THANKS TO OUR mild climate here in the Puget Sound Region, we can grow a larger variety of plants than almost anywhere else in the world. Nurseries carry amazing plants from all over the globe, but not surprisingly, some of these are only semi-hardy. Most survive without harm in mild winters, but it takes just one Arctic Express with temperatures in the low 20s or colder to maim or kill many of those slightly tender rarities that make our landscapes so fun and interesting.
There are measures you can take that will help keep your semi-hardy plants alive through a cold winter.
The first line of defense is to protect roots from freezing by applying a protective layer of organic mulch, such as compost or arborist wood chips, at least 3 inches deep over the roots of any broadleaf evergreen plant. Even if the top freezes to the ground, many broadleaf evergreens, such as bottle brush (Callistemon) and fringe flower (Loropetalum), just to mention a couple, will grow back as long as the roots don’t freeze.
Not all plants, however, will survive if the top is killed. California lilac (Ceanothus), rock rose (Cistus), New Zealand flax (Phormium) and many others can be killed or maimed beyond rejuvenation in very hard freezes. The best way to protect these plants is to cover them if temperatures are forecast to dip below the mid-20s. Covering plants with sheets or tarps will add about 4 degrees of warmth, which is usually enough to prevent serious damage.
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Don’t use clear plastic. If it’s sunny out, it will fry the plant, even on a freezing-cold day. Sheets and tarps must be removed as soon as temperatures moderate, but if it stays cold for long periods, plants might suffocate due to lack of air and light. A relatively new product called Frost Protek is available online from Charley’s Greenhouse in Mount Vernon. Made of polypropylene, Frost Protek is a permeable cover that allows air, water and light to penetrate while purportedly increasing the temperatures around the plant by up to 8 degrees. It’s available in several sizes, including sufficiently large for a 6-foot-tall shrub. It is equipped with cinch-cords, making it easy to secure around the plant. The literature recommends covering plants only during extremely cold weather, but I’ve left them on quite a number of broadleaf evergreens all winter long.
When I remove the covers in spring, the plants often look like a boxer that stayed in the ring too long, but most of them recover quickly once warmer spring temperatures arrive. No matter what you use to cover the plants, make sure to pound stakes in the ground around the plant to hold the cover up in case it snows. It’s a real bummer to save your plant from the cold, only to find it’s been flattened like a pancake by heavy snow.
Some plants are too tender to survive freezing temperatures, even if you cover them. If you don’t want to risk losing a prize specimen, dig it up by Thanksgiving, or sooner if freezing weather is forecast, and overwinter it in an unheated garage. A few tender plants, such as angel trumpet (Brugmansia) and red Abyssinian banana (Ensete ventricosum ‘Maurelii’), can be allowed to go dormant and stored in a dark corner, but most tender broad-leaved evergreens must be overwintered next to a bright window or under a grow light. Keep them in active growth by watering only when the soil surface feels dry, and forgo any fertilizer until spring.
Of course, if you’re a rare-plant addict like me, you’ll have to follow my example and convert the south wall and roof of your garage into windows to make sure there is adequate light for hundreds of tender plants. You’ll never be able to park your car in the garage during winter again, but think of the amazing plant collection you’ll be able to show off to your gardening friends and neighbors every spring and summer!