It’s funny how one day you can be out gardening in your favorite pair of shorts, and just a few days later you’re pulling on extra layers of polar fleece. Even though I do my best to mentally prepare, I’m never quite ready to see another gardening year come to an end.
Over the last few weeks, I’ve found myself checking the extended weather forecast multiple times each day, searching for borrowed time. But I’ve been a gardener long enough to know that Jack Frost will ultimately win the race.
If all I grew were locally native plants, I’d have no reason to fret over the weather forecast. These dependable stalwarts evolved to shake off big chills. But I’m a collector, adding peonies from China, sedges from New Zealand and maples from Japan. Will these transplants survive a deep freeze in Central Washington? Horticulturists at the U.S. Department of Agriculture can help us make these predictions.
The USDA Plant Hardiness Zone Map is an important guide in determining which plants are most likely to thrive at any location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones. It divides the United States into 13 zones, with Zone 1 marking the colder regions of Alaska and Canada, and Zone 13 including southern Mexico and Hawaii. Further divisions of “a” and “b” in a single zone indicate a 5-degree variability.
My Yakima urban-area garden is in USDA Zone 6b, and with some confidence I can choose perennials that survive temperatures as low as minus 5 to zero degrees. Some of my friends in West Valley and Selah occasionally record even lower readings and look for plants hardy to Zone 5b (minus 15 to minus 10 degrees) or Zone 6a (minus 10 to minus 5 degrees). If you’ve carefully chosen your plants, you’ll need the polar fleece more than they do.
Remember that soil temperatures drop slower than air temperatures. Insulated by the soil, underground roots have a measure of protection. Roots freeze even slower if the soil is well mulched or snow-covered, or if the soil was moist when it froze. Yet no matter how hardy a plant, few would survive even a mild Yakima winter without going through seasonal changes called “acclimation,” a process that leads to dormancy. Most plants are well on their way to their genetically determined winter hardiness by now, and stand well-prepared to face the freeze.
Hardiness zones are a guide and not the only factor to weigh when determining whether a plant will be perennial in Yakima. Gardens with south-facing brick walls or courtyards, or near concrete parking lots or bodies of water, may have microclimates that are significantly warmer than the surrounding area. I often visit urban gardens on the Buffalo, N.Y., where plants rated hardy to Zone 6b thrive, despite Buffalo’s overall Zone 5 rating. Brick houses built very close together, just blocks from Lake Erie and the Niagara River, offer protection to plants that will freeze to death in gardens just a few miles away.
Even without a microclimate, what gardener hasn’t gambled on zonal denial? (As you might guess, zonal denial refers to trying to grow plants that are not hardy in your climate zone.) In consecutive years with mild winters, plants with a tender pedigree may pull through, lulling us into believing we’ve fooled Mother Nature. It serves us right when, the very next winter, weather blows in straight from the Arctic, and May finds us shopping for replacements.
Just like plants, the birds that stay in Yakima for the winter are well-equipped to face the weather. Stored fat and fluffed feathers that trap heat help keep them warm during the long, cold nights. A slowed nighttime metabolism helps with energy conservation until they can find food in the morning.
A garden filled with plants for food and shelter becomes an oasis for these birds. While most rely on wild foods, they will supplement with seed, suet and fruit from feeders when it’s hard to find natural sources, especially after a snowfall. If that’s not reason enough to keep your feeders full, then the amazing sights and sounds of birds, just outside your window, should do it. In addition to food, try to provide as many different habitats as possible, whether in birdhouses, natural tree cavities, copses of ornamental grasses and dried perennials, or evergreens and shrubs.