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Get it Growing: Mastering plastic mulches
Plastic mulches can extend the growing season, increase yield and quality of vegetable crops, and in some cases, deter insects. Plastic mulches increase soil temperature, retain moisture, suppress weeds and may reduce disease caused by the splash of soil-borne pathogens during rains.
Research is somewhat limited and inconclusive, but studies show that the color of the plastic mulch, its thickness and its opacity (the amount of light passing through it) can improve production.
Black plastic mulch is the most widely used and least expensive. It suppresses weeds and can increase soil temperature 5 degrees at a 2-inch depth and up to 3 degrees at a 4-inch depth. Laying down black plastic in the fall reduces weed growth and leaching of the soil over winter. It also warms the soil for earlier spring planting.
Clear plastic warms the soil better than black. Temperatures under clear plastic can be 8 to 14 degrees higher than bare soil at a 2-inch depth. However, because it lets light through, clear plastic allows weeds to grow, so it is not recommended for the growing season.
Clear plastic could be put down for 7 to 10 days in the spring to warm the soil before planting; use one of the colored plastic mulches when planting.
Brown Infrared Transmitting (IRT) plastic is a relatively new technology. It combines the weed-suppression properties of black plastic and the enhanced heat-absorbing qualities of clear plastic. Make sure that the brown plastic is IRT and not a lower-grade copy.
White, white-on-black and silver mulches effectively deter insects. However they reflect rather than absorb the sun’s rays, cooling instead of increasing the soil temperature.
Other colored plastics have been shown to improve production of specific crops:
Blue: cantaloupe, summer squash, okra and cucumber
Red: tomato, eggplant, okra, strawberry and basil
There are a few things to remember when using plastic mulches in planting:
A thickness of 1 mil is appropriate for most vegetables. Apply compost before laying down plastic mulch. Drip irrigation is important and should be installed before the plastic mulch is put down.
Rows and raised beds should be slightly mounded in the center to allow water run off.
Place the mulch tight on the bed to provide the maximum amount of contact with the soil. Secure the plastic with rocks or landscape fabric staples to keep it taut.
Cut an “X” in the plastic where you will place each plant.
Make the hole only slightly larger than the seedling you are planting. Place the planting so that the plastic doesn’t touch the plant.
As the plant grows, enlarge the size of the opening. Work mid-season fertilizer into the soil and water generously to avoid burning the plant.
A light application of liquid fish emulsion is an effective mid-season fertilizer and is easier to apply than dry fertilizer.
Drawbacks to using plastic mulches include that they are relatively expensive and in general are unfriendly to the environment. With care plastic mulches can be reused for up to three growing seasons.
Alternative biodegradable plastics are available (see WSU Extension FS103E at https://pubs.wsu.edu for more information).
Amanda Rosenberg is a Washington State University-certified, Clallam County Master Gardener.