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Index : Plant Care
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- 31. Protecting Your Plants From The Cold - Top
- The pleasant chill of autumn has returned to Tucson. It reminds us that the cold of Winter can't be far behind. And while were nestled snug in our homes, our garden and landscape plants must endure the frosty weather, come what may.
Most of our landscape trees and shrubs are cold adapted. With the exception of some tender introduced species like bougainvillea and hibiscus, desert plants can take the cold. This is not the case, however, for tender vegetables and flowers.
Warm season vegetables like tomatoes and peppers will freeze at 32 degrees. Cool season vegetables like broccoli, lettuce and cauliflower can take slightly colder temperatures. But even they need protection when temperatures dip much below 30 degrees.
Flowers also have varying degrees of cold hardiness. Snapdragons and violas are the most cold hardy. They can tolerate temperatures in the upper teens and low 20 degree range. Petunias and calendulas are slightly less cold tolerant, withstanding temps from the low to mid 20s. Others, such as geraniums and begonias will be damaged with the slightest frost.
When in doubt about a vegetable or flower plant's cold hardiness, error on the side of caution. If a frost or freeze is forecast, take precautions to protect your plants. Like water, cold air drains down hill. It settles in low areas or pockets. These low areas could be at the bottom of washes and river beds, or in low spots around your home. Anything that restricts the downward movement of cold air, such as walls or fences, also traps cold air. These locations will be the coldest around your home, and the areas were plants are most likely to suffer freeze damage.
Areas exposed to direct winter winds from the North and Northwest will also be vulnerable to cold damage. South facing walls will provide the warmest location. Here plants are protected form winter winds and are exposed to the heating rays of the sun.
Our clear winter days here in Arizona give us a winter's edge for fighting cold. Radiation heating from the sun works to warm up sun exposed surfaces. Masonry walls, driveways and walks all absorb the heat of the sun during the day, and release that heat at night. The soil also absorbs heat from sunlight. The radiation, or release of this stored heat helps to moderate night-time temperatures, especially close to the ground where tender plants grow.
Moist soil does an even better job of storing the sun's energy. That's why one of the best ways to protect your plants from cold is to thoroughly water the garden soil the morning prior to a predicted frost or freeze. The sun will heat up the moist soil, storing it for release 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, newspaper several sheets thick, 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.
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 enough to encourage growth. New growth coming out now is more susceptible to freezing. Hold off fertilizing plants 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 17,1996
- Updated: November 17,1996
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