Northern California has abruptly entered into the cold season, and as such it’s time to consider how to protect frost sensitive plants. Sacramento and surrounding areas are located in USDA zones 9a and 9b. This means minimum low temperatures to be expected are 20 degrees and 25 degrees Fahrenheit, respectively. Some areas of the foothills are in zones 8a or 8b, with low temps of 15 or 20 degrees. As gardeners, we of course push the limits of our growing range — here are some tips on how to protect more tropical leafy plants (such as citrus, Bougainvillea, avocados) and succulents from frost damage.
Most cold-sensitive plants only need to be protected when temps drop below their tolerance level. It is important to know your plant’s zone and temp range, as there is variability within groups. For example, although both citrus and kumquats can handle temperatures down to 15 degrees, kaffir limes can only handle a low of 30. When temperatures are expected to dip down below the tolerance level, a few defensive steps can be taken.
For the sake of this article, I am lumping plants into two very large groups — leafy plants (literally those with leaves) and succulents (those containing significantly more water than their leafy counterparts). A good reference for finding USDA zones is planthardiness.ars.usda.gov.
Covering Leafy Plants
▪ The goal is to create a warm environment while allowing air flow.
▪ Frost protection covers can be an assortment of materials, such as cotton sheets or blankets (thrift stores are a good place to find them). Specific marketed “Frost Blankets” are sold at most nurseries
▪ Do not cover the plants all winter. I have seen plants covered all winter, but this can harm plants by impeding air flow and sunlight exposure.
▪ Avoid the lollipop look. Do not cover just the top of your plant, leaving the trunk exposed.
▪ Note that younger trees need more protection than fully mature ones — a best practice is to wrap young tree trunks with cardboard or burlap
▪ Holiday lights give off just enough heat that they can be used in conjunction with a frost covering.
▪ Do Not use LED lights, as they do not give off sufficient heat.
▪ Do use C7 or C9 lights for outdoor use. Carefully read the box, because some lights have the appearance of the larger C7/C9 bulbs, but are actually LEDs.
▪ Wrap the lights around the tree, but avoid direct contact between foliage and bulbs.
▪ This is often counter intuitive for people — leafy plants handle frost and freezing temps much better when well-watered. A wilting plant going into a frost is more likely to sustain damage.
▪ Moisture in the plants will insulate the cells, helping the plant avoid frost damage.
▪ Evenly moist soil radiates heat better than dry soil. A day or two before a projected frost, be sure to water leafy plants.
▪ Do not overwater. You just need to avoid the soil being bone dry.
“Succulent” is a descriptor as opposed to a distinct group of plants. They are found in many diverse plant families across the world. As with leafy plants, it is essential to know where your succulent comes from to know its temperature range, because the variation is substantial.
Case in point, Aloe vera requires temperatures to stay above 40 degrees (with allowance for infrequent dips into the upper 30s). Aloe striata can handle temperatures down to 20. While only slightly different geographies of origin, the functional temperature ranges are disparate. Because succulents tend to be smaller and more mobile than leafy plants, it is a good practice to move them closer to your house for additional cold protection.
▪ The same rules apply as with leafy plants.
▪ For large succulents, the same lights as for leafy plants can be used, but extreme caution must be applied to not allow bulbs to contact stems or foliage.
▪ Because they are so rich with water, you want to decrease watering of succulents to minimize frost damage — they run the risk of cellular bursting if they freeze.
▪ Do not let them sit in trays of water.
Ask The Plant Lady
Q: I have on my indoor orchid with mealybugs. What can I use to get rid of them?
A: Mealybugs are phloem suckers, meaning they will pierce into the sugar transport system of the plant (phloem) and pull out sugars. By removing carbohydrates, they can decrease the plant’s health and eventually kill it if their population is large enough.
Mealybugs are easily recognizable by the cotton-like covering they exude to protect themselves and their young. To control an infestation, first wipe off as many as possible with a wet paper towel, and second, spray with a horticultural soap spray (found at any nursery) or neem oil. Rubbing alcohol diluted to a 50 percent solution also works well.
Spray it onto the foliage, but keep in mind that this can dry leaves and should only be done once or twice. Alcohol will desiccate the adults as well as the younger generations. The key to permanently ridding the plant of mealybugs is vigilance.