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Winter 2013

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

February 19, 2013

Ivy: You say helix, I say hibernica

Ivy is just ivy to most people, but distinctions are important, especially if one species is blamed for another's bad behavior.

Part of the problem is that ivies are difficult to identify — but you can start with your nose. Crush Irish ivy, Hedera hibernica, and a honeyed scent rises. It's a fragrance that many a weed warrior has inhaled while ripping the invasive plant from local parks, where it blankets the ground and coats the trees. It may look similar to English ivy, Hedera helix, the one that gets fingered on many a least-wanted list, but Irish ivy is fundamentally different. (See another identification tip, right.)

Like us humans, H. helix is diploid; its somatic cells harbor two sets of chromosomes. But H. hibernica is a tetraploid freak, with four sets of chromosomes, explains Arthur O. Tucker, a research professor at Delaware State University. "This is usually the noxious weed." But not always. "In some places it is H. hibernica, but in other places it is H. helix," he says, citing a survey of herbarium curators. "The invasive potential also depends on the cultivar, so it is impossible to make generalizations."

"It isn't fair to attack all 500 cultivars of English ivy as invasive because of Irish ivy," says Suzanne Pierot, president of the American Ivy Society. The organization challenges the widely held assertions that any ivy covering the ground will exclude native plants or that climbing ivy will engulf and suppress trees and make them easier for winds to topple. The group says that ivy can provide a nitrogen boost to the forest floor and can protect trees from beetle attacks and bark damage from dramatic swings in winter temperatures.

However, people working to restore native plant communities say that the weed's detriments far outweigh any benefits. Invasive ivy has become a target for eradication, although efforts

have failed in state legislatures to ban Hedera helix in Maryland and "English ivy" in Virginia. In Oregon, however, the state's department of agriculture has banned the sale, transport and propagation of both H. helix and
H. hibernica.

"I'm not in favor af a ban against Hedera," says Richard Davis, founder of the Ivy Farm, near Locustville, Va. "Although it wouldn't cause us great financial pain, it's the principle of a ban that I'm opposed to." He cautions, "You need be careful of lumping Hedera. These pretty little variegated ivies that you can grow in a pot on your deck aren't going to go invasive."

Although Davis maintains one of the largest collections of ivy in the country, offering 100 cultivars of several species of Hedera, his nursery sells much more than just ivy. "If we had to live off of what we made selling Hedera," he jokes, "we'd be eating ivy."

"Hedera is a fascinating genus," says Sarah Reichard, director of the University of Washington Botanic Gardens in Seattle. "It flowers in fall and fruits in spring, which is opposite of almost every species, but brilliant for an invader — it provides fruits for birds in the spring when their other resources are depleted, thereby getting the fruits dispersed away from the mother plant. It mostly only reproduces when growing vertically on trees or walls — getting the fruits up where the birds are."

Reichard grew up in North Carolina, so she is familiar with aggressive East Coast ivy. But to really appreciate ivy's invasive potential, she says, "you need to come out here to western Washington, where it is devouring urban forests." Reichard helped analyze the genetics of invasive ivy in Washington state: 15 percent of the infestations were English ivy,
85 percent were Irish ivy.

Sources: Watsonia, Ivy Journal, American Nurseryman, Biological Invasions, John Peter Thompson, National Park Service

ivy, Hedera

Check your ivy
Grab a 10-power hand lens and look closely at young ivy stems and under young leaves, where you may find a white fuzz comprised of individual trichomes.

Irish ivy has flat trichomes, which look like tiny starfish with
5 to 8 rays.

English ivy trichomes are erect and bristly, with
8 to 12 rays.