We’ve had some very cold nights with temperatures falling below freezing into the high 20s this winter.
Temperatures below 32 degrees are low enough to cause severe freeze damage to exposed tropical and semitropical plants including bananas, cannas and bougainvillea and light freeze damage to hardier citrus varieties. The extent of the freeze damage to any plant depends how low temperatures drop and how long the freezing temperatures last as well as the plant variety and its location. Every garden has warmer microclimate spots as well as cold pockets that can alter the effects of freezing temperatures.
Freeze damage on semitropical and tropical plants causes leaves to turn brown and die and twigs and branches to die back. Our first impulse is to clean up the ugly leaves and to cut back dead twigs and branches, but that is exactly what we shouldn’t do. The dead leaves act as insulation, protecting (somewhat) the plant from further freeze damage. If you can stand the sight of ugly brown leaves in your garden, and if the neighbors don’t complain, leave the brown leaves on the bougainvillea branches and the fallen leaves on top of the ground covering the bases of banana and canna plants until late January when temperatures begin to warm. Continue to protect frost-tender, freeze-damaged plants on clear nights when freezes are predicted by covering them with cloth tarps, old towels or blankets. Our average last frost date in the Central Valley is Feb. 15, but we often experience a frosty night or two into the first week of March.
The full extent of the freeze damage to tropical plants won’t be apparent until new growth appears in spring. Cutting out dead wood now can cause further dieback in twigs and branches. As new growth appears in a couple of months, use your fingernail or a sharp knife to lightly scratch the bark on damaged plants. If the underlayer of bark tissue is green, the branch is still alive. Bougainvillea branches may die all the way back to the base, but the plant will send up new shoots if the roots are still viable. Canna rhizomes and underground banana roots regularly survive freezing temperatures and send up new shoots in spring.
Most citrus varieties can withstand cold temperatures of 28 degrees and above fairly well. Some leaves on the outer canopy and twig tips may die, but cleanup of the minor damage should wait until new growth flushes. Lime trees are among the varieties that are more cold sensitive. Again wait until spring to prune out damaged wood.
Wait to fertilize freeze damaged plants until the extent of dieback is clearly marked. Give heavily damaged plants a very light first feeding in spring with a lower nitrogen food. Heavy fertilization that forces new growth will further stress struggling plants.
One beneficial effect of below-freezing temperatures is that the cold kills the overwintering eggs of many soft-bodied insects. Populations of pest insects that proliferated during the past drought including aphids, whiteflies and mealybugs should be reduced this next growing season.
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