Cold-Adverse Plants Warm Up to a New Home

A Warley Pink hebe plant. i i

A hebe's plant color can vary greatly. The native New Zealand plant grows well in a warm and windy climate with no humidity. Mark Bolton/Corbis hide caption

itoggle caption Mark Bolton/Corbis
A Warley Pink hebe plant.

A hebe's plant color can vary greatly. The native New Zealand plant grows well in a warm and windy climate with no humidity.

Mark Bolton/Corbis
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Talking Plants is a new gardening blog from NPR's Ketzel Levine.

   

It's a place to meet new plants and cool plant people, tour incredible private gardens, savor inside-gardening industry gossip, swap dead plant stories and get the odd gardening question answered by your fellow "hort-heads."

   

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H. pimeleoides i i

My favorite hebe, H. pimeleoides 'Quicksilver,' with the double-flowered blossoms of a Dianthus 'Chomley Farran.' Ketzel Levine, NPR hide caption

itoggle caption Ketzel Levine, NPR
H. pimeleoides

My favorite hebe, H. pimeleoides 'Quicksilver,' with the double-flowered blossoms of a Dianthus 'Chomley Farran.'

Ketzel Levine, NPR
H.  speciosa i i

I found this H. speciosa hugging the security building on the grounds of Portland Community College. It's planted with a variety of ornamental grass called Pennisetum — rhymes with dentists eat them! Ketzel Levine, NPR hide caption

itoggle caption Ketzel Levine, NPR
H.  speciosa

I found this H. speciosa hugging the security building on the grounds of Portland Community College. It's planted with a variety of ornamental grass called Pennisetum — rhymes with dentists eat them!

Ketzel Levine, NPR
H.  pinguifolia on the left, and H. ochracea on the right. i i

From the Portland private garden, Bella Madrona: on the left, the very classic-looking hebe H. pinguifolia 'Pagei.' On the right, you'll see a hardy hebe that thinks it's a dwarf conifer, H. ochracea. Ketzel Levine, NPR hide caption

itoggle caption Ketzel Levine, NPR
H.  pinguifolia on the left, and H. ochracea on the right.

From the Portland private garden, Bella Madrona: on the left, the very classic-looking hebe H. pinguifolia 'Pagei.' On the right, you'll see a hardy hebe that thinks it's a dwarf conifer, H. ochracea.

Ketzel Levine, NPR
A variagated Hebe called silver queen. i i

Variegated hebes are unpredictable when it comes to hardiness, but this Hebe 'Silver Queen' is so superb-looking in both foliage and in flower, gardeners happily risk losing it. Ketzel Levine, NPR hide caption

itoggle caption Ketzel Levine, NPR
A variagated Hebe called silver queen.

Variegated hebes are unpredictable when it comes to hardiness, but this Hebe 'Silver Queen' is so superb-looking in both foliage and in flower, gardeners happily risk losing it.

Ketzel Levine, NPR
The white-flowered H. salicifolia i i

The white-flowered H. salicifolia has a lot going for it, including a really nice texture because of its long, narrow leaves and lounging habit. To boot, it loves to flower and is a fairly hardy species. Ketzel Levine, NPR hide caption

itoggle caption Ketzel Levine, NPR
The white-flowered H. salicifolia

The white-flowered H. salicifolia has a lot going for it, including a really nice texture because of its long, narrow leaves and lounging habit. To boot, it loves to flower and is a fairly hardy species.

Ketzel Levine, NPR
Neil Bell i i

Neil Bell is a community horticulturist at Oregon State University's extension service. He particularly enjoys cultivating hebes while singing like Jim Nabors. His colleagues are fortunate; Bell works alone! Ketzel Levine, NPR hide caption

itoggle caption Ketzel Levine, NPR
Neil Bell

Neil Bell is a community horticulturist at Oregon State University's extension service. He particularly enjoys cultivating hebes while singing like Jim Nabors. His colleagues are fortunate; Bell works alone!

Ketzel Levine, NPR
Midsummer Beauty i i

The stunning 'Midsummer Beauty' is one of the real lookers from Neil Bell's hebe hardiness trial. It can reach five feet and flower summer into autumn. Unfortunately, it's still hard to find in the U.S. Neil Bell hide caption

itoggle caption Neil Bell
Midsummer Beauty

The stunning 'Midsummer Beauty' is one of the real lookers from Neil Bell's hebe hardiness trial. It can reach five feet and flower summer into autumn. Unfortunately, it's still hard to find in the U.S.

