But It's Rated For My Zone . . .

But It's Rated For My Zone . . .


But It's Rated For My Zone . . .
by 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.

Another difficulty with zone ratings is that they only take into consideration the average temperatures in your area. Within your own yard you may discover microclimates that can actually be used to grow plants out of your average hardiness range. My bamboo happens to be planted behind a low stone wall that apparently provides enough reflected heat and protection that the lucky plant thinks it's in North Carolina.

There are so many differences in humidity, rainfall, days of sunlight, and more that the issue can get even more confusing. I grow magnolias here in zone 5 with ease; a friend in Colorado, also zone 5, simply can't do it. There isn't enough humidity in Denver.But the plant tag's zone information won't tell you that. Nor will it tell you if that "hardy-in-this-zone" plant will also tolerate your soil type. I tried for years to grow gypsophilia (baby's breath), til the light dawned and I realized that gypsophilia quite literally translates to "lime lover." Hardy in my zone, yes. Hardy in my acid soil? No.

Most frustrating of all, many plants simply haven't been tested to the limits of their hardiness. When I bought my first hellebore orientalis (Lenten rose) I had scant expectations that it would live, because it was rated as zone 7. Now, six years later, most catalogs give it a zone 5 rating because so many brave (or foolhardy) souls tried it and found that it survived. Not all catalogs have noticed this however.

Some catalogs are quite extravagant with their hardiness ratings, assuring most of the country that the plant in question will grow for them. Others, more responsible (or not wanting a flood of complaints about winter deaths), are extremely conservative in their hardiness ratings. Some simply haven't looked around and discovered that the hardiness range is greater or less than originally estimated. So going from catalog to catalog reading about the same plant can be mightily confusing.

So how do you really know which plants you can safely grow? If you trust your local nursery, ask for guidance. But also take a good look at other gardens in your area. Notice what is growing well for gardening friends. And experiment. Buy small plants, rather than investing in huge ones, and see how well they do. If they appear only marginally successful, try moving them to another spot--sunnier, more sheltered, more shady--whatever you think it lacks in its present position--and give it one more chance.

And if you really, really want to grow a plant that is rated one zone warmer than you are, give it a try. We'll never know exactly how hardy a plant is until people are willing to test it. I have many plants in my garden rated zone 6, and a few rated zone 7, that have lived here happily for years. Obviously you don't want to invest in a rare and expensiveplant for your tests--but a small, moderately priced one, well-planted and tended is worth the risk.

Check out these sites for more information on gardening and hardiness zones. Perry's Perennial Pages give another look at factors that affect plant hardiness.

The Weather Channel has some great weather maps for gardeners that you can study.

The Fuchsia Page shows how Europe compares to the US in terms of hardiness zones.

About the Author

About the Author

Carol is a garden writer and college professor in northeast Pennsylvania. She manages the Gardening section of Suite 101.com, where she also writes the column Virtually Gardening.

About this Author