Hardiness Zones

It has been a virtually frostless winter where the weather has only occasionally been freezing cold. So it won’t surprise many that the USDA has revised the Hardiness Zone Map it publishes to guide gardeners as to what plants have the best chances of survival.

But the professionals at Ward’s Nursery find the new map to be a bit premature. “We were kind of surprised,” said Jodi Cahillane who wears many hats at the popular garden center. “We feel we haven’t had enough of a history to say the zone has changed.”

The map puts Great Barrington in Zone 5b (-15 F to -10 degrees F for lowest temps) and the slightly warmer Zone 6a (-10 F to -5 F). The change is five to 10 degrees warmer than the previous designation.

“The USDA decided that, with climate change and rising temperatures, it was time to change the map,” continued Cahillane, “but generally speaking we think of the zones as guidelines. We have wide fluctuations of temperatures here and it can still get down to 17 degrees below zero. The Earth’s heating up doesn’t make that much difference yet.”

With an eye toward caution, Ward’s staff always guides customers toward plants tolerant of colder conditions. “If you live in a hill town, plants that can withstand Zone 4 temperatures are a better bet,” she said.

Zone 4 plants should be able to withstand temperatures of -30°F to -20°F with mid-May as the ideal outdoor planting time. Annual flowers will do well in the summer regardless of zone but perennials, shrubs, grasses and trees must be able to withstand the minimum winter averages to thrive each year.

The average minimum temperatures for Zone 6, such as those experienced in the Hudson Valley, fall between zero and -10 F degrees with a growing season from early spring to early fall. The last frost can be expected in mid-March with the first frost occurring in November. But even here, caution is needed.

“Spring frosts happen in low-lying areas,” said Cahillane explaining that cold air, which is heavier than warmer air, accumulates in low spots. As a cold air layer builds, it displaces warmer air forcing it upward. The development of a colder air layer below a layer of warmer is called a temperature inversion.

Spring frosts can be costly. “We encourage clients to look at a colder zone than they might think,” she said, “especially with larger perennials. If you want to try a change of pace, look at smaller plants, where there is less money involved. It’s easier to lose a plant that costs $20 than one that costs $500.

Rather than being adamant about the zone, she says the homeowner should look at where the plant will be placed. “I have a spot on the southwest corner of my house where plants are protected on the north and east,” she said. “My garden there barely froze because of the protection the house provided and because of the sun reaching it. That is where I would experiment.”

A broad range of plants can thrive in Southern New England, including cacti, evergreens, fruit trees, alpine plants and more. “It’s all about selection,” said Cahillane. “Breeders and vendors have developed cultivars that are hardier. What I wouldn’t have is a tropical banana tree. You would need a nice greenhouse to do that.”

Much of the decision about what to plant can be determined by elevation. Great Barrington (705 feet above sea level), for instance, is a little warmer than Monterey (1,244 feet).

Norfolk, just south of the Connecticut state line has an elevation of 1,230, high enough to earn it the sobriquet “the icebox of Connecticut.” The USDA shows it to be in Zone 6 but Nash Pradhan of Ginger Creek Nursery begs to differ. A horticulturalist with more than 40 years of experience in Norfolk’s chilly climes, he advises against “pushing the envelope” when using the new hardiness map to choose perennials.

“Norfolk is different because of all its microclimates,” he cautioned. “I’m not going to change anything. I will plant the same perennials, the same trees.”
The key to successfully raising plants is selecting the right plant for the right location and planting it correctly, he continued. “It depends on your location. What kind of exposure do you have? Are you in a valley? Are you near water?”

He now exclusively plants native plants which have a high survival rate because they have adapted and thrived under local conditions for millennia. “I will not plant invasives or potentially invasive plants,” he said. “Burning bush has beautiful fall foliage but it is so invasive that I removed mine 20 years ago and I still find seedlings in the woods.”

He admits that there is uncertainty about what the warming temperatures will mean in future years. “There are some rare plants that grow here. Will they disappear? We don’t know,” he said.

The rising temperatures have lengthened his season. “Things have certainly changed,” he observed. “There is almost no frost in the ground this year. I was still planting until the first week in January when my season usually ends in late October or early November.”

When planning for this year, Cahillane advises planters to expect another wet year. “It feels like it might be an early year,” she said, “but the chances are, it will be wet again. Our usual advice is that, if you had trouble last year, expect the same this year. If you are growing tomatoes, I would use a variety that can take more wet and adjust the planting to provide better drainage areas. That is one of the easiest things to do. Get compost and open up that soil. And sometimes it is a good idea to plan on replanting. If you put peas in on St. Patrick’s Day, plan on replanting in April.

Wards is located at 600 Main Street in Great Barrington MA; 413-528-0166. Ginger Creek Nursery is at 764 North Main Street in Norfolk, CT; 860-542-5143.