Cold-Adverse Plants Warm Up to a New Home
With apologies to most of the country, we gardeners in the Pacific Northwest are spoiled rotten. Our temperate climate enables us to grow a glut of the world's plants.
Walk down a leafy street and you'll see not only native or Asian plants, but less familiar species from South Africa, Chile and the South Pacific, particularly from New Zealand.
Phormium, the New Zealand flax, is found nearly everywhere. A popular foliage plant, it's grown for its big, strapping multicolored leaves. Hebe (pronounced hee-bee), however, is not so well-known; its native haunts include New Zealand's dramatic cliffs and tufted grasslands.
"The thing about hebes is that they come in so many different shapes and forms," says Fiona Eadie, head gardener at one of New Zealand's most popular public gardens, Larnach Castle. "You can get groundcovers, you can get little bushes and you can get small trees. And you can basically get a hebe in whatever color foliage you like."
Hebes range in color from plum purple and pewter blue to burnt orange and forest green. And, Eadie says, the upkeep for hebes is fairly standard.
"They need fast drainage," she says. "They loathe humidity, but they love the wind. Hebes love to nestle right into their environment. You can get the best-looking plants in the harshest conditions."
Harsh in New Zealand is not the same concept for some Americans. Some of the showiest hebes expire at 17 degrees Fahrenheit – a warm winter day for the Pacific Northwest. The plant's fatal relationship with the cold explains why the genus has been such a heartbreaker in the Northwest, where until recently, only plant nerds risked loving and losing the plant.
Different Temperatures, Different Home
Things have changed.
Just yesterday, I saw hebes growing happily at a local community college. And each week, it seems, our nurseries are selling new Hebe species and cultivars.
What's different is this: We now know a lot more about the hardiness of different Hebe species. Hardiness is a word typically used to describe a plant's tolerance for the cold. Also, hebes are particularly happy these days because winters as of late have been warmer.
And it's not just here in Oregon. Gardeners across the country now report growing plants they could never get away with before.
"A classic example for us would be windmill palm, Trachycarpus fortuneii," says Tony Avent, the well-known proprietor of Plant Delights Nursery in Raleigh, N.C.
"Up until 1996, I would consider those very marginal in our area," he says. "All of a sudden, they're almost completely reliable in our climate. Eucalyptus would be another plant. We used to be able to grow only one species here. Now that the climate has warmed, we can certainly grow quite a bit more."
And, sadly, a bit less.
Evidently, good old-fashioned peonies are now struggling to grow in Raleigh, denied of their preferred, invigorating winter cold.
Tony Avent likes to brag that he considers every plant hardy until he has killed it himself three times. But not all gardeners like to push the envelope.
The safer way to determine whether a plant is hardy is with the USDA's Plant Hardiness Zone Map, a tool the entire trade uses in selecting appropriate plants for a particular place.
The map most commonly used was developed in 1990, which was based on climate data gathered from 1974 to 1986. The map divides the country into lasagna-like layers of different-colored hardiness zones, with each zone determined by a region's average lowest temperature.
As it turns out, 1974 to 1986 was a colder period than any other since. Which means the USDA map, that holy writ of hardiness, has made us unwittingly cautious in our garden choices, growing plants well within our range.
It would seem to be time for a new hardiness map, which Avent says is in the works. "We've got our fingers crossed for delivery by the end of the year."
Though a consultant on the USDA project, Avent could reveal no details, but did say the map would reflect three decades of data instead of one. He also mentioned that it would confirm what Hebe lovers already know.
"The new map will certainly reflect that it's getting warmer," Avent says. "I don't think there's any question about that. We can grow plants now that we couldn't in the 80s."
But he adds this condition: "As long as people aren't surprised when we get a cold winter to come in and knock things back."
A Changing Landscape
Back in the rolling hills of Oregon, an hour south of Portland, horticulturist Neil Bell stands in the middle of a seven-year trial evaluating the hardiness of several hundred hebes.
"The Achilles' heel of Hebe is that many of them, particularly the showier ones that bloom in the fall, well, they'll just keep growing and blooming happily as long as it's relatively mild," Bell says. "And when we get those cold spells, they die really suddenly. The plants aren't prepared for it."
In his work for the Oregon State University's extension service, Bell is making the kinds of observations about previously unfamiliar genera that may one day alter the look of the Northwest landscape. (In addition to Hebe, he's also evaluating hardy species of Grevillea and Cistus). He shares his discoveries about Northwestern hardy plants both with home gardeners and eager nurseries looking for the next big trend.
"This one I like a lot," he says, pointing to a particularly robust plant. "It's Hebe corriganii and this one will just cover itself in flowers."
A few thousand hebes flowering in Oregon does not mean the Pacific Northwest fancies itself the new South Pacific. But we have reached a point where prejudices like "That won't grow here!" might deny our chlorophyllic friends a port in the storm.
With the climate changing globally, native plants from one part of the world could find themselves seeking shelter in another, and the hebe may someday need the global gardens of Oregon for this lyrical species to thrive.
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