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Gardening hangovers, Part 1: periwinkle

April 19, 2010 |  7:05 am
Invasive_Vinca Ecologist Christy Brigham stands amid willows hemming Medea Creek in the Santa Monica Mountains. The trees’ amber leaves glow in the morning light. She frowns at an ivy-like plant with violet-blue flowers. It’s blanketing a large swath of the creek. “Periwinkle is a common landscape ground cover,” she says. “It’s attractive to some people. I think it’s a green menace.”

Periwinkle (Vinca major) hails from the Mediterranean. Let loose in parts of Southern California, it smothers virtually all of the wildlife-supporting native plants in its path. California is home to many indigenous plants found nowhere else on Earth. Many are at risk of extinction. The main culprit is urbanization, but weedy exotic plants — even some that residents buy for their gardens — often share the blame. Able to rough it in the wild, runaway plants can throw entire ecosystems out of balance.

Each day this week, I'll highlight one of these rogue beauties. Today: periwinkle, one of about 300 nonnative plants that have taken root in state and federal parks in the Santa Monica Mountains. Most are fairly benign, but the National Park Service spends as much as $400,000 a year to keep 19 especially destructive weeds at bay. Statewide, the cost of controlling the most invasive weeds on public lands is estimated conservatively at more than $82 million a year.

A tiny piece of water-slurping periwinkle can launch an infestation that will take the National Park Service 10 years to quell. “I’m sure what’s happened is that someone upstream has this on a bank,” Bringham says. “This species has vegetative propagation; as stem and root fragments break off, they wash downstream and become established.”

Medea Creek is a tributary of Malibu Creek, a rare perennial stream that’s vital for wildlife. The periwinkle alters the creek’s flow and covers gravel beds.

“It’s stabilizing soils that should not be stable,” Brigham says. "There should be a lot of open spaces — open sediments that are used by amphibians and reptiles.” If the periwinkle washes downstream, it could destroy breeding pools for the endangered southern steelhead trout.

To Brigham’s dismay, some California nurseries still sell this periwinkle, Spanish broom, fountain grass and other aggressive exotic plants known as invasives.

The nonprofit California Invasive Plant Council hopes California gardeners will protect wildlands by choosing alternatives it promotes on its website. Periwinkle substitutes include ivory star jasmine, beach strawberry (Fragaria californica or Fragaria chiloensis) and wall germander. Dwarf periwinkle (Vinca minor) is less invasive, but it's still not recommended.

The invasive plant council advises gardeners to pass on two other ground covers: ice plant (Carpobrotus edulis) and ivy. The former has taken over miles of California’s coastal dunes, displacing rare plants; some ivy species not only shroud the ground, they climb trees and kill them.

Invasive plants don’t just threaten wild species, Brigham says. They rob people of a wilderness experience.

“You should be able to go to your park and see the native plants and how they interact with animals," she says. "When places become degraded and dominated by a plant from another landscape, you don’t have that experience.”

-- Ilsa Setziol

Coming Tuesday: The trouble with Mexican fan palms.


Gardening hangovers, Part 2: Mexican fan palms

Gardening hangovers, Part 3: fountain grass

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Photo credit: Los Angeles Times