Confusion between Zones & Climate
It amazes me what I read on my twitter stream sometimes. A couple days ago the topic of daffodils came up. I mentioned that the daffodils hadn’t started to grow in Toronto. Another “gardener” tweeted it couldn’t be so. Her sister-in-law who also lives in Zone 5 already had daffodils up.
Daffodils at Toronto Botanical Garden, May 6, 2011
I was flabbergasted. USDA Plant Hardiness Zones do not equal climate. It doesn’t even predict the last frost day in spring. Zones only let the gardener know how cold it will get in winter.
Let us compare the zones between Toronto (the closest large city to me) versus the daffodil growing city, Cincinnati. On closer inspection the USDA zones didn’t turn out to be the same. Toronto is Zone 5b, and Cincinnati 6a. But it’s close enough.
What is really different is last frost day in spring. And that’s what is making a difference in daffodils emerging from the soil. The Farmer’s Almanac lists May 9 as the last frost day in Toronto, whereas Cincinnati gets its last frost on April 13. Almost a month apart!
The difference in climate is subtle… these two cities are only 409 miles (658 kilometres) apart in continental North America. Let us examine three cities with the same USDA Plant Hardiness Zones that are further apart. Seattle, Washington; Norfolk, Virginia; and Big Spring, Texas are all 7B. Interesting, isn’t?
I don’t think anyone would argue that the daffodils are coming up at the same time in these three cities. Seattle has a moderate wet maritime climate, Norfolk, a humid subtropical climate and Big Spring swelters in its semi-arid steppe climate.
I don’t even think folks are looking at frost dates to compare these three puppies. But hey, I am going to give them to you anyway because I’m a smarty pants. Seattle, March 10; Norfolk, March 20; and Big Spring, March 27. These last spring frost dates are closer than the Toronto – Cincinnati comparison!
My conclusion? You can’t use climate and zones interchangeably, but if you are lucky you can occasionally get away with sweeping generalities. It’s complicated.
My plea: think before you tweet. On the other hand, no, don’t. It gives me material for blogs!
National Gardening Association’s USDA Plant Hardiness Zone Finder
The Old Farmer’s Almanac. Frost chart for Canada
The Old Farmer’s Almanac. Frost chart for United States