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Index : Plant Problems - Cultural
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- 3. Protecting Sensitive Plants for Winter Chills - Top
- Those chilly nights we’ve been having of late are a reminder that cold weather frosts and freezes may soon be upon us. Even here in Tucson it’s not unusual to have 6 or 8 nights of freezing temperatures coming anytime from early December to late March. It’s on these nights that frost sensitive plants need protection. So be ready to insulate your plants from the cold, and here’s how:
First, determine which plants need cold protection. In general, most sub-tropical and all tropical plants are cold sensitive. Most commonly grown among these are Bougainvillea, Plumbago, Hibiscus, Natal Plum, Crown-of-Thorns, Cape Honeysuckle, Tropical and White Birds of Paradise, and many non-native Cacti.
Many citrus trees are also frost and freeze sensitive. Mexican limes can be damaged with frost. Freezing temperatures below 32 degrees can harm lemons. Oranges need protection at 25 degrees and below. Grapefruit, mandarins and tangelos are the less prone to freezing, but can be damaged when temperatures approach 20 degrees. But, all newly planted citrus, regardless of the type, are cold sensitive and need protection from freezing temps.
The most cold sensitive of the winter flowers we grow are; Geraniums, Impatiens and Begonias. They can be damaged by light frosts. Lobelia, alyssum and calendula need protection when temperatures drop to 28 degrees. At 25 degrees it’s time to insulate snapdragons. Hardier still are petunias, and stock. They can tolerate temperatures down to 22 degrees. Finally, the most cold hardy of all are pansies. They’re happy as long as temps don’t fall below 20 degrees.
The best protection for plants is provided by simply covering them with sheets, blankets, cardboard boxes, paper grocery bags, canvas tarps or frost cloth (available at garden centers). These coverings provide anywhere from 3 to 6 degrees of additional warmth. If that’s not enough, then a heat lamp or utility lamp equipped with a 75 watt bulb should be placed under the covers. The lamp will keep the air under the cover up to 10 degrees warmer than the air outside. A word of caution - do not use holiday ‘twinkle’ light strings. They don’t give off enough heat to keep plants warm.
I do not recommend using plastic covers over plants! Plastic, be it clear or black, conducts the cold and plants will freeze underneath. Blankets should only be used to cover strong, woody trees or shrubs as their weight could beak weaker plants they are covering.
Frost blankets and covers made of cloth or paper can be left on plants for several days. Our winter cold fronts usually linger for two or three days, so it’s easier just to keep plants covered.
The Tucson Metro area has many winter microclimates, some rather warm, others very cold. Try determining what the microclimate is like in your area. Warmer locations are those in the city where pavement and buildings tend to hold the most heat. Warm areas also tend to have cold sensitive trees like Citrus, Jacaranda, Ironwood, Palo Brea trees growing successfully. Outlying areas are generally colder, especially in low areas along washes and rivers where cold air settles. Having and using an outdoor thermometer is one of the best ways to determine how cold your property gets relative to other parts of town.
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
- Updated: November 27, 2005
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