Because row-cover fabric reduces air movement, it traps warm air. The temperature under medium and heavy row covers can be 10 to 20 degrees warmer than the outside air. This is ideal for heat-loving and heat-tolerant plants, such as eggplant, zucchini, pepper, squash and potatoes, but not so good for cool-season crops, such as lettuce. They do best under a lightweight cover, which traps much less warmth.
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