January Frost in the Winter Garden
By Josh Reilly
In January, perhaps more than any month, I appreciate the big south-facing window looking out onto my garden. I can see almost the whole backyard without having to step outside, crunching through the film of frost that forms in the shade of my neighbor’s 120-foot Ponderosa pines. Inside of the window, it’s about 40 F warmer than out there. I know what cold feels like. I used to live in Indiana. In the first week of our last January there, the temperature never rose above 15 F, day and night, blue sky or cloudy. Aside from shoveling out, there is nothing to do when it’s like that (unless, God help you, your pipes freeze). As I recall, they closed the schools that week. Employers just told people to stay home.
Here in the Valley, we enjoy a far milder Winter. In some parts of the Valley, however, we can count on a few weeks in January with morning temperatures in the low 20s. If not a death sentence, these temperatures will prevent frost-tender or subtropical plants from thriving. Being determined, however, to own and successfully grow these beauties, you find yourself measuring the space in your sunnier rooms for pots, pot stands, and baker’s racks. You price grow lights at your nearby hardware and garden centers. You get out the stud finder to figure out where to hang potted specimens. A 1-gal plastic pot, fully loaded, weighs about 6 lbs, dry. A ceramic pot that size is about twice the weight. A single, small eye hook, any old place in the sheetrock ceiling, will not do. But you are a gardener. You remain undaunted. If you can just get your orchids and begonias through January, you’ll be alright.
The USDA Hardiness map shows Ben Lomond in USDA Zone 9a (low extremes of 20 – 25 F). I don’t know how many Z9a-rated plants I’ve lost after about a week of January frost. I do know that Sunset Magazine has its own hardiness map, which places my neighborhood in Z7 (low extremes of 5 to 10 F). We’ve never registered temperatures lower than 20 F here, but the Lane family, the original publishers of Sunset Magazine, had a nice, rustic horse property right over the hill. When they left it to the County of Santa Cruz, it became Quail Hollow County Park. They’d have known as much about our microclimates as anybody.
Hardiness zones can be geographically tiny. I can see the edges of our zone (across Newell Creek) from my front yard. And of course, some years, it doesn’t drop below the high 20s at all. I suspect that our location in the hollow formed by the Newell Creek riparian corridor explains all this. Moist air descends from the slopes in the early hours of morning, slows down, and picks up more moisture over the river. When temperatures drop, water vapor condenses out as frost on cold surfaces like soil and rocks. And on your beloved subtropicals. That same moisture-laden air tends to gather in the bottom of the slopes on either side of the creek. Frost lingers in the shade of the forest, well into the morning, extending the period of contact with plant tissue and increasing the damage.
You could give up on frost-tender plants. You could just buy new ones each year, in March, and settle for specimens that don’t get big. And save the floor space around your sunny windows for more sensible things, like, you know, furniture.
Not me, though. I just keep trying. Mountain Feed and Farm Supply and Ace Hardware carry frost-protective tarps that’ll help. Drape and secure them over a trestle of garden stakes to prevent tarp contact with plant tissue. It just has to admit light. The airspace under the cloth may also be slightly warmer than above. A few degrees of protection is often all you need.
Josh Reilly, aka Uncle Skip, writes about seasonal gardening from his home in beautiful Ben Lomond, California.
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