The average first frost date in the Fresno/Clovis region is Nov. 15. Our winter season is extremely short, lasting only about eight weeks with the coldest nighttime temperatures usually occurring right around the winter holiday season in late December.
California native plants, Mediterranean natives and other drought-tolerant plants tend to be frost- and freeze-tolerant as well. It’s the subtropical and tropical plants that we persist in planting in our near-desert climate which are most damaged by freezing temperatures.
How much damage is caused by frost or freezing depends on how low temperatures drop and for how long. When temperatures fall to 30 degrees or just below freezing for only a few hours, tropical and subtropical plants will sustain minor damage. When temperatures fall below 28 degrees for more than a few hours (what’s known as a hard freeze), damage can be severe.
Clouds covering the sky at night retain the earth’s heat; when we see the stars on a cold, crisp winter evening, we can expect freezing temperatures. Monitor weather forecasts and be prepared to protect frost-tender plants whenever freezing temperatures are predicted.
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Wet soil holds heat and creates a warmer microclimate around plants. Irrigate the soil around frost-tender plants both in the ground and in containers before a predicted freeze. Hang outdoor-approved lights on freeze-tender citrus trees, wrap citrus trunks with burlap or old towels, and drape old blankets or towels over frost-tender trees and plants using stakes to hold the material off the plants. Remove any coverings during the day to allow light and air into the plants. Plastic tarps transmit cold to plants’ leaves and can cause freeze burn. (Wrap outdoor water pipes with insulating materials as well; burst pipes will waste water).
Move container plants to spots next to south- or west-facing walls which retain daytime heat or under patio eaves or even under large tree canopies.
Bananas, bougainvillea, citrus and cannas are examples of tropical and subtropical plants that are severely damaged by freezing temperatures. Banana bougainvillea and canna leaves turn brown and die after a freeze. Leave the unsightly leaves on the plants or on the soil surface; they’ll provide some freeze protection for the underground rhizomes or root systems. You can also cover the root area with blankets or tarps.
Citrus can suffer some moderate freeze damage when temperatures fall to freezing or just below for a few hours; severe damage or death occurs during a prolonged period of hard freezes. Outside branch tips and tree tops die first. Like bougainvillea, the extent of the freeze damage won’t be obvious until next spring when new growth appears. Wait to prune any freeze damage from citrus, bougainvillea and other frost- and freeze-tender plants until mid-spring (March or April). Scratch bark surfaces lightly with a sharp knife. If the tissue underneath the brown bark is green, that part of the branch is still alive.
Send Elinor Teague plant questions at firstname.lastname@example.org or email@example.com (“plants” in the subject line).