In the early 1900s, Hawaiʻi imported a thorny European shrub to feed sheep and fence pastures; it now forms 6-foot walls that fuel fires and sprout again

Imagine a plant so tough that fire doesn't kill it. Instead, fire wakes it up. That’s gorse, a spiny shrub currently invading the mountains of Hawai‘i. The plant was introduced to Hawaiʻi around 1900 to feed sheep and serve as a living fence, according to a January 2021 report submitted to the state legislature by the Hawaiʻi Gorse Task Force. Over a century later, it has become one of the toughest invasive species problems on the islands.

A sheep fodder experiment gone wrong

Gorse, or Ulex europaeus, is native to Western Europe and was used by ranchers as fodder and as a natural barbed-wire fence. It made sense to introduce it to the Hawaiʻi Islands, where sheep could graze the young shoots and the spiny stems would keep the animals within the fence boundary.


However, the scheme backfired. Hawaiʻi's climate had no natural checks on the plant such as insects or disease, which could slow its spread. As the task force report notes, ranching later changed from sheep to cattle in high-elevation pastures on Hawaiʻi Island, which made things worse, because cattle tend to ignore gorse. Without the sheep to nibble it back, the gorse took over.

Six-foot walls of thorns

According to the Hawaiʻi Invasive Species Council, a multi-agency council housed within the state's Department of Land and Natural Resources, gorse can form very thick clumps that grow up to six feet high and are full of one-inch spines. The branches grow so densely that almost nothing else can force its way through. Native vegetation gets overshadowed, and grazing animals cannot penetrate the ground below. Even the bright yellow flowers, which DLNR says have a faint coconut smell, cannot offset the damage the plant causes below ground.

The task force report said gorse now covers more than 10,000 acres of former pasture on Mauna Kea, at elevations between 5,000 and 8,000 feet. The report says it has eaten up at least 1,000 acres along the slopes of Haleakalā on Maui. Those are current totals, not a spread rate, but they hint at how much room the plant still has to grow: the report estimates up to 500,000 acres on Hawaiʻi Island alone provide the right conditions for gorse to thrive.

Why fire makes it worse, not better

Wildfire might seem like a natural way to clear out an invasive shrub. With gorse, the opposite is true: fire doesn't just fail to kill it; the plant is actually built to exploit it.

According to a 2019 study, ‘A World of Gorse: Persistence of Ulex europaeus in Managed Landscapes,’ in the journal Plants, gorse seeds actually germinate better after they’ve been subjected to heat. The study showed that brief exposure to about 110 degrees Celsius for only five minutes is enough to increase the number of seeds that germinate. The soil serves as a buffer, the researchers point out, shielding seeds from the fire's most extreme, lethal heat while still allowing enough warmth to reach them to trigger germination. Thus, after the fire rips through the gorse bush, the land underneath becomes ready for growing new seeds.

The plant also contains flammable oils in its leaves and branches, helping fires spread faster and burn hotter. Gorse is easy to ignite and fast to regrow, a combination that makes it a major wildfire threat in Hawaiʻi’s dry upcountry areas.

Seeds that could outlive you

Not only does gorse have the ability to survive, but its reproductive biology is also built for long-term persistence. This was reported in a 2021 paper published in the journal Weed Research. According to the research findings, a mature gorse plant has the potential to generate more than 18,000 seeds within one year. The paper also found that seeds can remain viable in the soil for more than 30 years, waiting for favorable conditions such as fire, disturbed soil, or scratching by animals to trigger germination.

This is why cutting gorse is not a lasting solution. Even if you remove the entire visible part of the plant, there will be hundreds of thousands of seeds lying dormant in the soil.

Can anything stop it?

Hawaiʻi land managers are experimenting with a combination of methods, the task force report says: mechanical removal, herbicides, planting fast-growing native trees such as koa to shade out young gorse, and trying goats where cattle have failed. Some of the management approach, the report says, comes from New Zealand and Australia, where gorse has been a problem for decades.

But experts agree there's no quick fix. The task force report recommends follow-up treatment of five to twenty-five years, not months, given how long the seeds can wait underground. What began as a well-meaning import for sheep and fences became a century-long lesson in how invasive species can persist even after fire.