USDA Hardiness Zone “Upgrade”
If you think that the weather at your place has grown warmer over time, you may be right. Evidence it has could lie in the recent “upgrade” (if that’s the right word) of Vashon’s plant-hardiness zone by the U. S. Department of Agriculture. Island landowners might want to consider this news when thinking about what plants they can and can’t grow, as well as the needs of native plants, animals, and insects on their property.
Periodically, the USDA updates its map of so-called hardiness zones, areas that share average lowest-minimum temperatures over the previous 30 years. (That’s average minimum temperatures, not average temperatures.) The intent of the map is to allow folks anywhere in the U.S. to see the worst cold that any plants in their region might have to be “hardy” enough to survive. The newest update of the map, issued late last year, appears at https://planthardiness.ars.usda.gov/.
Enlarge the Vashon area of the map, and you’ll see most of the Island now lies in zone 9a—average low temps of 20-25 degrees F. (Parts of southern Vashon and southeastern Maury show as lying in 8b—average low temps of 15-20 degrees F.) On the 2012 hardiness map—the most recent version until this latest update—all of Vashon was shown as being in zone 8b.
This upzoning doesn’t prove anything about climate warming by itself, but the temperature increase it reflects might add evidence of such a trend. A recent federal report states the average temperature in the Northwest has risen two degrees since 1900. Maybe the USDA data on rising low temps corroborates this general increase.
So what does this hardiness-zone change mean for what landowners can and can’t grow? Regional media reports on the change have featured gardeners and nursery personnel seeming upbeat at what could, and probably should, be cause instead for alarm in this news of temperature rise. Rather than worrying about native plants being stressed by increased summer heat, interviewees have mused about what exotics these warmer winter temps might now allow Northwesterners to grow. More citrus! More subtropical flowers!
Everyone likes opportunities to grow new things, but think for a moment about how warmer temperatures may mean lots of other, not-so-pleasant effects. Take as one example the earlier bloom times of native flowering shrubs caused by temperature rise. Many native insects and birds evolved to feed on native blossoms, timing their own emergence or arrival to coincide with these plants blooming. Accelerate native blossom times with warmer temps, and you’ve potentially starved native fauna showing up to feed at their normal time, only to find blossoms already finished and therefore unavailable as food. Also, consider how earlier bloom times mean flowers may be spent by the time fauna arrive to fertilize them by moving pollen from flower to flower, a process that normally produces the seeds and fruit we and other animals depend on to live.
Next week: how the Land Trust’s native-plant sale—now in full swing—can offer guidance in planting for this seemingly new set of challenges.
