The Oregon Department of Agriculture said the plant was found in October on private property in Jackson County. It grows long, slender branches with serrated leaves that form dense mats capable of blocking waterways and overwhelming native ecosystems.

"It's widely regarded as the worst invasive weed in the world," said Troy Abercrombie, who manages the department's noxious weed control and native plant conservation programs. "It's one of very few federally listed noxious weeds."

Abercrombie described the discovery as a best-case scenario: the infestation was caught early and appears isolated to small ponds. Hydrilla, which thrives in freshwater, is classified in Oregon as an A-rated invasive species, meaning it is not yet widespread and can be targeted for eradication with herbicides.

The department is consulting with experts from Florida, where the weed has been fought for years at enormous expense. Abercrombie said Florida has spent hundreds of millions of dollars over the past decade trying to control it.

Native to tropical regions of Asia, Africa and Australia, Hydrilla verticillata was originally sold as an ornamental aquarium plant because of its bright green color and elegant stems. Once it escaped into the wild, its spread proved impossible to contain.

Dense mats of the weed can be nearly impassable for boats and swimmers and can block the movement of aquatic organisms. In some cases, its growth can alter a river's hydrology by slowing water flow and causing organic matter and soil to accumulate.

"You can have these cascading effects where the eventual impact is much larger than the singular impact from the species itself," Abercrombie said.

Samuel Chan, a watershed health expert at Oregon State University and a member of the Oregon Invasive Species Council, said Hydrilla — sometimes called water thyme — is exceptionally difficult to manage.

"It's a species that in my whole career, I've never seen spread so fast," Chan said. "It grows at least 1 inch a day."

Chan said the plant is highly adaptable because it can reproduce from tiny fragments or dormant buds. Even if the visible vegetation is treated with herbicides, he said, those buds — known as turions and tubers — remain in the sediment and can sprout again the following season. That means eradication will require years of vigilance.

Abercrombie urged the public to watch for the weed and report any sightings to oregoninvasiveshotline.org.

The discovery places Oregon among a growing list of states grappling with a species that can transform freshwater systems. For Jackson County residents, the immediate risk is limited to the small ponds where it was found, but the long-term stakes are high: once established in a river or lake, Hydrilla can be nearly impossible to remove.