Neil Bell

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.

Plant Mappers Push to Keep Up with Climate Swings

Tony Avent at home in his garden in Raleigh, N.C.

Tony Avent stands at his garden in Raleigh, N.C. hide caption

itoggle caption
USDA  Plant Hardiness Zone Map i i

The USDA's Plant Hardiness Zone Map helps gardeners, growers and planners determine which plants will best suit a climate across the United States. The USDA hopes to release a new map by the end of this year. hide caption

itoggle caption
USDA  Plant Hardiness Zone Map

The USDA's Plant Hardiness Zone Map helps gardeners, growers and planners determine which plants will best suit a climate across the United States. The USDA hopes to release a new map by the end of this year.

Tony Avent has been an adviser on a long-awaited revision of the USDA Plant Hardiness Zone Map. The map divides the country into climate zones based on average low temperatures, and is a much-consulted guide for professional and amateur gardeners who want to know which plants will thrive in their region. Avent, of the well-known Plants Delight Nursery in Raleigh, N.C., talks here about the history of the map, its strengths and shortcomings, and how a changing climate may soon bring about yet another revision of the map.

Ketzel Levine: I've always figured that those of us who care about the zone map were plant geeks. True?

Tony Avent: I'd thought so to, but during our map meetings, I was surprised at the diversity of people that use the map: universities, academics, forestry people, fruit growers. There were people from all over.

As long as I've been gardening, people have been complaining about the zone maps. You, too?

Definitely. I remember there was a great controversy between the first hardiness map from the Arnold Arboretum — which used 40 years of climate data — and the new USDA map, which used only 13 years. And this is the map the baby boomers have grown up with, the 1990 map.

I think we typically see our climate running 15 to 20 years hot, 15 to 20 years cold. And that 1990 map happened to use data from a particularly cold period. It became controversial because it dramatically changed the zones downward from the Arnold Arboretum map.

So one day you were happily zone 7, and the next you were zone 6... 10 degrees colder?

Exactly. Now fast-forward to 2003, and a new map by the American Horticulture Society (AHS). Those folks did something very bad, they actually deleted the half zones!

For example, they got rid of zone 7 "a" and "b."

Right. Most people might think a half zone is not a big difference, but it's a dramatic difference. Fortunately, there was enough uproar that the map was withdrawn.

The other problem was that the 2003 map captured a much warmer period, and indicated that people could grow less hardy plants that they couldn't before. But one cold winter and those plants would all be dead!

Any other ill-fated maps since?

Last year, the Arbor Day Foundation also got rid of half zones and moved everybody upward a zone. For example, Chicago went from zone 5 to zone 6. If people in Chicago had been planting trees according to that map, they would all have been killed this past winter.

So inevitably the question is, how will the new USDA map avert all these problems?

The USDA formed a group of stakeholders, including myself, who reached a consensus on what kind of criteria we wanted on the map. We've returned to "a" and "b" zones, and we'll be using a 30-year data set. Then once this map's out, the process will begin again on an even more detailed version.

Including?

We intend to include factors like the number of days over a certain temperature, humidity, night temperatures and other factors that affect heat adaptation. After all, hardiness is not just about winter lows.

In other words, a plant may do just fine in Raleigh during winter, but die during the summer. Like Hebe, which loathes your humidity.

Right.

So when's the next map due?

The best we can do now is keep our fingers crossed for year's end.

So let's wrap up with some advice for gardeners. What's there to learn here? How much should we rely on zone maps at all?

A map is only the first tool one should look at when selecting plants. Other things to consider are a plant's moisture needs, tolerance for humidity, amount of sun, and summer temperatures.

This is pretty conservative advice from a man whose tagline reads, "I consider every plant hardy until I've killed it myself three times!"

Well, people often limit themselves because they buy a plant and kill it, and assume it's not good for their climate. That's not necessarily the case. Many times there are more factors involved. So often it's a microclimate, a soil condition, just something else that you haven't got right. Don't be afraid to experiment!

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