One of the attractions of my house, when I first saw it for sale some decades ago, was a fragrant, lush Meyer lemon tree abloom in the side yard. Unfortunately, by the time I moved in, the Great Blue Norther of 1990-’91 had transformed it into a blackened mass of lifeless twigs and shriveled leaves.
The previous owners had neglected to cover it at night, and the unusually cold weather that winter had done its malign work, killing the tree. It actually looked as if it had been scorched, not frozen. I learned that the plant cells had solidified in the cold, which meant that the nutrition cycle had been blocked. The lemon tree appeared deflated, which was exactly how I felt at the time.
The best way to protect your plants during an unusual cold spell is through preparation. Assume it will freeze sometime this winter. (It takes only one night to do major damage.) Pay attention to weather reports. Identify which plants need protection. Assemble a collection of old sheets, blankets, towels, burlap, cardboard boxes, baskets and/or buckets. Do it now to avoid the horrid midnight rush to save the avocado (when it’s probably too late anyway).
What plants will need attention? Young fruit trees with thin bark, avocado trees and citrus trees are the main candidates in Napa Valley. Certain tender shrubs, such as brugmansia, fuchsia, mandevilla, daphne and bougainvillea should be covered. Some cacti and succulents need protection. If you have plantings in a lower elevation of your yard, these plants should certainly be protected, as those parts of a yard are always a few degrees colder than the rest of it.
Water the plants before you cover them, which should be before dusk (except for succulents). Some of the water will evaporate during the night and warm the surrounding air.
For small plants, you can fill old plastic milk jugs with water, which will insulate the plants a bit, and then use those vessels to support a draped cloth.
Small plants will do fine with an overturned bucket, cardboard box or basket for protection. If you have a lot of small potted plants, move them to a protected spot or next to a wall. You can also protect small plants by covering them with mulch.
If you use a sheet or a blanket as a cover, it shouldn’t touch the plant. You can build a frame to support the cloth, or create your own supports with lawn furniture, clothes drying racks, trellises or bean poles. If the cloth touches the plant, it can transmit cold, so use a frame. Make sure the cloth reaches the ground, for better insulation. Remove the cloth in the morning, after the air warms up.
You can wrap young tree trunks with burlap or old blankets or towels for insulation.
Nurseries sell plant covers in all sizes and shapes, including row covers. They are made of a nonwoven synthetic, and they allow light to penetrate so you can leave them on during the day. This fabric — similar to the interfacing used by home sewing enthusiasts — can touch a plant’s leaves without doing harm.
If the worst happens and your plant freezes, don’t lose hope. It just might recover. I thought I had killed two large brugmansia shrubs last winter. A surprise freeze had left them drooping and shriveled.
I consulted Sunset’s Western Garden Book and learned that, instead of hacking the shrubs down to the ground or throwing them out, I should wait until spring to see if any new growth occurred. And it did!
At that point, I pruned the brugmansia down to the green leaves, put them in large pots (so I could move them near the house in the winter) and saw them put forth spectacular peach-colored trumpet-shaped flowers in late summer. They are so sturdy they even survived a walnut limb landing on them and being blown over by the windstorm that preceded the October wildfires.
When spring returns, rethink some of your plant placement. Perhaps you have citrus trees in planters. You could put the planters on wheels and roll the trees to a sheltered spot during cold spells. Maybe you should move plants that are in sunken areas of your yard.
A fragile plant can be placed against a south-facing wall, and the warmth that radiates from the wall will help the plant get through the winter. I have noticed how many bougainvillea are trained against south walls in Napa Valley. That’s where my new lemon tree is, and it has survived several cold spells.