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Does Black Plastic Mulch Protect Tomatoes from Frost?

A native of South America, the tomato (Lycopersicon esculentum) is a frost-tender perennial that rewards diligent gardeners with a prolific bounty of juicy fruits in the summer. The warm weather-loving plant requires soil temperatures of at least 60 degrees Fahrenheit before it can be planted in the garden. Covering the soil with black plastic mulch allows for an earlier harvest, protecting tomatoes against frost by raising the daytime soil temperatures by about 5 F. Black plastic mulch also suppresses weeds and retains moisture in the soil.

  1. Plastic Preparation

    • Pull the black plastic mulch taut against the soil in the planting bed because gaps between the soil and the plastic trap cool air and make the mulch less effective. Bury the edges of the plastic 2 to 4 inches in the soil, or if you're using a raised planting box, staple the plastic to the edges. Cut holes into the plastic for your tomato plants. Holes should be wide enough that plastic will not touch and burn the plants.

    Coverage Is King

    • Though black plastic mulch can give tomatoes a significant boost in frost protection, it may be necessary to provide additional protection before an especially deep freeze. The afternoon before a predicted frost, cover your crop with a blanket or clear plastic, supported on stakes or a hoop. Uncover the plant the following morning to allow the plant to ventilate because temperatures above 80 F can harm the crop. Irrigating before a frost is also helpful because moist soil can hold four times as much heat as dry soil, according to the Cornell University Cooperative Extension.

    Considerations and Concerns

    • When using plastic mulch, it is important to plant tomato crops early so that plants can grow large enough to cast shade onto the plastic in the summer. Plants that are too small in the summer will overheat. In the fall after your tomato plants are spent, remove and throw away the plastic mulch. Do not plow the plastic into the soil because it will not decompose. Black plastic mulches generally can only be used for one season.

    Culture and Care

    • Tomatoes are usually grown as warm season annuals, though they may survive the winters in U.S. Department of Agriculture plant hardiness zones 10 and 11. Tomatoes prefer a bright, sunny location with deep, well-draining soil that is rich with humus. Plants supported by cages or stakes can be spaced 1 1/2 feet apart, while unsupported tomatoes should be given 2 to 4 feet of space. Soil dries out slower under black plastic mulch, so water carefully to keep the soil moist but not waterlogged.