For Immediate Release March 29, 2002
When Can Illinois Expect the Last Spring Frost?
Now that Illinois has had a few days with freezing rain and snow, gardeners and farmers alike are eager to
begin their spring planting, but they still need to wait a bit longer and keep frost dates in mind.
"The last spring frost usually occurs between April 7 (southern Illinois) and April 28 (northern Illinois), and
April 14-21 (central Illinois) based on 1971-2000 averages," says State Climatologist Jim Angel of the Illinois State
Water Survey (http://www.sws.uiuc.edu), a division of the Illinois Department of Natural Resources.
"The actual frost date varies quite a bit from year to year," says Angel. He suggests adding two weeks to the
average frost date for your area before planting tender plants to protect them against the possibility of a late season
frost. "By doing so, the odds are only 1 in 10 that frost will occur later in spring" continues Angel.
Although 32 degrees Fahrenheit is the temperature traditionally used to identify frost, visible frost can be seen
on the ground and on objects at slightly warmer temperatures on calm, clear nights that allow cold, dense air to collect
near the ground. Under these conditions, the temperature near the ground actually can be a few degrees cooler than at
the 5-foot height of the official National Weather Service thermometer.
Open, grassy areas usually experience frost first while areas under trees are more protected because the trees
help prevent the heat from escaping. Covering tender plants when a frost is expected can provide this same type of
protection. Plants near heated buildings sometimes are spared too. An abundance of warm buildings and trees in town
means that urban areas tend to have frost less often than rural areas.
According to Angel, average high temperatures in spring (March-May) range from the upper 50s (north), to
the low 60s (central), to the upper 60s (south), while the average low temperatures range from the upper 30s (north),
to the lower 40s (central), to the upper 40s (south). Data from the Water Survey WARM Network also indicate that
soil moisture across the state is near to above average for this time of year.
"Don't let the mild winter we've had fool you into thinking we'll have a warm spring. Historical data indicate
that warm springs do not follow warm winters. Although there is a tenuous relationship between warm Aprils and
earlier dates at which the last spring frost occurs, it only takes a day or two of unseasonably cold weather to produce a
late spring frost," cautions Angel.