Forest restoration efforts on Kaua'i are focusing on eradicating an aggressive "superweed" through an unprecedented ground and air attack.

The target: Australian tree ferns, which, like pale-green lace parasols, dot the landscape of central Kaua'i.

Attractive, but cancer-like in their impact, their wind-borne spores leapfrog natural barriers. The ferns climb the mountain ridges, descend into valleys, and form dense, tall canopies that shade out the native understory.

"It can cause severe changes in an ecosystem. It is far more dispersible than even miconia," said U.S. Geological Survey botanist Jim Jacobi, referring to the plant that previously had been considered the state's worst forest weed.

This may be the worst weed in the Kaua'i wet forest environment, said Trae Menard, director of the Kaua'i program for the Nature Conservancy of Hawai'i. It is found on other islands as well, but nowhere as widespread as on Kaua'i.

Menard's office has been contracted by the Kaua'i Watershed Alliance — a consortium of forest landowners and the Kaua'i Water Department — to coordinate a response.

The plan, said Menard, is to send crews on foot to hike into the uplands of Wainiha and the Lumaha'i valley to attack the weed where they can reach it.

And in more remote and dangerous areas, teams have been experimenting with a helicopter-based treatment system.

"We're going to hit it with everything we've got — ground crews, helicopter control, plant nurseries and help from individuals — but it's going to take everyone," said Chipper Wichman, director of the National Tropical Botanical Garden, a member of the the Kaua'i Watershed Alliance.

The helicopter-based system is designed for those areas where humans can't reach, or where it is too steep for them to work safely.

A helicopter flies into the forest with two tanks of herbicide under its belly, and a 100-foot-long cable with a weighted spray nozzle at the end.

The pilot can drop the spray nozzle directly onto the center of a tree fern, give it a brief shot of herbicide, and fly on. A dye in the herbicide confirms that the plant was sprayed, and how wide the spray pattern was.

"It's so precise, it doesn't seem like there's much overspray, but we're trying to make this applicator even more precise. We're going to get better at it," Menard said.

"We're finding that we can treat 300 or more plants in a 50-acre area within about two hours. A good crew on the ground would have taken a week to 10 days, if they could have gotten to the place," Menard said.

Menard said he recognized how serious the infestation was during the aerial mapping from 2003 to 2006 of Kaua'i's deep central mountains — the most pristine native forest on the island.

"We found that Australian tree fern was distributed throughout this core of our watershed," Menard said. "Pilots are telling us that where 10 years ago they saw five of them, now they see 500."

The introduced fern, which several major nurseries have now stopped selling because of its threat, is much more aggressive, faster-growing and taller than the native hapu'u tree fern. And while the hapu'u coexists with other native plants, the Australian introduction quickly forms one-species stands, shading out everything under it.

"Australian tree fern is an extremely serious incipient invasive weed because of its unique reproductive biology. The plant can produce thousands of propagules that wind-spread and can begin growing in places like crotches of trees where invasive trees might not be able to," said Wichman.

Fern spores appear to have blown on the trade winds from the residential garden plantings of Wailua into the forests of upland Wailua. There, now dense plants are producing spores that waft up over the knife-edge ridges of Hanalei, Lumaha'i and Wainiha and into the mile-high Alaka'i Swamp wilderness preserve.

Menard said his team experimented to find the herbicide that is most effective, but has the least collateral impact. They settled on a comparatively weak solution of triclopyr, an herbicide sold as Garlon 3A that is mixed with water and a blue dye and sprayed directly on the heart of the plant. He said it breaks down quite quickly in both soil and water.

And while the label permits it to be used at a rate of 3 gallons per acre per year, the team expects to spray no more than a half gallon per acre per year.

"The safety of this herbicide is the message we want to get out the most," Menard said.

That may be, but the public remains short of information and wary, said Maka'ala Ka'aumoana, executive director of the Hanalei Watershed Hui.

"As it relates to (the impact of the spraying on) a watershed, we don't know whether we're concerned or not. All we have is questions and no answers," she said.

Members of the Kaua'i Watershed Alliance have scheduled a public meeting in Hanalei tomorrow to discuss the project and its safeguards. Among them will be regular monitoring of the water downstream from areas where spraying takes place, to see if any detectable chemical residue is getting into the water.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.