Here in the Central Valley of California, we normally enjoy mild winters. However, once every few years temperatures dip below freezing. This causes widespread panic among gardeners, who have forgotten (or perhaps did not know) that many plants in our garden are subtropical and need protection. These plants include citrus, avocado, anything with succulent leaves and bougainvillea, to name a few.
Young plants are most susceptible to frost, but luckily easier to protect because of their small size. For protection, build a frame around each tree (use wood, pvc pipe, etc) and throw a cover of burlap, clear plastic or an old bed sheet over it. Make sure the top and sides are covered. If you use plastic, you can leave the cover on during the day. Otherwise, remove the cover to allow sunlight to reach the plant. (note: draping covers over a tree without a frame will not protect it, as foliage touching the cover will be damaged by the cold.)
Normally, trees and fruits aren’t damaged unless temperatures stay low for 30 minutes to several hours. Ripe oranges, grapefruit, and mandarins are injured at 27° to 28° F. Ripe lemons are less cold tolerant, and may be damaged at 30° to 31° F. Unfortunately, picking fruit before it ripens won’t save it. It’s best to leave fruit on the tree as long as possible. If fruit is damaged by frost, remove it as soon as possible to save the tree energy.
When planting subtropicals in the landscape, plant near buildings and choose southwest exposures when possible. And remember, not all citrus have the same cold tolerance. Kumquats and Satsuma mandarins are the most cold resistant. The least resistant are limes and lemons. Oranges, grapefruit, and mandarin hybrids are intermediate in tolerance. Interestingly enough, avocado trees can grow successfully in our area. Choose Zutano and Bacon varieties as they are the most cold tolerant.
If you think a plant was damaged by frost, do not prune. Wait until spring to see how much damage was actually caused. Also, avoid over-fertilizing subtropical plants, and apply fertilizer during late winter and early spring. Plants actively growing during a frost can be easily damaged.