Providing a delightfully pungent or sweet flavor to many family dinners, onions (Allium cepa) are easy-to-grow garden vegetables. As their bulbs swell in response to day length, onions mature, developing their characteristic papery brown skins. This cool-season crop reaches the end of its growing season with the first hard freeze.
1. A Frost or a Freeze?
Even though the words "frost" and "freeze" are often used interchangeably, they mean different things to your onions. Frost happens on a clear, calm night as the temperature briefly drops to a few degrees below freezing. Your onions usually survive this bit of cold with relative ease. A freeze, on the other hand, is prolonged cold below 28 degrees Fahrenheit that damages onions by allowing ice crystals to form in the tissues. Semi-hardy vegetables like onions can recover from one or two light frosts of around 31 degrees F.
2. Frost Protection
While you cannot do much to protect your onions from a freeze, frosts are another matter. Tucked into the still-warm soil, onions typically survive one or two frosts. You can up their survival chances by capturing that soil warmth with a covering of at least 2 inches of straw, leaves or grass clippings. Floating row covers, bed sheets and blankets all provide similar protection. Watering the soil around your onions the day before a frost provides even more insurance against frost -- moist soil absorbs more daytime heat, holding it in place for an evening release. Remove all the coverings during the day to allow the soil to recharge itself with more of the sun’s heat.
3. Harvesting Before a Freeze
While onions can withstand frost with protection, a hard freeze -- where the temperature drops below 28 degrees F -- means you need to harvest your crop or risk losing it. Even coverings won’t protect your onions. Loosen the soil around the bulbs with a hand trowel or shovel, pull them from the ground and gently brush away any loose soil. Bruised onions are prone to rot, so be gentle. Spread the bulbs on a thick layer of newspaper, allowing them to dry for several weeks. They are now ready for long-term storage. According to The Old Farmer’s Almanac, onions do best when kept in a dry location with a temperature of 40 to 50 degrees F.
To keep your onions healthy and growing strong right up to the first freeze, tend to the pests that bother them. Deal with tiny, tan-colored thrips -- insects no thicker than a sewing needle -- with two sprays of insecticidal soap, spaced two days apart and applied to the aboveground part of the plant. Mix at a rate of 1 tablespoon of soap per quart of water. Keep onion maggots away from your crop with a floating row cover. Don’t mulch near your onion plants during the growing season to avoid providing a nesting ground for other insects.
- The Old Farmer’s Almanac: Onions
- Colorado State University Extension: Frost Protection and Extending the Growing Season
- University of California Cooperative Extension Vegetable Research and Information Center: Imperial County Agricultural Briefs
- Iowa State University Extension: Wash Away Houseplant Pests With Insecticidal Soap
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