A common sight on autumn mornings is that of plants shrouded in white. These are life-giving shrouds -- sheets, newspapers, and old blankets -- protecting such cold-tender plants as basil, pepper, and impatiens from nighttime frost.
Signs of imminent frost are unmistakable, and worth knowing if you want to give your plants some protection. Carried unscathed through the first frosts of autumn, tender plants can continue their offerings for a few more weeks of Indian summer.
A cool afternoon, with temperatures in the 40s or lower, sets the stage for an even chillier night. Then frost is likely if the air turns eerily still at sunset, and stars glow brightly against a cloudless sky. The cloudless sky is like a giant vacuum, sucking back the day's warmth.
The soil, with its great capacity to store heat, keeps releasing heat to warm the air near ground level, perhaps enough to stave off frost on nearby plants. Plants growing in lawn or mulched ground are more likely to freeze than are plants growing in bare soil, because lawn and mulch are insulators, containing the ground's heat. Old-timers used to loosen the surface of the soil in the garden with a hoe or a tiller to help protect nearby plants from a few degrees of frost.
Heat lost from the earth's surface does not necessarily escape unimpeded. Water molecules in the air absorb some of this outgoing radiation, so frost is less likely when the air is moist. And anything between clear sky and the ground can reflect heat rays back to the ground like a mirror, keeping plants warm. So although open lawn might be covered with hoar frost after a chilly night, lawn under a tree might still look lush and green. Similarly, houseplants on a covered porch might come through a frosty night unscathed, while those out in the open might be limp with frost.
Anything draped over plants to block heat loss offers some protection from frost. Hence autumn's shrouds. Better insulators offer greater degrees of protection. Thus the temperature stays warmer beneath a blanket than beneath a fabric sheet, and warmer beneath the fabric sheet than beneath plastic sheeting. Reflective materials, shiny side down, bounce outgoing radiation back downward.
The best covering of all is clouds, quietly rolling in during the night. The mercury stops plummeting once this fleecy blanket covers the sky, and might even rise before morning.
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