Frost Damage on Landscape Plants
Proper plant selection and placement of plants is helpful in preventing frost damage. If some plants are killed during winter freezes, replace them with hardier plants that are more tolerant of the cold. Think about using microclimate areas in your yard to plant tender, frost sensitive vegetation. Areas close to your house or under the shade of its roof or eaves tend to be warmer than areas that are exposed and farther away from your house. Bring smaller potted plants inside or place them in the garage during cold weather periods. Western and southern exposures are usually the warmest areas. Block and masonry walls, in addition to rocks and patios, collect and reflect the heat of the sun. Remember, a little protection will go a long way.
In the winter of 2007, populated areas around Tucson, Phoenix, and Yuma experienced some of the coldest temperatures in more than twenty-five years; and again in the winters of 2011 and 2013, nighttime temperatures fell to the low twenties and teens for several consecutive days. Around Tucson, temperatures fell below seventeen degrees Fahrenheit, killing or causing major damage to frost-tender plants and many native trees. During that freeze, even hardier landscape plants, including the hopseed bush, tuna cactus, aloe species, palm varieties, agave, cereus species, succulent, and prickly pear cactus, suffered extreme frost damage. Many of our common trees also suffered frost damage, including the willow acacia, shoestring acacia, sweet acacia, palo blanco, palo verde, bottle tree, Texas olive, lemon and weeping bottlebrush, Indian rosewood, desert ironwood, Texas ebony, Indian weeping fig, rubber tree, Indian laurel fig, silk oak, lysiloma, Canary Island pine, purple orchid tree, African sumac, eucalyptus, and citrus. A lot of our tender trees shrubs and groundcovers did not recover until late spring and after the summer monsoons.
Most frost sensitive trees and shrubs can survive a night or two of below-freezing temperatures. But when the temperatures drop below freezing for several nights, the results are deadly to native and non-native landscape plants.
Frosts are more likely to occur when the air is dry and subject to high day-to-night fluctuations. Cold air settles downward to the lowest lying areas, while hot air rises. That is why locations along riverbanks and dry washes in the Southwest desert often have the lowest temperature readings on a very cold night. There are low-lying areas as well as thermal belts throughout the Southwest desert. If you are living in a community surrounded by native ironwood trees, jojoba shrubs, citrus groves, and mature bougainvilleas, you likely are in a warmer microclimate. If you live along a riverbank or low-lying wash, these locations are more susceptible to frosts during the winter. Every town and city has microclimates—areas where temperatures are much colder than other locations. In these areas, think twice about growing semi-tropical plants that are subject to yearly winter freezes. It only takes one or two nights of a hard freeze to destroy plants that took many years to mature.
A freeze is often more severe than a frost. Fortunately, severe freezes are relatively uncommon in southern Arizona. Most arctic, cold fronts stay north of southern Arizona or travel east of the Rocky Mountains. However, less severe freezes do occur and may last for several nights. While gardeners can cover sensitive plants with protective sheets or blankets, it is often difficult to provide full protection. If the freeze occurs in combination with a winter storm that produces rain and high winds, it can shift the sheets or blankets used to cover plants or leave them soaking wet.
If nighttime temperatures drop to the low or mid twenties, covering plants with blankets, old sheets, or burlap sacks does offer some protection if they are fastened down. This helps conserve heat accumulated during the day. A more effective method is to apply a heat source to the plant. Place Christmas lights or a shop light with an incandescent light bulb in the canopy of smaller or medium-sized trees. This method works best on trees that have been covered. Just be careful that the light does not come into contact with water or the covering.
When the soil around a frost-sensitive shrub or tree is kept wet, it also retains some of the soil’s heat. Sensitive vegetation can be sprinkled with water throughout a frost. As water freezes around leaves and branches, it sometimes adds enough heat to maintain the plant tissue temperature at about the freezing point.
Many exotic succulents used as landscape plants in the Southwest desert are native to areas where freezing temperatures are nonexistent. Therefore, when temperatures drop below freezing for several hours, they suffer major frost damage. A few days after the frost damage occurs, plants may develop a black appearance on their outer, exposed parts. After a few weeks, the blackened area dries out and becomes crisp. Sometimes, new growth occurs weeks or months later, but some plants may turn yellow, shrivel and die.
Cold damage on agaves becomes evident when the leaves turn black quickly after a freeze. Wait until the damaged part of the plant turns brittle before pruning the frosted growth. The plant might seal off the frozen leaf, protecting itself from more damage and possible infection that could spread through the plant. There are ways to protect your frost-sensitive cactus. Cover the tops of Cereus species with Styrofoam cups or paper bags, protecting the top parts. Covering entire plants with cotton sheets also helps. Place a sixty-watt light bulb under the sheet, making sure the bulb doesn’t touch the covering. Do not ever cover plants with plastic sheeting. Plastic is a poor insulator and transfers the cold directly to the plant, causing more extensive frost damage. Plants in containers can be moved indoors or closer to the house, under roofs or eves. Placing potted plants in brightly lit areas also helps, as the light emits some warmth. Frost-damaged barrel cacti show damage on the margins of their ribs, and the area will yellow. If temperatures don’t drop too low, then only the plant epidermis may be damaged. The plant will outgrow the damaged area in a few years, because the damage is only cosmetic.
When frost occurs, damage to plants is not immediate. Wait until late February to mid March to prune. If you prune right away, another frost could worsen the damage. In Tucson, the last frost date is usually March 15. In Phoenix, the last frost day is about a month earlier. However, some reported frosts occurred in early March. Plants such as the lantana, red bird of paradise, asparagus fern, plumbago, bougainvillea, dwarf oleander, orange jubilee, and yellow bells freeze nearly every year, but grow back quickly in the spring. Fortunately, the dead, frozen leaves that appear after a frost provide some insulation against additional freezes. In these circumstances, the foliage works like a winter coat.
After a frost, some smaller shrubs and groundcovers can be radically pruned back to stubs if the damage is present. When cut back severely, these plants will grow back more uniformly than trying to cut only the damaged branches. Understand the plant’s growth habit before pruning frozen branches. Start at the smallest, dead branches. Use your thumb or a knife and lightly scratch the surface of the branch. Determine the color of the underlying plant tissue. If the cambium below the bark is green, the tissue is alive. If it is black or brown, it is frozen tissue and probably dead. When you find green tissue, you will know that it is safe to cut back to that point.
Flowers and other herbaceous plants such as the geranium, vinca, and begonia, as well as succulents and some soft tissue plants, will need to be removed. If the damaged stems are left on, they will rot and decay, spreading disease to undamaged portions of the plant. Frost damage is difficult to detect on some trees and shrubs. Wait until the warm days of spring when the plant begins to produce green sprouts from the live wood. The point where the new bud breaks will tell you how deeply it has been damaged. Eventually, hot weather and the summer monsoons hasten new growth, helping to regenerate plants. If you have the time and inclination, always attempt cold protection strategies. Move frost-sensitive plants that are in pots and cover those in the ground with proper protection—it should save you from having to replace expensive or cherished landscaping.