Exotic Intruders Threaten Native Oak Saplings : Nature: Officials say palm, olive and ash trees need to be weeded out of a Conejo Valley Botanic Garden creek bed.

For nearly two decades, the Conejo Valley Botanic Garden in Thousand Oaks has been a place for volunteers to nurture budding plants to life.

But an invasion of palm, olive and ash trees into a creek bed in the garden has spawned a botanical battle between the intruders and a grove of native oaks, volunteer gardener John Oblinger said. Young oak saplings are being pushed out of the sunlight by the larger, fast-growing trees.

The only choice is to uproot those exotic trees, he said.

“A layman would say it looks like a natural site as it is,” Oblinger said. “But if you let it go, in 20 or 30 years this would be an ash tree grove instead of an oak. They’re not used to competing.”

Sunday, volunteers will open the garden to public tours in hopes of recruiting help for a project to preserve the 100-year-old oak grove and clean up the tiny creek that they have adopted as their own.

Oblinger said his vision of the project amounts to a massive weeding job. But instead of dandelions, it’s palm trees.

“It’s like weeds growing in a garden,” he said. “We don’t want them to get out of hand.”

In addition to plucking out non-native saplings, the group wants to reintroduce native species of shrubs and flowers and fix areas of the creek that show signs of erosion.

On a bank close to a road, erosion has cut so deeply into the sandstone that a large oak tree has fallen into the creek. Soil around a fence is gradually being eaten away, and vandals have left behind trash and graffiti.

The 30-acre botanic garden actually belongs to the Conejo Valley Recreation and Park District. It was donated by the Klingbiel Corp. in 1973 and taken over under a 50-year lease by the Gregor Mendel Botanic Foundation, a nonprofit, volunteer group responsible for the garden’s maintenance.

A dozen dedicated volunteers keep the garden cultivated, about 200 plant species catalogued and trails cleared.

The creek itself is actually a large drainage channel that feeds into the Arroyo Conejo Creek and eventually spills into the Pacific Ocean, Tex Ward, the park’s district manager, said.

It owes its year-round flow to runoff from the hundreds of residences near Gainsborough Road and Tuolumne Avenue, Ward said. Keeping exotic plants out of the creek is difficult, considering that it’s in the heart of Thousand Oaks, he said.

“That stream is the recipient of whatever flows from surrounding urbanized areas,” Ward said. “It picks up everything that is deposited on the streets from yards: seeds and phosphates and oils.”

The creek is one of the few refuges within walking distance of houses and shopping centers, said Rorie Skei, chairwoman of the Conejo Open Space Conservation Agency.

Unfortunately, she added, it is the destination for shrubs “that have escaped from people’s gardens.”

Despite its unnatural source, the creek is home to hundreds of plants and bird species. Occasionally, coyotes cross the garden searching for food, and endangered plants, such as the Conejo buckwheat, have managed to survive alongside non-native shrubs.

In some spots, the brackish stream is little more than two feet wide. In others, cattails rise up from a marshy bed stretching about 10 feet. The county Environmental Health Department monitors the creek to keep mosquitoes from thriving in the stagnant water, but “until now, the creek has remained untouched,” said John Dick, a horticulture expert working on the restoration project.

Dick has compiled a list of 15 trees and shrubs that will be planted near the creek bed. It includes wild gooseberry and roses that provide food for birds and other wildlife in the garden.

It will probably take years to completely remove exotic plants and to replant the area with native species, Dick said. Most of the work will depend on volunteers, he said.

On a recent tour of the garden, Oblinger pointed to a crop of bright yellow flowers called California mustard. According to legend, mustard was planted by colonizing Spaniards who wanted to leave a trail to mark their settlements, he said. Now they grow in abundance.

“That’s the kind of plant we want to clean up,” Oblinger said.