How Do You Know If A Plant Is Hardy?

January 21st, 2010 · No Comments

I was just reading Graham Rice’s musings on plants that grow in the coldest climates. He observed that the resources he consulted did not agree on which plants were tough enough to take USDA zone 2. If you’ve been gardening for any length of time (which of course Graham has), this won’t surprise you.

First, there is the matter of statistics. The fewer people gardening in a certain zone, the less data available on any particular plant. I imagine with a lot of plants, to say it’s hardy to zone 2 is as much an educated guess as it is a tested hypothesis.

And then there are so many factors affecting plant hardiness besides air temperature that some people think the whole concept of hardiness zones is a joke. Ellen Hornig of Seneca Hill Perennials admits, “We include hardiness zones largely to pander to popular prejudice and give you a small degree of guidance.” Tony Avent provides a detailed analysis of the problems inherent in the concept of hardiness zones.

But you’ve got to start somewhere. That’s where other local gardeners are a tremendous help. If anyone is going to know what will make it in your garden, it’s someone with the same growing conditions. And many people who are still too isolated to find a local garden buddy now have the benefit of internet access, and can consult and befriend others in similar conditions.

This caryopteris was a gamble that I lost. It failed to return after its first winter.

In the end, the only one who’s an expert on what grows in your garden is you, and you become the expert by trial and error. That means you will kill plants. You will kill plants. After a while, you won’t kill as many, because you’ll develop a better understanding of your extremely local growing conditions. By patient observation you’ll know, for example, that a certain corner is windy, the snow always melts first by the walkway, and the area by the gutter downspout is always a little bit damper.

On the other hand, I suppose you might kill more, because you may become more willing to take chances. I find my willingness to experiment is constrained by the limits of my pocketbook. I will try a perennial rated a zone or even two zones warmer if I think I can provide the other conditions it needs, perhaps shade and moist, acid soil. But I am more conservative when it comes to an expensive tree that requires a lot of digging to plant–and to dig out again when it dies. Gardeners with a bigger acquisitions budget will be braver. Wayne Winterrowd and Joe Eck, in A Year at North Hill: Four Seasons in a Vermont Garden, describe many rhododendrons in their garden that aren’t considered hardy in their zone. But there they are, grown to maturity. Winterrowd and Eck were not assured of success when they planted them.

Graham Rice has a tough job, writing about plants for an area in which he’s never gardened. I imagine after consulting reference works, he’ll talk to friends and colleagues more familiar with gardening in that rigorous climate, and maybe seek out a gardening blog or two. Northscaping, which I’ve mentioned before, is another good source of hands-on cold climate gardening information.