“When that plant is very young, the leaves are very fleshy,” said Greg Friel, vice president/livestock manager for the ranch. “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.”

While the ranch is constantly dealing with new invasive species that come in by boat, plane or person, its biggest foe has been around for more than 100 years. Now the ranch is hoping native koa trees will be the latest answer to the longtime invasive.

Gorse is native to Western Europe and the British Isles, and the thick shrubbery was used as natural fencing for livestock, said Jordan Jokiel, vice president of land management for the ranch. The plant was brought to Hawaii in the late 1800s when Hawaii was trying to develop a sheep industry. Growing in a totally different climate without its natural predators, the gorse flourished.

Gorse shrubs contain bright yellow flowers that “smell faintly of coconut,” according to the Hawaii Invasive Species Council. But they also come with inch-long spines and grow in dense, impenetrable thickets that prevent other plants from growing — proving why it’s a problem for landowners like Haleakala Ranch. The seeds can stay viable in the soil for more than 30 years, and not only can the plant survive fires with its deep roots, but the fire actually helps the seeds sprout.

Gorse, a thorny shrub, is pictured growing in Haleakala Ranch pastureland about a mile below the entrance to Haleakala National Park. The plant grows quickly and has taken up more than 1,000 acres of Haleakala Ranch land.

Gorse has taken up well over 1,000 acres of Haleakala Ranch’s mauka pastures, said Jokiel, adding that it’s even worse on Mauna Kea, where it’s consumed thousands of acres.

In the past, the ranch introduced goats and sheep to graze gorse, and staff would follow the herd and do a combination of mechanical and chemical control that involves mowing and “spot spraying” herbicides. In recent years, however, the ranch has tried planting acacia koa.

“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.”

In 2015, the ranch started planting on a relatively small scale — 15 acres at a time, and usually between the 4,000- to 6,000-foot elevation, where the koa trees do well and the gorse tends to be located. Over the past three years, the ranch has planted close to 15,000 trees. Not only does the koa impede the growth of the gorse, the tall trees also help capture water vapor from clouds drifting across the mountain. The water eventually makes its way through the roots, into the soil “and hopefully into the water table and aquifer,” Jokiel said. Koa trees can also attract other species, like native forest birds and endangered native bats.

And, of course, there’s the commercial value of koa trees. Perhaps 30 years from now the ranch could harvest the koa trees to be used in valued hardwood products.

Cattle graze on Haleakala Ranch pastureland. The animals are key to the ranch’s invasive plant-controlling efforts, but even they can’t stomach fully grown gorse, the thorny shrub that causes the ranch the most problems.

“It takes the pressure off the harvesting of wild koa trees, which can sometimes be done inappropriately when they’re not replaced,” Jokiel said.

“Our plan is to try to keep planting small amounts until we get really good at it,” he added. “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.”

Friel and Jokiel said the trees are still too young to shade out the gorse. However, Jokiel said some older koa groves have helped “reduce gorse density, occurrence and vigor.” The ranch planted some koa trees on its land in 1985 as part of former Hawaii first lady Jean Ariyoshi’s statewide koa planting project, “A Million Trees of Aloha,” Friel explained.

As for other invasives, the 1,200-plus cattle of Haleakala Ranch are happy to help. Friel teaches the livestock to eat the problem plants by bringing in truckloads of the invasive and spraying a mixture of molasses and water over the leaves. After the cattle have grown accustomed to foraging on the invasive plant for about a week, Friel weans the cattle off of the molasses spray, and they’ll continue to eat the plant. The ranch has done this with the invasive black wattle and faya trees.

“We’ve done it with small bunches of cattle,” Friel said. “Now it’s time to ramp it up to a big herd.”

For the cattle, “it becomes a learned habit, and through the rest of their life they’ll continue to eat it, and the calves learn from their mothers,” he added. “Hopefully some day they’ll graze it to a point where these trees are a minimal problem for us.”

Using local resources to combat invasives is key, especially when biocontrol — bringing in predators of the invasive — is complex and risky. The new species has to pass many stages of testing to make sure it doesn’t become a threat itself, and the state Department of Agriculture doesn’t have a big enough budget to do much of this testing, Friel said.

For more than 40 years, the ranch hired a summer crew to control gorse and eucalyptus. Now it has three full-time field employees dedicated to controlling invasives.

“With hundreds of airplanes landing every day . . . and container ships being unloaded all the time, there’s always something that’s going to be loaded in,” Friel said. “Every year there’s just something else that’s going to show up.”

* Colleen Uechi can be reached at cuechi@mauinews.com.