A well-grown cactus accentuates the landscape, with interesting form and showy flowers. Cacti also make excellent container plants. This large plant family has around 2,500 species. Most cacti are slow-growing, so a small cactus may represent a decade or more of carefully invested time and effort. Larger landscaping plants can be costly to replace or may suffer permanent damage. In locations with occasional frosts, it’s essential to protect cacti from potentially damaging cold.
Knowing where a cactus grows in the wild indicates how much cold it can withstand. Some cacti native to cold winter areas tolerate temperatures below zero or the equivalent of U.S. Department of Agriculture plant hardness zone 5. However most cacti native to the warm regions such as our Sonoran Desert tolerate less cold. A good example is the widely-grown golden barrel (Echinocactus grusonii), which is hardy in USDA zones 9 through 11. Click here for an interactive USDA temperature zone map.
Duration and Length of Cold
To assess how much at risk your cactus could be in the face of predicted cold weather, take into account more than just the minimum temperature your plant is rated to withstand. If the cold is predicted for just a few hours, the risk of severe damage isn’t as great as it is for a freeze that lasts for 12 hours or longer. The more the temperature goes below the threshold of your specific cactus, the greater the damage it will experience. Take greater protective measures for temperatures 24 degrees Fahrenheit or below and for long-lasting freezes.
Like all plants, cacti tolerate cold better if they are healthy and have gradually acclimated to cold weather. Beginning in early fall, reduce watering and stop fertilizing cacti so they stop active growth and become dormant. Locate cacti that might need protection in favorable microhabitats that will protect them from the most severe cold. Choose the south side of a building, alongside a sheltering rock that retains heat, or on a sloped area where the cold air drains away. Depending on the type of cactus and the severity of the predicted freeze, further protection might be needed.
Types of Protection
For container plants, take the cactus inside. For landscaping plants, cover the plants with frost blankets, burlap or bed sheets put in place in late afternoon. Arranging a supporting framework can be very helpful. Remove the coverings the next day after temperatures rise above freezing again. When severe cold is expected, place a 60-watt light bulb under the covering and leave it on all night. For columnar, or spear cacti in light freezes, cover just the more tender newer growth at stem ends with polystyrene cups. Yes, Santa hats do just as great of a job or better!♥♥♥