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What to do on extra-chilly nights: Protect sensitive plants from killer frost

Freezing temperatures can be a major threat to succulents and other tender plants. Garden experts recommend covering them with frost cloths or other insulators when temperatures below 32 degrees are forecast.
Freezing temperatures can be a major threat to succulents and other tender plants. Garden experts recommend covering them with frost cloths or other insulators when temperatures below 32 degrees are forecast. aseng@sacbee.com

Every winter, our sensitive plants face the life-threatening possibility of killer frost.

A moderate freeze with temperatures in the 25- to 28-degree Fahrenheit range can be widely destructive to vegetation. According to the “Old Farmer’s Almanac,” the greatest frost danger in the valley falls between Dec. 4 and Feb. 10, although historically killer frost can hit as early as Nov. 1 and as late as March 27.

Freezing temperatures can be a major threat to succulents, tropical plants, ferns and citrus as well as such cold-tender favorites as begonias and geraniums. Frost protection such as insulating covers also can help some summer vegetables, such as peppers, survive through winter months.

As a reminder, Cheryl Hawes of the Plant Foundry in Sacramento suggests marking plants that need special attention to survive frost with a Popsicle stick (perhaps painted blue) or other visual cue. The Oak Park nursery where she works protects its large selection of succulents and cacti with lightweight cloth insulation blankets, tucked in before sunset and removed in the morning. Held in place by clothespins, the covers can be suspended from roof gutters and tented over tender plants close to the house.

Here are some more tips for helping your garden cope with frosty nights:

▪ If temperatures below 32 degrees are forecast, water your plants lightly in the late afternoon or early evening before frost hits. Wet soil holds more heat than dry soil and this also raises the humidity level. Well-hydrated plants can cope better with frost “burn.”

▪ The exceptions to that “water before sundown” rule are succulents. They retain a lot of water in their leaves and, if just irrigated, may pull up even more moisture into their foliage. Instead, cover them with frost cloths or other insulators.

▪ Pull back mulch away from plants so the ground can radiate any stored heat.

▪ If overnight frost is expected, move potted plants to protected areas indoors or a covered patio.

▪ Cover sensitive plants before sunset; that helps capture any ground heat and keeps it close to the plants. Cloth sheets or blankets work better than clear plastic and can increase the temperature 5 degrees. Allow a little room for air circulation under the cover; that helps keep in warmth, too. Remember to remove the covers by mid-morning or else you risk suffocating the plant.

▪ Use heat caps or row covers to protect tender vegetable transplants.

▪ Plants in raised beds or on mounds stay warmer than those planted in sunken areas, where cold air collects.

▪ String old-fashioned Christmas lights – the ones that get hot – on the trunks and limbs of citrus, avocado and other frost damage-prone trees and bushes. LEDs offer no heat for plant protection.

▪ Wrap the trunks of tender trees or shrubs with rags, towels, blankets or pipe insulation.

▪ If temperatures are expected to go below 30 degrees, harvest ripe citrus fruit to avoid potential damage.

▪ If a plant shows frost burn, don’t cut off the damaged foliage. It will help protect the plant from further harm. Remove the browned leaves in spring.

Debbie Arrington: 916-321-1075, @debarrington

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