First if you are unsure about how much cold your plants can endure you can check in The Sunset Western Garden Book for the recommended climate zones for specific plants. The Santa Cruz area is in Sunset’s zone 17. Average winter temperatures go from the high 20s and low 30s. Record freezes plummet down to the low 20s. The pre-Christmas freeze of December 1990 went down to a numbing 18 degrees for several days. There were many dead poinsettias that week. If you’re more comfortable in front of a computer, you could also find that information on line.
Make sure the soil is moist. Drought-stressed plants are more likely to be damaged (or killed) by the cold. Cover any marginally hardy plants like Princess Flower and Angel’s Trumpet with a woven fabric. There are light-weight, spun polyester sheets on the market specifically made to insulate plants. They can keep the temperature beneath them about 5 to 9 degrees warmer than the outside air. They let in light and water so they can be left on during the entire duration of a freeze. These frost blankets need to be removed when warmer temperatures return.
If a fabric cover isn’t adequate to cover your prize Bougainvillea there are liquid acrylic sprays that help insulate tender foliage from the cold. It works by coating the leaves with thin layer of what is essentially plastic. This would involve using some sort of sprayer. It’s also available in a spray bottle for small jobs.
For small plants like that rare blue Impatiens from Annie’s Annuals there is what used to be called the Wall-Of-Water. It is a plastic cylinder made up of connected bags. You fill each compartment with water. At some point after it becomes evident that you are dealing with something that’s starting to resemble a very deep doughnut, but before the receptacles are completely filled and unwieldy, it’s a good idea to place the whole apparatus around the plant in need of protection. Continue filling until each container is full. Even if the water freezes the plant within will be a few degrees warmer.
It will be Christmas in a couple of weeks. Have you put up your holiday lights yet? Providing that you are not yet using the new cool LED lights consider wrapping your strings of old incandescent lights around your lemon or orange trees. If you’re going to waste energy over the holidays at least put it to good use. It’s not called a warm glow for nothing. Sadly menorahs don’t seem to work as well.
As for your marginally hardy potted plants; whatever you can’t bring indoors huddle up against the walls beneath the eaves or under some other solid overhead shelter like a carport. Clustering them together also helps them from dissipating any residual heat. Covering them with the aforementioned frost blanket would also help.
Good Luck! keeping in mind that often you can make your own luck.
