Our winters in Portland are generally mild with just a few days of true cold, which typically don’t require any extra effort on our part as gardeners. But every so often, temperatures will dip below our typical winter lows — which tend to hover just above 32°F — into the 20s, prompting us to protect certain plants in our gardens.
When temperatures drop significantly below freezing for extended periods, less hardy plants can sustain damage, which can sometimes be severe enough to kill them. Having an outdoor thermometer or two posted in your garden will help you to pinpoint the temperature of your site's specific microclimates, which is helpful as you try to decide how much you may need to protect certain plants in your own yard.
Below, you will find a rundown on what to do when you spot unusually low temperatures in the forecast.
Don’t panic. Most garden plants will bounce back just fine from a cold snap, especially if they are reliably cold hardy in our Zone 8 climate.
Focus your energy on the plants that need it. Plants that are only marginally hardy in our zone will need the most help, along with those exhibiting tender new growth or swollen buds, which are more susceptible to damage from the cold than other plant tissues. Cold-hardy woody plants will likely survive a cold snap on their own, but this spring’s floral display may not fare as well without a little help.
Move what you can. Relocate containers planted with tender plant material to a sheltered location — ideally a space that is bright and stays above freezing, but is not so warm as to shock your plants. A garage or shed works great, as does a cool interior room, but even a location up against an exterior wall or under an overhang can provide some protection. Place hanging baskets on the ground, or move them inside to protect them from chilling winds.
Keep in mind that because their roots are more exposed to the elements above-ground, plants are less winter hardy when planted in containers than when grown directly in the ground. As a general rule of thumb, anticipate your plants to exhibit two full zones’ less cold hardiness in containers. For example, a containerized plant normally winter hardy to Zone 6b would likely fare just fine without protection during a typical Zone 8b winter, whereas a Zone 7b plant might struggle, even though it is nominally winter hardy here. If we are forecasted to experience a stretch of unseasonably low nighttime temperatures closer to the average minimum a colder zone — say, Zone 7b’s chilly 5 - 10°F as opposed to our more typical 15 - 20°F — change your calculations accordingly when deciding what to protect.
Protect tender plants in the landscape with 2-3 inches of mulch or compost to help insulate their roots, being sure not to mound any around the stem or trunk of the plants. Frost cloth can also be used to provide an additional layer of protection, but for plants often treated as annuals, this may not be enough.
Protect plants in windy exposures from desiccating winds with frost cloth. In a pinch, old sheets and towels also work well as an insulating barrier! When cold temperatures are accompanied by the notorious east wind, the leaves of plants are more susceptible to desiccation and burn. A few plants to protect the foliage on would be Gardenias, Bamboo, Schefflera, Pittosporum, Nandina, and hardy Bananas and Palms.
Give your plants a good drink before freezing temperatures arrive. A well-hydrated plant is one that's better able to withstand whatever Mother Nature can throw at it, and water itself can help to insulate a plant’s roots.
With timely action, you can successfully protect most plants in your landscape from unseasonably cold temperatures. And even if hardy plants sustain damage from leaf burn, new growth will generally cover this in the spring. Of course, for a variety of reasons, this isn't always possible. Sometimes, in spite of our best efforts, a plant might not survive a late cold snap. All gardeners — no matter their experience level — have been there, and there's no shame in being bested by Mother Nature. In the event that this does happen, we always like to think of it as a good excuse to change things up and experiment with new plants in our gardens!
Not sure which plants to protect? Feel free to ask us by phone, or email a photo to firstname.lastname@example.org.