WAILUKU >> A Maui ranch has planted thousands of acacia koa trees to help stave off the spread of an invasive plant species.

The Haleakala Ranch has planted nearly 15,000 koa trees in the past three years to impede the growth of the thorny gorse shrub, The Maui News reported Monday.

The European plant was introduced to Hawaii in the late 1800s, said Jordan Jokiel, the ranch’s vice president of land management. Lacking local predators, the species has flourished, creating impenetrable thickets that harm native plant growth. The shrub has covered more than 1,000 acres (405 hectares) of the ranch’s mountain side pastures, he said.

The ranch has used cattle to consume and fight other invasive plants, but cows aren’t effective on the shrub, said Greg Friel, the ranch’s livestock manager.

“When that plant is very young, the leaves are very fleshy,” Friel said. “But it hardens up very quickly and every little leaf becomes a thorn. By the time that thing is a foot tall, they physically can’t eat it.”

To contain the shrub in the past, the ranch introduced goats and sheep. Workers also mowed the shrubs and use herbicides. The ranch then decided to take a different approach and began planting koa in 2015.

“We’re just trying to shade it out by planting native koa trees over gorse to try to shade it out and reduce its competitive edge,” Jokiel said. “Gorse doesn’t do really well in a shady environment. It will grow, but not as vigorous.”

The trees can also be harvested and sold for use in hardwood products.

“Our plan is to try to keep planting small amounts until we get really good at it,” Jokiel said. “We’ll be able to maintain a standing forest hopefully of koa, because we plant regularly every year. When we harvest, we can just replant. It is potentially very sustainable.”