From Frost to Freeze
By: Julie Barnes
In the course of autumn, frost warnings send chills up the spines of die-hard gardeners. To cover, or, not to cover is the question? E.B. White, best known for the childhood classic Charlotte's Web, had a northern agricultural perspective on this. "Those of us who garden in places with just so many frost free days must do so concisely. Tender plants have a limited life span and seasons no matter how we try to lengthen them, must come to an end." With that said, just remember there are many hardy or halfhardy plants that can tolerate light or moderate frosts before the "killing frost" terminates our gardening season. The first nip of cold weather will damage tender annuals such as begonia, coleus, impatiens, geranium or vinca which originate from warm winter regions. But, hardier plants, such as, sweet alyssum, calendula, snapdragon, dianthus, viola, or dusty miller come from cold winter areas making them better early frost survivors. Frost does not "fall" but, is actually the natural reaction of warm objects contacting cool air. If you hope to protect your plants there are signs to look for to gauge the likelihood of frost. This begins with knowing the average first fall frost date in the area where frost occurs 50% of the time. According to the National Climatic Data Center, our frost free growing season lasts about 170 days with an "almost" guarantee of no frost from May 14 thru October 3rd.
Frost Forecasters
❖ The temperature is cool at sunset. Plants soak up heat from daytime sun. Once temperatures dip to 32°F, frost then forms on leaves releasing warmth into cooler air.
❖ The air is calm. As cold air collects near the ground, warmer air will remain trapped above it. A frost can be averted when a steady blowing night breeze mixes the air.
❖ The sky is clear overnight. Lack of cloud cover is a definite frost indicator as the temperature drops considerably from rapid heat loss; in contrast thick clouds that stretch across the horizon will work like a blanket to block frost and retain heat.
❖ The soil is cold and dry. Additional moisture around plants can prevent frost formation when the soil is warm and wet instead.
❖ The night is long. A longer night provides extra time for temperatures to fall. Sometimes, daytime clouds may start to diminish at dusk. With overnight sky clearing, temperatures can drop enough for frost.
❖ The dew point is close to freezing. Dew point measures the humidity which can affect frost. A lower dew point temperature produces drier air, increasing the likelihood of an overnight frost. With higher humidity, an air temperature drop is improbable.
❖ The Moon is new or full. Frost is more likely in the week before, or, under a new moon.
❖ An arctic mass follows a cold front passage. A huge mass of freezing air from the North will kill everything but the hardiest of plants.
The "killing frost" of fall is actually a killing freeze that dramatically blackens plant foliage after turning their water-filled tissues to ice. During the fall season, frost slows us down before bringing on the closure and rest needed after another gardening season. It's time to weather that transition.