Sierra Vista, Arizona
A. Even plants that are hardy under normal conditions for your region may be damaged by unusually cold temperatures that last for more than a few hours. In addition, cold injury tends to be more severe if very low temperatures occur early in the winter when plants are not fully hardened against the cold or in spring as they are beginning to emerge from dormancy, says Jack Kelly, a retired commercial horticulture agent with the University of Arizona's Pima County Cooperative Extension.
Be patient when assessing plants in the wake of damaging cold, Kelly advises. Wait until all frost danger has passed and plants are starting to put forth new growth. Severely freeze-damaged cacti will "melt down to mush," says Kelly, but if there's any green left, such plants may come back from their roots. Succulents like agave and yucca may have blackened foliage that turns dry and crisp after a hard freeze but can recover if the damage is not too extensive. Grab the central leaves of agave and pull, Kelly suggests.
"If they come out, it's not salvageable." Cold injury on columnar cacti such as Cereus spp. shows up as straw-colored patches on the ribs that a plant may gradually outgrow. Watch saguaro cacti for signs of bacterial rot following cold damage; use a clean, sharp knife to cut out blackened spots that are less than 2 inches in diameter. Allow the wound to heal on its own and clean your knife with a solution of 1 part bleach to 9 parts water between cuts. When finished, spray the open wound with the same bleach solution. Some species, says Kelly, may take nearly 6 months to recover after suffering injury from extreme cold.
In late summer, help your plants prepare for the cold weather ahead by backing off on watering and fertilizing, Kelly says. And, to avoid stimulating new growth, don't prune plants after mid-September, or feed cacti or succulents after late September. When unusually cold weather threatens, take steps to protect mature cacti and other hard-to-replace landscape plants. Covering native cacti and succulents with cotton sheets or row covers often provides enough protection, Kelly says. For citrus or other marginally hardy plants, heavier covers, such as the "mover's quilts" available at home centers, may be in order. These can be augmented with a utility lamp with a 60-watt light bulb placed under the covering when extended periods of cold are forecast. Don't cover plants with plastic, he warns, because it offers little insulation.
If you wind up having to replace plants that have been killed or irreparably damaged by the cold, Kelly suggests taking a drive around your neighborhood to see what survived. Talk to local nurseries that specialize in native plants, and don't be tempted by showy tropicals unless you're willing and able to give them adequate protection each time temperatures plummet.
Ask Organic Gardening is edited by Deb Martin.
Photo: (cc) mikebaird/flickr