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But it's rated for my zone. . . .

© Carol Wallace

At the back of my property is a large stand of black bamboo (phyllostachys nigra) which I planted in an attempt to hide an apartment complex which sprouted, most inconveniently, right in my backyard. The apartments have proved to be completely hardy--neither freezing temperatures nor drought nor pesticides will eradicate them. The bamboo is allegedly hardy only to zone 7--but it doesn't seem to know that. It grows quite happily in my zone 5 garden, where temperatures can drop to -20 F in winter.

While my "tender" bamboo flourishes, a friend in Ohio repeatedly loses plantings of cornus canadensis, which is rated hardy to zone 2. Zone 2 means winters roughly as cold as those we imagine for Nome, Alaska. But my friend's garden is also in a comparatively balmy zone 5.

Neither of us are beginning gardeners, and both of us are sometimes confounded by zone ratings. So it's no wonder they can totally frustrate the beginning gardener.

Some of us may be old enough to remember having a zone in our addresses before they were replaced by zip codes (I do--barely.). But that's not the zone I refer to here. Hardiness zones are indications of the average minimum temperatures for different areas of the country. To find your zone, try The Zonefinder at Virtual garden.

Obviously, knowing how low temperatures can go won't tell you all you need to know about what plants you can grow. But knowing your zone provides a guideline that may keep you from planting things that will be killed by that first nip of frost. Check the catalog description, or the care tag on most nursery plants and you will see a hardiness rating which may (or may not) include the zone in which you live.

The trouble with hardiness zones is that they tell us only that one thing. If a plant is rated as hardy to zone 2 we know how low a temperature plunge that plant is likely to survive. Not just survive, but survive well. Grow. Maybe even flourish--all other things being right for it.

So why didn't my friend's cornus canadensis survive in a higher zone? It died because, while it can survive at a very low temperature, the heat does it in. When the thermometer rises much over 70 degrees F c. canadensis acts like it's been placed under the broiler at high flame. But the zone rating says nothing about how high a temperature a plant can tolerate. The USDA allegedly will be issuing a chart sometime in 1998 that will list average high temps, as a supplement to their current zone information, but it will be a long time before all the nurseries and catalogs can really use that information.

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