When freezing plants damage our plants
When freezing temperatures cause damage to our desert gardens, horticulturists generally have a few responses. One is the surprise. We never seem to expect it to get terribly cold here in the Mojave, although it does happen every so often. Most years, there are a few nights of chilling, but rarely does the temperature drop much below the freezing point. If it becomes excessively cool and damage is severe, perennial plants can appear to be so damaged that they look dead. Any gardener, no matter how experienced, can have a moment of near panic when that happens. When a landscape that was lovely only a few months ago overnight turns to a yard with brown foliage and blackened mush, it can be seriously distressing.
Fortunately, this kind of injury only happens occasionally. Covering raised beds with a sheet of clear plastic or row cover is usually enough to prevent frozen plants, but those nights when the soil or nighttime temperature is less than 28°, even this layer of protection might not be enough. That is when the results of very cold temperatures appear. Since water expands when it freezes, plant cells expand to the bursting point.
Well-established perennial plants usually have roots that survive the cold. A plant that has thrived in place for a few years will tend to have a robust root system insulated by several inches of soil. With this kind of protection, the roots of perennials are usually safe from death, unless temperatures have dropped to an extremely low point.
While the roots may be healthy, the above ground plant parts might not be. Dealing with these problems will vary, depending on the type of plant.
Woody perennials can show damage on overwintering foliage, but as long as the roots are in good shape, many of these shrubs can be pruned down to a few inches tall, and the plant will produce new growth as soon as the weather permits.
Many desert landscape plants grow as rosettes from a central point, but visual similarities do not mean they respond to extreme cold in the same fashion. The site where they evolved helps predict what the response will be.
Many aloes, for instance, originated in Africa, where icy temperatures are extraordinarily infrequent. When they experience a deep freeze, the center rosette becomes mushy, and comes out of the ground with only a slight pull. It is a good idea to remove the entire aboveground rosette and leave the roots in place. When spring and summer return, the plant will almost certainly return as well.
Agaves might look like aloes, but they are native to North American deserts where they experience a wild range of climatic variations. When they suffer cold injury, it is the outer (lower) leaves that show the damage, becoming brownish black and either limp or crisp. Their rosettes do not collapse from cold, but if they do collapse, it is frequently a sign of the dreaded agave weevil. When that occurs, the entire plant must be removed and agaves must not be placed there again for many years.
Dr. Angela O’Callaghan is the Social Horticulture Specialist for University of Nevada Cooperative Extension. Contact firstname.lastname@example.org or 702-257-5581.