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…