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Index : Plant Problems - Cultural
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- 11. Protecting Plants From Severe Cold - Top
- Our early cold weather may be a good indication of very cold weather to come in Southern Arizona this winter. Recent mild winters have lulled many gardeners into a false sense of security. Exotic plants from tropical climates, plants like Malaysian ixora, Brazilian alamanda, African pentas, and tropical bananas are being purchased from local garden centers and planted. Certainly if cold comes these plants will perish without considerable protection. But more importantly, many commonly grown plants such as Queen palm, bougainvillea, citrus trees, jacaranda, guava, carissa and others may be lost from very cold temperatures.
There are ways to protect plants from the severe cold we could experience - temperatures in the teens or single digits. With these low temperatures, simply covering plants with sheets or frost blankets won't be enough. It will be necessary to use a heat source underneath covering materials to keep the temperatures high enough to prevent severe damage.
The best heat source to use outside is an incandescent bulb with sufficient wattage to warm the air under the cover. Low covers over low plants will require lower wattage bulbs than when covering larger plants. When warming bed plantings of annual flowers, ground covers or individual shrubs a 40 watt bulb should provide enough heat. Large vines, shrubs or small trees will require heat from a bulb of 75 watts or more. Heat lamps can also be used for larger plants. Christmas lights, the small twinkle sizes, do not give off enough heat to protect plants even when placed under a cover.
Utility lamp fixtures, also known as trouble lights are best for placing under plants. That's because the bulb is housed in a plastic or metal cage, protecting the bulb from breakage. Heat lamps can also be purchased in fixtures with protective cage enclosures for the bulb. These fixtures are rated for a certain wattage bulb. Do not exceed the rating by using a higher wattage bulb than the fixture allows. If extra heat is needed, two bulbs in separate fixtures could be used under one cover. Fixtures such as these are available at all hardware and home supply stores.
It is important that the heat source be placed at or near ground level. Warm air rises, and plants need to be keep warm from the ground up. It's also important that the cover you use be placed all the way to the ground and anchored. If not securely attached to the ground, wind or even a light breeze can pick the cover up, allowing cold air. If not well secured the cover may lift partially or completely off the plant.
The best material to use for covering is cloth or other fabric materials such as sheets or frost blankets. Large tarps may be used for small trees. If tarp is heavy, thing about supporting it with a frame. A simple, but sturdy frame can be constructed out of PVC irrigation piping. It can be formed into a box by using elbows and tees to couple the piping together. Bamboo or wood stakes can be used to hold the covering off of smaller plants. Avoid using plastic coverings. Plastic conducts cold, so even with a light source underneath the air may not stay warm enough.
For trees that are too large to cover, wrap the trunk and larger limbs with an insulating material. You can use newspapers, wrapping the thickness of about one newspaper section. You can also use blankets, foam rubber or paper-backed fiberglass insulation. The wrapping maintains heat in the core of the tree. Even if unwrapped stems and branches die, new shoots can sprout from the protected branches in the spring. It is important to unwrap the branches and trunk during the day, to allow sunlight to heat up the wood before rewrapping as the sun sets.
Finally, gather and store the materials you will need to protect your plants now. Making plans in advance will make it easier to cover and protect your plants when the time comes.
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Written by John Begeman, College of Agriculture, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on Nov. 26, 2000
- Updated: April 20, 2001
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