Because frost forms only on the surface of a plant, and it generally occurs when daytime temperatures are still above freezing, you can keep your planter going for a few more weeks if you keep the frost off!
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Frost is frozen dew, so frost depends on both humidity and temperature. Under the right conditions, a super-cool layer of air can form very close to the ground even though the air temperature 4-6 feet above the ground is 5-10 degrees warmer.
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