When will Jack Frost arrive in Union County?

Oct. 14, 2013 @ 02:42 PM

Historically, our average frost date (the date when the temperature drops below 32 degrees F) is around October 12th. Although the long-range forecast doesn't show a frost to be imminent, we know it's coming. Your garden's location, the surrounding landscape and man-made structures that border your garden can make the difference between frost damaged vegetable garden plants and plants that will hang on a bit longer before a hard freeze (below 25 degrees F) sends them to the compost pile.

Gardeners and farmers in the pre-Weather Channel days watched the natural signs to predict when temperatures would dip into frost range and used several techniques to extend the season. Locating the garden to avoid "frost pockets" and take advantage of solar radiation is the first consideration for creating a garden that delays frost damage. For instance, a garden located on a gentle slope facing south receives more heat and light for longer and with more intensity than other garden sites. Cold air flows down slopes so gardens at the top of slopes will get frost later than those at the bottom or on level spots.

Locating a garden that is surrounded on two to three sides by trees will survive frosts longer than open gardens.  Trees exude moisture, raise the dew point and act like a blanket that prevents ground heat from escaping into the atmosphere. Trees growing on a slope also warm the cold, downward flowing air. However, walls, fences and hedges can collect cold air flowing down a slope in frost pockets, so frost occurs sooner at the base of these areas. Providing an opening in the wall or hedge so the cold air can drain may delay frost on the plants growing beside a barrier. On the other hand, a south-facing stone or brick wall can serve as a "heat sink." During the day, the sun warms the wall, then releases the heat at night, making the plants on the south side of the wall less prone to frost.

We can look to the clouds and feel the breeze to help determine if frost will settle on our garden plants. During the day, the earth absorbs heat from the sun and at night, it radiates that heat back into the atmosphere. Cloudy skies act like a blanket to hold heat in and prevent the soil's stored heat from radiating into the atmosphere. If the temperature is falling, the sky is clear and the air is dry, frost is more likely to affect your garden. A slight breeze can help mix the cooler night temperatures with the warmer air near the soil. Plants that grow near to the ground, like lettuce, are less likely to suffer frost damage that plants that grow tall, like tomatoes, or hanging baskets.

The amount of moisture in the air can also help us predict frost. Ever hear the weather forecaster mention the "dew point"? As the temperature begins to fall in the evening, the air holds less and less moisture until the moisture condenses and forms dew. The dew point is the temperature at which dew forms. When dew forms, heat is released and that heat helps to keep the air temperature at or slightly below the dew point. The more moisture in the air at sunset, the less likely that frost will occur during the night. When frost is predicted, commercial growers watch the dew point and use sprinklers to add moisture to the air, which raises the dew point around plants. As a general rule, don't worry about frost if the dew point is above 45 degrees.


• Mary Roberts and Ray Tarlton are owners and managers of  Windcrest Farm, a USDA Certified Organic farm and greenhouse in Monroe, NC . Visit www.WindcrestOrganics.com for more information about plants, produce and classes. Facebook: Windcrest Farm Organics