Usually, our winters are maritime mild with just a few days of cold that don’t necessarily need any extra effort to shelter our outdoor plants.
But every so often, four or five years, the Arctic Blast blows from Canada into the U.S. and down the Columbia Gorge into Portland and temperatures dip below our average winter temperature of 35 degrees F. Forecasts for this weekend are for temperatures in the low 20s, which can damage some of the the less hardy garden plants. Having an outdoor thermometer will pinpoint your microclimate temperature to help you decide how much you may or may not need to protect certain plants. Make sure plants are well watered before the cold settles in to make sure they are well-hydrated to withstand the freezing, thawing day temperatures and any wind. Snow is actually insulation from dry cold.
If your plants are listed hardy to Zone 8b or lower, you should not need to cover or protect them UNLESS we get the notorious east wind on top of the cold temperature. Desiccation can occur which results in leaf burn, which if bad enough, causes the leaves to eventually fall off and be replaced in later spring with fresh new ones.
If your landscape is in a particularly windy area, you may want to wrap the plants with frost cloth or burlap to prevent leaf desiccation.
A few plants to protect the foliage on would be Gardenias, Bamboo (very slow to re-leaf out after winter damage), Schefflera, Pittosporum, Nandina, and hardy Bananas and Palms.
Landscapes which normally receive winter wind may wish to protect plants by tying on frost cloth, tarps, plastic, anything to keep the wind off. The advantage to frost cloth is that it lets in sunlight and if the cold lasts more than a day or two, the plant isn’t losing the benefit of chlorophyll. Another option is to spray foliage with an anti-dessicant such as Wilt Stop to keep leaves looking good and moisture in the leaves.
More tender plants in containers (less winter hardy, USDA Zone 9 and above), should be moved to a sheltered location (if in containers) such as a garage, nestled close to the house on a porch or even cooler indoor area with as much light as possible. If planted in the landscape, mulch with 2-3 inches of mulch or compost (always not touching the stem or trunk). Frost cloth or even a bath towel can offer significant protection, but it may not be enough to save the plant.
If the weather ends up worse than predicted, new growth in the spring will generally cover up winter foliage damage on the hardy plants. Tender plants may not survive, Gardening 101 lesson learned.