University of Vermont Extension 
Department of Plant and Soil Science

Fall or Spring News Article
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FROSTS IN THE GARDEN
 
Dr. Leonard Perry, Extension Professor
University of Vermont
 
 
Understanding some facts about frosts may help prolong your gardening season, and help to protect your plants from injury. 

The most common question I get concerns frost dates.  Generally, and this will vary with area, the first frost date in fall in USDA hardiness zone 5 is around the first week of October, about 10 days earlier in zone 4.  In the spring, the last frost date is around the second week of May in zone 5, and about 10 days later in zone 4.  These zones refer to average annual minimum temperatures, and are shown on a map seen in many publications (www.usna.usda.gov).

Within these map zones or large geographic areas, however, are smaller “mesoclimate” zones which may vary in their temperatures.  Areas near a lake, in valleys, or on mountain foothills for instance can have quite different temperatures and frost conditions.

Since cold air is heavier than warm air, it tends to sink into valleys.  Mountaintops, too, are generally colder than lower elevations.  For these reasons, frosts usually come first in these areas while hillsides remain frost-free.  Similarly, on even a smaller or “microclimate” scale, some parts of a particular property such as low areas may be more prone to frost than areas near warmer pavement or buildings.

There are two types of frosts to watch for: “advective” and “radiation.”  Radiation frosts usually are the first of the season, and happen on clear, calm nights.  During the day the sun warms the soil, and if clouds are present at night, they trap this heat near the ground as it radiates upward.  If the night is calm, and there are no clouds, this heat isn’t trapped but rather keeps rising into the upper atmosphere.  The temperature near the ground becomes cold and a radiation frost forms.  This is the inverse of daytime when the ground is warmer from solar heating, while the upper atmosphere is colder.  Radiation frosts often follow the passing of a cold front.

Advective frost forms when a cold front moves through an area, usually later in fall, and typically is accompanied by clouds and gusty winds.  In these situations, cold air may reach a mile high.

Since often there are a couple weeks or more of growing season after the first radiation frost of the season, if you can minimize the effects of this frost you can get more enjoyment from flowers and a longer harvest season.  Here are several methods to protect tender plants from frost.
*Watch both the calendar, and the daily forecasts during the times for first frosts in your area.  If you look up data online on weather sites, date of first frost in fall (and last in spring) is when there is a 50 percent chance.  This is important to note, in that some years frosts may come before this date, and some years after. You also may see freeze severity listed.  Light freeze (29 to 32 degrees F) kills only tender plants, moderate freeze (25 to 28 degrees) will be widely destructive to plants and fruits, and severe freeze (24 degrees and colder) damages most plants.
*Of course the main prevention most think of is covering plants.  Woven fabrics are better than solid ones such as plastic.  You can tell a frost may be coming when you see yards showing a
colorful patchwork of sheets, blankets, and other materials. There are special white fabrics you can find at complete garden stores just for frost protection, often referred to as “floating row  
covers.”  There is a lighter weight or thickness, giving perhaps two degrees protection, a thicker one giving up to five degrees protection.  Whatever fabric is used, greater protection comes from not having the material rest directly on the plants.  Apply covers in early evening as winds die down, and remove the next morning as the sun warms the plants. For a few smaller plants you can make “hot caps” from recycled milk or soda bottles with the bottoms cut out, paper bags, or newspaper tents.
*One easy method to afford some frost protection that many don’t think of is irrigation.  Moist soil can hold up to four times more heat than a dry soil, conducting heat faster to the soil surface, and keeping the air above it about five degrees (F) warmer.  So water well before a frost.   A variation on this water theme is milk jugs, painted black, full of water in the garden.  These absorb heat during the day, releasing it at night.
*If you can’t protect sensitive crops like tomatoes, harvest them early.  Green tomatoes don’t need light to ripen, and in fact ripening can be slowed by light.  Keep fruit 55 and 65 degrees (F) for best ripening.  Tender crops that can’t withstand frost include tomatoes, eggplant, peppers, beans, cucumber, sweet corn, squash and melons.  Beets, carrots, lettuce, cauliflower, and potatoes will withstand a light frost.  Cool-season crops such as cabbage, broccoli, onions, parsley, peas, radish, spinach, turnips, and Brussels sprouts will withstand a hard frost.
*For the future, consider cold frames for your garden, either portable or permanent. 
           
More on frosts, as well as other effects of weather on gardening, can be found in articles from Cornell Cooperative Extension (www.gardening.cornell.edu/weather/).
             

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