Many garden and landscape plants we grow in Southern Arizona are tropical or subtropical by nature and need protection from freezing temperatures.
Some we commonly grow include lemons and limes, young citrus trees, exotic cacti, bougainvillea, plumbago, hibiscus, natal plum and tropical bird-of-paradise. Frost-tender flowers include geranium, impatiens, begonia and cyclamen.
A few supplies should be readied now to protect these plants from approaching cold snaps. Topping the list: sheets and blankets. Old sheets provide excellent cold protection when draped over frost-sensitive flower beds and container plants, or less hardy varieties of trees, shrubs, cacti and succulents.
Cloth and paper provide the best insulating qualities to hold in heat. Consider purchasing frost blankets from garden centers and nurseries. These are made of non-woven polypropylene that is lightweight and can be left on plants for several days without causing them harm.
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Newspaper also makes and excellent frost blanket material. The covering should be several sheets thick to maximize cold protection. Paper grocery bags are excellent to use as frost caps to place over the tops of tender plants. Cold-sensitive columnar forms of cactus can be protected by placing foam cups over the top of each column. The cups insulate and protect the delicate growing tips from cold.
Plastic should not be used to cover plants unless a heat source is provided underneath. Plastic actually conducts cold, so heat must be generated inside the plastic cover to protect the plants within. When covering small areas, a low-wattage incandescent light bulb (40 to 60 watts), contained in a simple utility lamp fixture, can be placed under the plastic to provide adequate heating. When protecting larger plants — such as coverings over tender shrubs or small fruit trees — a higher-wattage heat lamp will be more effective. Heat lamps can also be used under sheet coverings and frost cloth to increase heat. Heat under plant covers is critically important if temperatures fall into the teens.
When frost covers of any type are used, it's important that they be secured properly. Cold snaps are usually accompanied by windy conditions. If plant covers are not secured properly, warmth will escape from under the cover. Cold air will also be forced under the cover on windy nights.
Tender plants growing in containers may not need covers if they can be moved under a patio roof or a shade tree. These spots stay warmer because the overhead cover traps heat radiating up from the ground. Areas close to south-facing walls also provide protection. The sun's rays heat the wall surface during the day; this stored heat is then released at night, keeping adjacent areas warmer. Added protection can be provided by placing cold-sensitive container-grown plants in the garage. Temperatures there will be as much as 20 degrees warmer than outside air temperatures.