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Seesawing temperatures fool plants and gardeners and
remind us that March and April weather is unpredictable. As temperatures
get warmer, many gardeners want to start planting, but they need to
understand their plants’ ability to withstand the cold. The average date of the last frost in central Illinois
(Zone 5B) is April 15. This is an average, meaning that the chance of frost
on this date is 50 percent or lower. The odds of frost drop by an
additional 10 percentage points for each week after this date. The
frost-free date for Zone 5B is May 12. So how do you know when it’s safe to plant an
annual? It depends. Certain flowering annuals and vegetables can withstand
frost, but others cannot. You can tell by the plant’s classification,
which may range from hardy to warmth-loving. Cool-season plants, classified as very hardy and frost-tolerant, develop and grow best
in cooler weather. The seeds germinate in cool soil, and the plants
withstand frost. Very hardy annuals
withstand freezing temperatures and hard frosts without injury. These
species can be planted as soon as the ground can be prepared, about four to
six weeks before the average frost-free date. Examples include pansies,
leaf lettuce, spinach, cabbage, and broccoli. Frost-tolerant annuals
can withstand light frosts, long periods of cold, and damp weather. These
plants can be planted two to three weeks before the average frost-free
date. Examples include cleome, carrots, and beets. Warm-season plants, classified as tender and warmth-loving, develop and grow best
in warm weather. Their seeds germinate in warm soil.
Tender annuals are injured or killed by frost and so
should be planted around the average frost-free date. Examples include
snapdragon, marigold, snap beans, tomatoes, and sweet corn. Warmth-loving annuals, intolerant of frost and cold,
require warm soil and air temperatures for germination and good growth, and
most are tolerant of high summer temperatures. Examples include zinnias,
cosmos, sunflowers, cucumbers, watermelon, and peppers. Temperatures can fluctuate with little warning. If a
frost advisory is in the forecast, it may be possible to limit cold injury
to plants by covering them with old sheets, blankets, or lightweight rugs.
Avoid heavy materials, which can damage plants. Also avoid plastic, which
will transfer the cold to the plants and therefore provides little
insulation. It may not be possible to protect plants from temperatures
lower than 25 degrees Fahrenheit. As temperatures begin to warm again, don’t
forget: Mother Nature may still have some surprises in store.
Jennifer Fishburn is a horticulture educator
with
the University of Illinois Extension
Sangamon-Menard Unit. Contact her
at www.extension.uiuc.edu/Sangamon.
This article appears in Apr 12-18, 2007.
