The cause of a runaway brush fire in August that forced thousands of Waikoloa residents from their homes, schools and businesses remains unknown, but officials said the reason it spread so quickly to consume 25,000 acres is no mystery: Blame it on fountain grass.

The grass is a highly aggressive, drought-resistant species that has been declared a noxious weed by the state Department of Agriculture. As it withers and quickly springs back to life with only the slightest encouragement, more and more dead plant material builds up, creating ready and abundant fuel for wildfires. And fountain grass is one of the few invasive species that can grow in young lava flows, which means fires can spread unchecked into surrounding subdivisions, pastures and forests.

"It is one of the more plentiful fuels available on the west side (of the Big Island) because it's primarily open land and there are very few trees. The majority of ground cover is fountain grass," said acting Battalion Chief Al Tobosa of the Hawai'i County Fire Department.

"Only the tops burn and it can run pretty quick," Tobosa said. "The winds ... make it that much more difficult to catch it. You have to get in front of the fire with bulldozers, aircraft or something else that can move quickly."

Like many of the state's plant pests, fountain grass was first brought to the Islands as an ornamental for landscaping purposes before it spread into undeveloped areas.

On the Big Island, it covers an estimated 200,000 acres in the arid North Kona and South Kohala regions, and the plant has begun invading subdivisions in Ka'u to the south. More than $500,000 a year is spent controlling fountain grass in Hawai'i County, and campaigns also are under way to keep it in check on Maui and O'ahu, where it has become established on Diamond Head and in other leeward areas.

In Ka'u, small populations have been found throughout Hawaiian Ocean View Estates and Hawaiian Ranchos and on adjacent lava flows. Because the clumps of grass normally exist in small groups, control efforts can be effective, said vegetation ecologist Rhonda Loh of Hawai'i Volcanoes National Park, which has been waging its own war against fountain grass for at least 15 years.

Although the grass is found throughout 50,000 to 100,000 acres in the park, it grows in very low-density satellite populations, Loh said. Helicopter surveys have been able to pinpoint the location of these patches so they can be uprooted. The park conducts aerial surveys twice a year, and Loh said the fountain grass population has gone from about 10,000 individuals when control efforts first started to only about 1,000 now.

But with the park's 2003 acquisition of 116,000 acres of Kahuku Ranch land, staff are starting to see small populations of fountain grass on the newly purchased property and in the subdivisions adjacent to the park, mostly on the roadsides.

In Kona, the grass grows over vast expanses.

"It looks like a sea," said research ecologist Susan Cordell of the U.S. Forest Service, who remains optimistic that effective control strategies can be put into practice.

The Forest Service has been studying the impact of fountain grass on the native ecosystem in Kona for five years.

The fire threat is the biggest danger to native forest, which includes trees such as wiliwili, kauila, 'iliahi (sandalwood), lama and alahe'e. But Cordell said fountain grass also outcompetes other species for sunlight, water and soil nutrients.

With fountain grass comprising 90 percent of the understory in Kona's tropical dry forest, "native species don't have much of a chance," she said.

"The reason it is so successful is its incredible ability to withstand drought, and it responds really quickly to rainfall."

At a 120-acre restoration project in Pu'u Wa'awa'a in North Kona, Cordell and other researchers have found that once an area is cleared of fountain grass, there is a window during which other species can be planted and take hold, preventing the noxious weed's return.

Reach Christie Wilson at cwilson@honoluluadvertiser.com.