While some observers see a Global Warming angle, others see a 15 year trend measurement. The government statistics use 100 years or more. The USDA has chosen to stay with its longer measurement period, while the American Horticultural Society and the National Arbor Day foundation go with the shorter and warmer period.
Some suppliers prefer not to advertise the use of plants outside their recommended zones, they could be asked to refund payments if the weather turns colder, for example. Others warn customers that the plants are "pushing the line" by offering boundary creep.
Alice Longfellow, president of Longfellow's Garden Center in Centertown, Mo., says the warmer climate "has allowed us to have more diversity in what we can sell." According to the updated map, her area now is in Zone 6. That means she can safely sell boxwood. "Fifteen years ago boxwood would burn out in the winter," she says.
The bottom line: You may want to experiment, but first do some research. Compare the two hardiness maps at arborday.org and consult a local garden center; staff there should know which out-of-zone varieties have been successful. If you decide to try some, plant them in the warmer parts of your yard, such as next to the south side of the house or on the sunny side of a stone wall. Such "microclimates" can be a full growing zone warmer.
Some plant-industry experts caution about pushing zones. "There is no question that we are experiencing zone creep, which is reflected in people's orders," says Tony Avent, co-owner of mail-order retailer Plant Delights Nursery. But Mr. Avent says gardeners shouldn't take chances unless they're willing to lose plants. "Anyone who has studied historical climate change understands that we swing back and forth from cold to warm."