Nothing sends gardeners running faster than a weather forecast of FROST. Cool air, clear skies and light or calm winds are necessary for frost to occur. Cool air permits temperatures to drop low enough to freeze moisture in the air which would otherwise form dew. When skies are clear, heat from the soil is able to rise, allowing the cool air to settle close to the ground and chilling the plants as they lose heat. Calm winds allow the cool air to settle without mixing it with warm air.
Frost (the sparkling ice crystals that form on all surfaces) can occur without severely damaging plants. The critical feature is the internal temperature within plant tissues. If temperatures within these tissues are cold enough to break cell walls or disrupt cell constituents beyond repair, damage, wilting, and dying will occur in those tissues affected.
Some plants are more tolerant of frost than others. Woody plants are less affected than succulent plants. Fruits and flowers may be more sensitive than leaves. Sudden and prolonged freezing will be more damaging than gradual cooling of short duration. Plants already exposed to cool temperatures will be more resistant. Within our own properties we can find variations on different sides of the house, under trees, on south or north facing slopes, or low lying areas. Cool air settles at the bottom of slopes because it is heavier than warm air. Frost pockets will then form in valleys where cool air becomes trapped. Hilltops are also susceptible to cool temperatures. Hillsides often remain frost free until a more severe frost occurs.
How can we protect plants from that first cold snap? The two most common methods are covering to keep the plants warm or to warm the plants by sprinkling with water. Covering is the most effective for most people. Covering plants the night before with a sheet, blanket, or tarp will trap the warmth from the soil over the plants thus preventing freezing. This type of covering will usually protect plants when temperatures drop into the upper 20's. Plastic used as a covering usually doesn't work as well as the other coverings mentioned. Sprinkling the plants with water is often used as a "morning after" solution. When water cools and crystallizes into ice, heat is released which may prevent internal damage before freezing occurs within plant cells. The time when the internal plant temperature is coldest is in the morning. If the drop in temperature is not too great (more than a few degrees), watering plants in the early morning may protect tender plants that were left uncovered. Of course it never hurts to wish for cloud cover and a good breeze on those first cool nights of autumn to help prevent damage.
This article originally appeared in the October 11, 1996 issue, p. 163.
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