Fast-growing, fire-resistant grasses thrive in Hawaii

It only took a few days before new grass grew in the scorched earth left by the fires that raged across Maui and the Big Island starting August 8.

This is perhaps not surprising considering that for 200 years Hawaii has been importing and promoting many types of grasses intended to grow quickly to help improve agricultural production and enhance livestock operations.

Now, invasive grasses cover an estimated 25% of Hawaii’s landscape.

Forest fires only spread them. Many are adapted to fire, and carry traits that help them regrow faster and on a larger scale than native species.

But when this grass grows unchecked and is ignited — 99% of Hawaiian fires are set by humans — it becomes a high-octane fuel.

Meanwhile, the effects of fires have increased fourfold in recent decades, according to an examination by the University of Hawaii.

Herbal roots

At the turn of the 20th century, ranchers began introducing grasses to replace species native to Hawaii but lacking in nutrients, all to improve livestock growth.

“I would say every grass in the world has been introduced to Hawaii,” said Mark Thorne, UH rangeland and livestock specialist.

The 1939 UH publication “Weeds of the Hawaiian Ranges” listed 239 introduced grasses throughout the state, ranging from the arrival of sugarcane in the era of Polynesian settlement to wheat and corn in 1772.

But it wasn’t until the early to mid-20th century that herbs intended to maximize livestock growth began to appear.

“Grasses from the Eurasian steppe, all the way down to Africa, were introduced into the state because they were trying to find grasses that could support livestock grazing,” Thorne said.

The weeds represent some of the 10,000 alien species of plants and animals that scientists estimate have been introduced to Hawaii — often called the invasive capital of the world.

Over the past 100 years, ranchers have found a mix of grasses most suitable for raising livestock, including guinea grasses and Kikuyu grasses from Africa.

Both grow quickly, ensuring a constant supply of livestock. Hawaiian ranchers rotate cattle between pastures to keep the grass growing.

Kikuyu grasses are usually found in inland grasslands, while guinea grass is a hardy species found near the coast, along with other grasses imported for their nutritional value.

All grasses burn, but many imported pasture grasses are fire-adapted species, such as buffalo grass – another grass used by ranchers.

They burn quickly, which can lead to more frequent fires, and then regenerate faster than other species, crowding them out and fueling what researchers call the grass fire cycle.

All herbs have different chemical and physical traits that help them survive or thrive in fire. For example, guinea grass grows in dense bunches, accumulating dead foliage as it reaches 10 feet tall at a rate of up to six inches per day.

Thorne doesn’t call it invasive, because it has a benefit.

“Ninety percent of these grasses are extremely valuable to our livestock industry,” Thorne said.

Not all grasses are introduced to livestock.

The most problematic grass species—fountain grass—was introduced as an ornamental plant in about 1914.

Fountain grass is found on every major Hawaiian island. Its weak leaves catch fire easily and the fire spreads quickly.

Fountain grass rates 0.99 on a 0 to 1 scale for fire susceptibility, according to the Hawaii Invasive Species Council’s Weed Risk Assessment Database. The database was created last year by University of Houston biology professor Curtis Deehler and graduate student Kevin Facenda, and includes more than 360 weed species.

It is based on previous research on weed growth patterns and biological composition.

Buffelgrass, guinea grass, and molasses also ranked high on the fire risk scale, but still lower than fountain grass.

“That’s pretty much the height you get to,” Dyler said.

Invasive breeding

When grass is left to grow without grazing or mowing, wildfires become more dangerous in a landscape that has at least 1,400 introduced plant species.

Some of these species have negative impacts, such as invasive weeds, while others may be beneficial, which is why the Hawaii Invasive Species Council doesn’t target many of them.

“We don’t know if some of them…will become invasive species. New ones will emerge,” Dyler said.

Many scientists agree that getting rid of weeds completely is impossible. That’s why fire officials and researchers are taking a comprehensive approach.

“There is no one silver bullet,” said Chelsea Arnott, supervisor of the Invasive Species Council program. “It’s about integrated pest management.”

Measures include mowing or grazing fallow farmland, building more firebreaks and fuel barriers and strengthening community defenses against wildfires.

One tool the state has not used for at least 30 years is the state Department of Agriculture’s Noxious Weed Rules and List, which was created to help control noxious plants statewide.

These rules allowed Department of Agriculture staff to manage and eliminate listed species, even on private property.

Fountain grass is on that list. Guinea grass, buffalo grass, and molasses grass are not.

The list has not been updated since 1992.

“Hawaii Grown” is funded in part by grants from the Ulupono Fund at the Hawaii Community Foundation and the Frost Family Foundation.

Civil Beat’s climate change coverage is supported by the Hawaii Community Foundation’s Environmental Funders Group, the Hawaii Community Foundation’s Marisla Fund, and the Frost Family Foundation.

Civil Beat’s coverage of Maui County is supported in part by grants from the Nuestro Futuro Foundation.