Twelve days ago coastal gardeners awoke to find many of their garden plants withered and burned after one of the coldest nights in recent memory. Frost damage to tender plants was obvious as weather stations hit record lows: Costa Mesa went to 29 degrees. Upper Newport Bay, western portions of Irvine and downtown Laguna Beach dropped to 30 degrees. Bolsa Chica, Talbert Village in Huntington Beach and UCI recorded 31 degrees. Even Balboa Island, probably the warmest spot in the county, dipped to 36 degrees.
But it’s not necessarily the end for frost-frizzled plants. Phones were ringing off the hook last week at Roger’s Gardens and other nurseries for advice on what to do. Gardeners were seeking advice on how to save tender hibiscus, bougainvillea, lantana, ficus, cape honeysuckle, geraniums and others.
The best advice, as hard as it might be to accept, is to do nothing at all, but to wait until early spring to see what the real damage might be. There is nothing you can do to salvage cold-damaged plants other than to wait and see if they recover. Pruning now would expose still-healthy plant tissue to the cold and stimulate new, tender growth, even more susceptible to injury than the already-damaged older growth. Now is not the time to prune damaged plants or fertilize them; wait until new growth emerges in the spring. Fertilizing would promote new growth, which would be tender and subject to future frost damage.
Along the coast of Orange County, frost is a rare occurrence, blamed for damage than it causes. Orange County is a land of microclimates, one gardener’s low temperature is not the same as another’s a few doors away. Even in your own yard, changes in elevation, canopy and exposure will affect the temperature around tender plants. The same plant, growing in different locations in a garden will react quite differently to what appears to be the same temperatures. In fact, the temperatures may be quite different just a few feet away.
To understand frost, remember that cold air sinks and warm air rises. During the day, energy from the sun warms the soil. At night, this heat rises back into the air. On cloudy nights, heat stored in the soil rises to the clouds and is reflected back to the earth’s surface. High humidity moderates these temperature changes, while low humidity, like we had last week, will accelerate these changes.
As warm air rises, colder air takes its place. Temperatures near the ground are often five degrees cooler than the temperature just two or three feet higher. When you see a “frosty” lawn in the morning you are seeing this effect of elevation on temperature. If, on a cold night, we are fortunate to have a gentle wind it will mix the rising warm air with the sinking cool air, resulting in warmer temperatures. During winter, our air is often dry, the air still, and clouds absent. These are the conditions that pose the greatest danger of frost damage to plants.
Easy strategies that will keep plants warmer include moving potted plants closer to the house or under a tree with a large canopy. Stringing Christmas tree lights (the big-bulb type) around a plant will generate heat, especially if a plastic sheet is placed on top. For plants that can’t be moved putting a couple layers of old bed sheets over them will add a few degrees.
Unfortunately, the cold nights of last week may have caught you by surprise and it may be too late for many plants. If you noticed leaves hopelessly wilted or turning black, that’s a sure sign that it took a hit.
Under no circumstances should the dead portions be pruned back until consistent warm weather returns in the spring. Pruning now would expose still-healthy parts of the plant to the cold and also stimulate new, tender growth which is even more susceptible to injury. Those ugly, frost damaged leaves and stems will help trap warm air and protect dormant buds that will sprout once growth resumes. In a couple of months, or sooner, the frost danger will have passed and new growth will begin. At this time, trim off the damaged leaves and cut back any dead tips.
As awful as it may look and in spite of wanting to do something, you’ll be better off leaving frosted plants as they are. You’ll be surprised that most of those burned plants or scraggly twigs are still alive. All is not lost. Spring will be here soon.
Ron Vanderhoff is the Nursery Manager at Roger’s Gardens, Corona del Mar