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Wednesday, April 18, 2007 06:01 am

Some annuals still at risk

Don’t let fickle weather put you in a pickle

Untitled Document 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.
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