About the author: Jacqueline Houston is a member of the Wedgwood Garden Club and a Master Gardener of King County. She has been gardening in Seattle for over 30 years. Wedgwood Garden Club always welcomes new members – contact Jacqueline at firstname.lastname@example.org for more information on their monthly meetings.
What zone are we actually gardening in, here in Seattle? The answer usually given is either USDA zone 7 or 8, but one of the single most useful pieces of advice I got from our Wedgwood Garden Club presenters was to play it safe and garden as if we are zone 6.
USDA zones are based on minimum winter temperatures that are likely to occur in a given area. Zone 6b has minimums between -5 and 0 degrees F, so plants need to be hardy to at least 0 degrees. Zone 7 minimum temperatures range from 0 to 10 degrees F, and zone 8 has minimum temperatures of 10-15 degrees F.
Now my trusty weather app. never records overnight lows of anything lower than around 23 degrees F, so you might think this is all rather academic. That is, until you take a spring walk around your garden after a winter with some hard frosts and a good snowstorm or two, and count the graves of your dearly departed plants. And the most likely victims are your zone 8 treasures, followed by zone 7 (all those hardy fuchsias that are only hardy to 10 degrees!), while zone 6 specimens soldier bravely on – for the most part (we all have our failures).
One cold winter I lost 4 star jasmines (trachelospermum jasminoides – zone 8), and have learned to increase their chances of survival by planting them against a wall, the warmer the better, and piling leaves over the roots in winter. But even a west-facing wall was not enough to save my 7 foot high, stunning California flannel bush (Fremontodendron californicum – zone 8) which died over the winter two years ago. I have planted a precious, small pineapple broom (argyrocytisus battandieri – zone 7) against that same west-facing wall and am crossing my fingers.
Of course, we all like to gamble now and then, and some gardens are just plain warmer than others. You know your own microclimate best. But when I do take a gamble, I take it with a gallon-size plant if I can find one, rather than a large, expensive specimen. So if you are thinking of investing some serious money in a big shrub, or a new tree, think twice before going beyond a zone 6 purchase!