Dr. Grow: When Jack Frost comes nipping at your garden

At this time of the year, I'm asked the same question 10 or more times a day, "Is it too early to plant tomatoes or impatiens?" My response is always the same, "Tell me what the weather is going to do and I'll answer your question." I know this is not the answer that most people want to hear.

The answer to the question does depend upon the weather. Although we can have freezing temperatures at this time of the year, a gardener's major concern is whether it is going to frost.

To plants that are susceptible, frost is very detrimental or even fatal. Frost is ice that is formed into crystals on a plant's surface. These ice crystals have sharp edges, which cut into plant cells. When the ice crystals melt in the morning, the plant cells rupture and death may occur.

Most gardeners know that when the air temperature is going to drop below 40 degrees F on a clear, still night, frost will probably occur. Let me explain. During the day the sun radiates energy to the earth. All of the objects on earth such as cars, rooftops and plants absorb this radiant energy. At night when the sun goes down, these objects reradiate this energy back into space.

On a clear night, when most of the energy from a plant is reradiated into the atmosphere, the temperature on a leaf surface can be up to 10 degrees F lower than the ambient air temperature. This means if the air temperature is 40 degrees F, the temperature on a leaf surface can be 30 degrees F.

As you know, when the air temperature drops in the evening and night, excess atmospheric moisture is deposited on the surface of plants, cars and other objects. This moisture is called dew. If the surface on which the dew is deposited has a temperature below 32 degrees F, frost on ice is formed.

As mentioned above, frost is formed on a clear, still night. If clouds cover the sky, frost is not formed. At night the plant surface radiates energy into space. If a cloud cover exists, the energy bounces off of the clouds, reradiates back to the plant surface, and the temperature does not drop significantly. Therefore dew forms, but it doesn't freeze into ice crystals.

Frost is not formed on a night when wind blows. Under calm conditions, when the air temperature is 40 degrees F, the temperature on a leaf surface may be 30 degrees F due to radiant energy loss. A wind will mix the warmer ambient air with the colder air at the leaf surface. The mixed air at the leaf surface will stay above 32 degree F. Therefore no frost will form.

Orchardists will quite often hire helicopters to hover over their orchard if the trees are in bloom when there is a threat of frost. This artificial wind mixes air in the orchard just enough to prevent frost from forming.

If you want to plant tomatoes and impatiens early in the season, you have two choices. First, you can wait until the threat of frost is past and then plant. In this area, very few frost days have been recorded after May 1. Your second choice is to cover your plants on nights when conditions are right for frost to form. In effect, you are artificially creating a cloud cover over your plants.

There are several items you can use to cover your plants. Many gardeners use hot caps to cover their frost-susceptible plants. These caps are made of a paper that transmits some light through it. These covers should be removed from the plant each day and put in place each evening if the threat of frost exists.

You can use a milk carton or a rose cone to cover plants. These covers must be removed daily and replaced at night if there is a frost warning. If you use a plastic milk carton, be sure that no part of the plant touches the carton. Cold can be conducted through the carton and some leaf damage may occur.

I like to use a frost blanket. This lightweight material allows light and moisture to go through it. It can be left in place over plants for days at a time without really affecting the growth of the covered plant.

If you are a gardener who wants to get a jump on your neighbors and produce that first ripe tomato, then go ahead and plant early. Just remember that if you do plant before the threat of frost is gone, you will have to keep your eye on the weather forecast and be prepared to cover your plants when needed.