Many of the plants we have in our Central Valley gardens are frost-tender. In order to create lush vegetation in our desert climate, we’ve planted citrus, bananas and other subtropical species that cannot tolerate long periods of temperatures below freezing without suffering some damage. The coldest temperatures of the year here generally arrive in late December, but our average first frost date is Nov. 15. It’s a good idea to collect frost protection materials now and have them ready to go whenever temperatures are predicted to fall below 30 degrees.
Citrus trees can be severely damaged during a hard freeze when temperatures drop below 28 degrees. Deeply irrigating the soil a couple of days before the hard freeze and right after the freeze will create a warmer microclimate since moist soil holds heat better. Mature citrus trees can often survive short periods of really cold temperatures with minor damage if irrigated, but young citrus are especially susceptible to freeze damage. Avoid overhead sprinkling; water on the leaves can turn to ice on freezing nights, weighing down the branches. Wrap the trunks of young or new trees with heavy cardboard, straw bundles or several layers of newspaper. Don’t cover the bud union (the bulge on the lower trunk where the tree was grafted onto hardier root stock) and don’t use plastic materials since plastic transmits the cold. A market umbrella or wood and canvas structure can be used to cover and protect the leaf canopy as well. Old towels can be stretched over smaller citrus trees and removed during the day to allow sunlight and air into the canopy, then dried in the clothes dryer and replaced on the next frosty night.
Bananas often set fruit but it rarely fully ripens in our climate. Freezing winter temperatures arrive just as the fruit is nearly edible. Bananas, cannas and bougainvillea often die down to the ground at 30 degrees or below. Cut off browned banana and canna stalks and leaves and cover the stumps with tarps, blankets or towels to protect the underground rhizomes from damage. You can also leave the brown leaves and stalks until early spring (if the neighbors don’t complain); the dead leaves actually provide some protection.
Many garden centers, nurseries and online garden supply companies carry row cover fabric that provides frost protection and extends the harvest season of summer fruits and vegetables. The thinner fabric row covers can protect cold-sensitive plants to 28 degrees; thicker fabrics will protect plants to 24 degrees. Most of the fabrics are fairly fragile, lasting for just one season, and require attachment to hoops or to earth staples. The thinner fabrics transmit on average 85 percent of the sunlight and the thicker fabrics transmit around 70 percent. Heat caps and water walls are more durable but need to be removed during the day.
For those of us who can’t stand not having fresh, homegrown tender-leaf herbs like basil and tarragon or green beans, tomatoes, peppers and eggplant available for more than a few summer months, protecting the plants with covers can give us at least another month of good eating.
Send Elinor Teague plant questions at firstname.lastname@example.org.