University of Vermont Extension
Fall or Spring News
Department of Plant and Soil Science
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
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
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.
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.
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
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.
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
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.
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,
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
the future, consider cold frames for your garden, either portable or
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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