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Index : Plant Care
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- 30. Protecting Plants from Winter's Chill - Top
- The first killing frost here in Tucson usually occurs during the week of Thanksgiving. While temperatures are still relatively warm, make preparations to protect your cold sensitive plants.
Cold protection can help extend the life of some warm season garden plants. Covering tomatoes a night or two can provide you with a few more weeks of vine ripened tomatoes. You can also pick green mature tomatoes to ripen up inside. Just make sure the tomatoes you pick have either a pink blush, or a white star on the bottom.
Most of our winter flowers and vegetables are cold hardy, to a degree! A light frost is likely only to damage begonias and impatiens. Geraniums are almost as sensitive. Then comes calendula, lobelia, alyssum, and snaps which tolerate temperatures down to 28 degrees. The cold hardiest are petunia, stock, and of course, pansy. Protection for these are needed only when temperatures dip below 20 degrees. Here at the Extension Center temperatures dipped to 17 degrees twice last winter. Admittedly, our location along the Rillito River is one of the coldest around town, but many low areas, along the Rivers and Washes, can experience the same.
Leafy winter veggies, including broccoli and cauliflower, have no problem with cold down to the upper teens. Beets were the only crop we experienced injury on at those temperatures.
Many tropical types of landscape plants are injured with freezing temperatures. Some of those most commonly planted around Tucson include: bougainvillea, hibiscus, tropical bird-of-paradise, cape honeysuckle, and philodendron.
Plants in pots are easily moved to warm locations during the winter or on cold nights. South facing walls provide a degree of protection. The warmth of the sun is stored in the masonry wall during the day, and released at night. This can provide several additional degrees of warmth overnight. Placing plants under patio roofs or building overhangs can also provide protection. Warmth radiating up from the ground is trapped by overhead structures. Even placing plants under the canopy of a dense tree or shrub can offer some protection.
Our bright Arizona sun is an ally when it comes to cold protecting plants. During the day, the sun's energy is trapped and stored in paving materials, masonry walls, surface mulches, and the soil itself. At night this energy is released around plants, keeping them warmer.
Moist soil stores more heat than dry soil. Early in the morning before a predicted freeze, thoroughly wet the soil around plants. By doing so, the soil stores up the sun's energy over the entire day, maximizing the warmth to be released at night.
Coverings placed over plants help to hold in radiant heat released from the soil, and from the plants themselves. These coverings can be cloth sheets, blankets, several sheets of newspaper, paper grocery sacks, or black plastic. The thicker the black plastic, the better it will resist tearing in the wind. Specially made frost blankets, made from spun polyester, can also be purchased from garden suppliers.
Regardless of the covering, it is preferred they not contact plant leaves and flowers, especially black plastic. Cold damage can occur to the parts of the plant in contact with the covering. Make sure that the cover extends to the ground, and it tacked down to prevent lifting from wind or loss of escaping radiated heat.
On very cold nights, a heat source can be placed under the frost covering. For smaller areas, a low wattage incandescent light bulb (40 to 60 watts) can be used. If you are protecting larger areas, such as coverings over tender shrubs or small fruit trees a heat lamp will be more effective. Christmas lights, especially the small twinkle light variety, should not be used as they give off very little heat.
When the sun rises and temperatures warm above freezing, covers should be removed. Even if a freeze is predicted on consecutive days, removing the cover will allow the sun’s warmth to once again heat the soil and provide a source of radiant heat.
Finally, make sure plants are going into the Winter season healthy. Stressed plants are prone to cold injury. Nutrient levels should be adequate to keep plants green, but not too high to push excessive growth. Hold off fertilizing landscape plants and fruit trees until March, when temperatures begin to warm.
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Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on November 15, 1998
- Updated: November 15, 1998
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