When the first frost warnings came out for the third week of October, did you find yourself frequently checking the extended weather forecast, hoping there had been some mistake? Who wants to give up fresh tomatoes and long-stemmed dahlias before Halloween, for crying out loud?
Once again, the meteorologists were right. With an early cold front fast approaching, I found myself flying out the door just before sundown with all the old blankets, sheets and tablecloths I could find. Tossing them over my pots of tender annuals, I hoped to buy more time before finally facing that long list of tasks that come at the end of every gardening season. For someone who talks about frost as much as I do, you would think I would be better prepared. But I’ve become an expert frost dodger, and once again, a crisis was averted in my hilltop garden that night.
I had time to take cuttings of annuals like begonia, coleus and plectranthus. I lifted tender succulents from their mixed containers and repotted them individually. They’ll keep the annuals company until spring on open shelving in a cool spare bedroom that gets a few hours of morning sun. Even a west- or south-facing glassed area has only the light intensity in winter of a shady outdoor area in the summer. These plants definitely lose their luster spending a long winter in imperfect indoor growing conditions, but as long as I can keep them alive, there’s a good chance I can revive them in the spring to live on for another season.
But there’s still more work to do. These days, almost anything goes in containers, including small trees and shrubs, conifers and herbaceous perennials. Hardy plants left outside in pots will need protection if they are to make it through a Yakima winter.
The best way to help plants survive is to choose the right varieties from the start. I garden in USDA Zone 6a, and perennials rated hardy to one or two zones colder (Zone 4 or 5) have the best chance for overwintering here in a container. Since the soil in a pot is colder than the soil in the garden, a plant’s roots (the least hardy part) are more vulnerable to injury and death. We’ve all seen how the roots of container plants tend to circle around the thin walls of the pot. Roots in this environment freeze sooner, and are more likely to suffer damage as they thaw and refreeze as temperatures change. Big pots with a large soil mass give plants a better chance of surviving than small ones.
Another way of protecting outdoor container plants is to move them into an unheated shed or garage with moderate light. Water just enough to keep the soil from completely drying out, and no fertilizing. The key is to maintain an even temperature, ideally just above freezing, where plants are kept cold and alive, but not actively growing.
If you can’t bring them in, try clustering the pots in a sheltered area near the house or garage. In a northern or eastern exposure, temperatures tend to stay fairly stable. A southern exposure can warm up container soil on a sunny winter’s day, and when it refreezes at night, the roots can suffer. Set the containers on soil rather than pavement, which can also heat up in the sun. After outside temperatures dip into the 20s for several consecutive nights, covering the containers with a protective layer of straw, leaves or even snow can provide additional protection.
Another option, though not particularly pretty, works for plants that are too big to move or group together. Wrap the container in bubble wrap or an old blanket, and drop a cardboard box with both ends cut out over the bundle. Adding straw between the wrapping and container adds additional insulation.
If your containers are small enough, they’ll get the most protection if you bury them up to their rims in the ground, perhaps in the empty vegetable garden. After it gets very cold, cover the area with a layer of mulch.
If you’re a procrastinator like me, the good news is that there’s still time to try any of these strategies. Wait until plants are fully dormant, usually after temperatures have dropped below freezing for several successive nights. Regardless of the method you use, leave the plants protected until spring and all danger of cold injury has passed. Although there’s no guarantee that any potted plant will survive outside in a Yakima winter, it may be worth a try.