bed.

Bury the other edges of the plastic in the trenches on the other sides of the beds, keeping the plastic flat and tight against the soil.

Floating row covers are made from lightweight material and are laid over the tops of plants without the use of supports; burying or weighting their edges keeps the covers in place. These covers are permeable, allowing sunlight and water to pass through to plants, but heat becomes trapped beneath the covers, and they help to keep out pests. Many floating row covers are made of bonded synthetic fibers, but some are made from perforated plastic.

Floating row covers can be used early in the growing season to warm the soil and give cool-season crops a head start. During periods of freezing temperatures, however, plants that touch the covers may be damaged by the cold; so floating row covers are not a good choice when temperatures are very low.

Even though floating covers can be effective at keeping insect pests away from raised beds' plants, the beds must be free of insects before the covers are laid. Also, the covers' edges need to be adequately sealed in place, or else insects that manage to get underneath the covers will damage plants.

Heavier plastic covers must be supported with wire or plastic hoops so that the covers don't rest directly on plants. These covers require more work to install and maintain than floating row covers, but their thicker plastic provides more protection against cold temperatures. The covers rest on semi-circular hoops over the plants, forming a tunnel over each raised bed, and the edges of the covers are weighted, pinned or buried to keep them in place. Hoop-supported covers are the best choice to cover plants such as tomatoes (Solanum lycopersicum) and peppers (Capsicum spp.), which can be damaged by floating row covers.

For more information on prepping a raised vegetable bed for winter, see Raised Bed Vegetable Gardening Winter Care.

Plastic sheeting can raise the temperature under it as much as 30 degrees Fahrenheit above the temperature outside it. So providing ventilation for covered raised beds when the sun is out and outside temperatures are warm is important. Remove the covers during the day, and put them back in place over the beds at night. Using slitted or perforated covers reduces the need to remove them. In hot climates, colored covers may be a better choice than clear or white covers because the darker covers cause less heat buildup.

In general, it's best to remove covers three to four weeks after plants are transplanted to the raised beds to allow insect pollinators access to the plants. Hoop-supported covers may be used over warm-season crops before the first frost in fall to protect the plants from cool temperatures and to encourage crop ripening.