Tropical plants aren't just for the tropics anymore

Magnolia trees blooming in Wisconsin? Banana palms shading porches in central Ohio? Cactus surviving Ontario winters? All true, actually. All that and more as people bend hardiness zones by growing plants in areas where they've seldom if ever been seen.

Garden gamblers are increasingly producing winning blooms in challenging climates and the practice isn't altogether a byproduct of global warming, says Patrick Cullina, vice president, Horticulture and Facilities, at the Brooklyn Botanic Garden.

"The options are more extensive than they ever were," Cullina said. "There is this sort of maneuver within the nursery industry to widen the palette as much as possible. They're being very creative about the spread of the plant options they're offering.

"The other thing that's happening is that people are trying as many things as they can," he said. "They're becoming aware of the microclimates in their areas."

You can chance it and place exotic plants randomly around the yard or you can boost the odds by learning which cultivars might make it above the Mason-Dixon line.

Not all magnolia trees, for example, are native to Mississippi. Hardier varieties have been imported for years from the Pacific Rim.

The Musa Basjoo banana tree can shrug off temperatures to 20 degrees below zero under the protective cover of mulch.

Prickly pear cactus similar to that growing in the high desert country of Utah can thrive in protected pockets of northern British Columbia.

Conifers from the higher slopes of the southern Appalachians can grow without much risk in New England.

But before running off to a nearby nursery to buy plants similar to those you remember fondly from a childhood spent elsewhere, do what many prudent growers have been doing since the early 1960s -- study a USDA Hardiness Zone Map. Such things as soil moisture, sunlight duration and plant diversity play important roles in performance, but hardiness -- the ability to survive winters -- is the primary test.

The USDA map is divided into 11 sections ranging from Zone 1, which meanders from Canada's Northwest Territories down into mountainous sections of Montana, to Zone 11, a frost-free area encompassing Honolulu and Mexico's Yucatan Peninsula. Each zone represents a certain average winter temperature minimum. There are 10-degree separations between zones.

Most plants when sold carry care labels based on the lowest temperatures they can survive, such as "Hardy to Zone 3," or "Good to Zone 7." "They're (zones) based on absolute low readings and you need to have them as accurate as possible," said Scott Aker, a horticulturist with the U.S. National Arboretum in Washington, where some of his colleagues are testing the temperature averages on yet another climatologic zone update.

"Any hardiness zone will have its limitations. They're not magic lines. They're more of a continuum." By that Aker means growing conditions can differ greatly even within the same zone.

Gardeners also should consider a plant's tolerance to heat -- especially during droughts -- and you can look to the American Horticultural Society Plant Heat Zone Map for answers about that. "The effects of heat damage are more subtle than those of extreme cold, which will kill a plant instantly," said H. Marc Cathey, president emeritus of the American Horticultural Society in a fact sheet on the society's Web site.

"Heat damage can first appear in many different parts of the plant: Flower buds may wither, leaves may droop or become more attractive to insects, chlorophyll may disappear so that leaves appear white or brown, or roots may cease growing. Plant death from heat is slow and lingering."

Peruse the society's heat zone map much as you would the USDA's cold hardiness version. Find your location within one of its 12 zones and determine the average number of days per year you would experience "heat days," or temperatures above 86 degrees. That, the society said, is where plants begin suffering physiological damage from the heat. Living within Zone 1 would give you less than one heat day per year while people located within Zone 12 would sweat through 210 heat days.

Bending zones around new plants is an uneven exercise. Many introductions will succumb to cold. Others will shrivel under an unnatural sun. Some plants may root but not bloom. A feisty few may become invasive in new surroundings.

"I don't think we'll ever be able to grow palms in Minnesota or peonies in Florida. That just wasn't meant to happen," Aker said. "It was never intended that you could grow everything everywhere. That would make it a pretty dull world."