Leafy Invaders Cut Down to Size During Conejo Creek Restoration : Environment: State crews make mulch out of non-native trees that have caused erosion and flooding along crowded banks in botanical garden.

Palms, peppers and ashes, all beautiful trees when they line city streets, have taken root in a portion of the Conejo Creek, choking out native oaks and willows and causing erosion and winter flooding.

Now, in an effort to restore a stretch of the creek that runs through the Conejo Valley Botanic Garden, workers from the California Conservation Corps are bringing in chain saws to reduce the invaders to mulch.

“These trees block the flow of the creek, and then debris starts to collect and the water level rises,” said Carmen Oblinger, a member of the Gregor Mendel Botanic Foundation that operates the botanical garden. “The water eats away at the banks and doesn’t allow the natural vegetation to grow.”

The non-native trees, which germinate from seeds washed down storm drains dumping into the creek, are even harder on the creek in the botanical garden area than they are elsewhere in the city, she said.

“Unlike areas where the banks are reinforced, we’ve left a more natural setting in the garden, so the trees are more apt to take root,” Oblinger said.

As each tree was cut down, its limbs were dragged along the creek-side nature trail to a waiting “chipper,” which the Conejo Recreation and Park District provided to turn the branches into mulch.

The district owns the 33 acres of land where the botanical garden has created hillside trails to wind through native plantings as well as plants from other Mediterranean climates around the world.

Oblinger acknowledged that it might seem ironic for a botanical organization to uproot trees. But the California Native Plant Society is all for it, said Rick Burgess, a society member and a planner for the city of Thousand Oaks.

“Those plants are adapted to grow in any area where they can get a foothold,” he said. “And they reproduce well. So if you do allow them to stay, pretty soon you’ve got a grove of peppers and the native plants are locked out.”

Indeed, along the banks of the creek at the botanical garden, young palms with bright green fronds compete for water and nutrients with twisted old oaks. Ever-adaptable ash trees shoot straight up to the top of the oaks’ gray-green leaves to gain an edge for more sunlight.

The efforts to eradicate the non-native trees in the area have built-in problems. The intruders are bound to return.

“It’s hard to look at one area without looking at the whole watershed,” Burgess said. “And if you’ve got a source of exotics upstream, they are going to continue to be a problem downstream.”

Oblinger acknowledged the problem.

“There’s no way we can keep the seeds out,” she said. “We just have to keep them in check.”

The portion of the creek that runs through the garden drains into the main branch of the Conejo Creek, which receives runoff from 90% of Thousand Oaks, said city civil engineer Mohammad Fatemi.

The area of the creek that runs through the garden gets runoff directly from 46 acres, he said.

That portion of the creek runs below ground before it reaches the adjacent park, and runs at surface level through the park and botanical garden before its confluence with the main branch of the Conejo Creek. The main branch winds around the city and descends the Conejo Grade where it meets with Calleguas Creek, which then flows out to sea at Point Mugu.

Although Burgess said the cleanup efforts will have to continue, he applauded the work to eradicate the invasive plants.

“It’s not an ideal world, and if there is an opportunity to clean up one area, it’s worthwhile,” he said.

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FYI

The Conejo Valley Botanic Garden is open daily from dawn to dusk with no charge for admission. It is located next to the Conejo Community Park off Gainsborough Road. For information, call 494-7630.