By Paul James
I sometimes chuckle when someone asks me if a plant is hardy, because often what they’re actually asking me is if a plant is tough enough to survive Oklahoma summers. And that’s not what hardiness is all about. Hardiness has to do with a plant’s ability to survive the winter in a given area, and Hardiness Zones are established by the USDA based on historical weather data. So, do you know which Zone you’re in?
Before answering that question, I should explain that Plant Hardiness Zones are first numerically divided into 10-degree increments, and there are 13 of them (Zone 1 is bone-chilling cold, whereas Zone 13 is tropical). They’re then further divided into 5-degree “a” and “b” increments. The numbers and letters represent the average annual extreme minimum temperature for a given area. The key word here is average, because the zone designations do not reflect the coldest temperature ever recorded at a specific location.
(FYI – the coldest temperature ever recorded in Tulsa was 16-below zero, set in 1930. In 2011, we hit minus 12.)
Most of Oklahoma consists of three Zones that run somewhat parallel from east to west. Zone 6b (-5 to 0 degrees) is in the northernmost area of the state, roughly from just south of Bartlesville to the Kansas border. Zone 7a (0 to 5 degrees) includes Tulsa and extends south just beyond Okmulgee. And Zone 7b (5 to 10 degrees) covers an area from just north of McAlester to the Texas border.
Other than annuals – which by definition aren’t hardy – and tropicals, which I wish were hardy, it’s safe to assume that fully 99% of the plants sold by nurseries in your area, including here at Southwood, are hardy. Otherwise, they (and we) would have to deal with a lot of angry customers returning plants that didn’t make it through the winter.
But circling back to my first sentence, just because a plant is hardy in the winter here doesn’t necessarily mean it’ll survive the summer. Take hemlock, for example, one of my favorite conifers. It’s perfectly hardy (down to minus 30!), but it can’t take the heat in our neck of the woods. Ditto firs, many rhododendrons, and a host of other plants.
The American Horticultural Society years ago developed a Heat Zone Map that shows whether a plant is likely to survive the summer, but it never really caught on, perhaps because people just zoned out due to the confusion created by having to understand not one but two zone maps. Still, it’s a useful tool that you can find online at http://ahsgardening.org/gardening-resources/gardening-maps/heat-zone-map. (And by the way, we’re in Zone 8 in terms of heat).
Fortunately for us, we live in an area where we can grow a staggering number of plants that are dependably hardy. And yes, that includes laurels.