The invasive kelp Undaria pinnatifida is has now spread from Los Angeles to San Francisco Bay, despite eradication efforts.

Earlier, the northward spread of this sea weed – which can grow an inch a day and forms dense underwater forests – was believed to have been stopped at Monterey Bay, but this assumption turned out to be wrong when a biologist at the Smithsonian Environmental Research Center happened to notice a six-foot long piece of kelp attached to a boat in a yacht harbor in San Francisco Bay.

“I was walking in San Francisco Marina, and that’s when I saw the kelp attached to a boat,” said Chela Zabin, biologist at the Smithsonian Environmental Research Center in Tiburon, California. “It was six-foot long, and there is nothing here in the bay that gets to that size. I didn’t want to believe what it was, it’s depressing.”

Further investigation showed U. pinnatifida clinging not only to boat hulls in the marina but to docks and pier pilings as well.

U. pinnatifida was discovered in Los Angeles Harbour in 2000 and within a year reports of its presence had arrived from Catalina Island and Monterey Bay. A federal eradication program was put in place, but the funding dried up last year. Since then, volunteer divers have been the only ones combating the kelp.

Five quick facts about Undaria pinnatifida

• Undaria pinnatifida is a fast growing kelp native to the waters of Japan, China and South and North Korea.

• Within its native range it is an appreciate source of food and if you’ve ever tasted miso soup, this is what you were eating. The Japanese name for this species is wakame.

• U. pinnatifida has managed to establish itself in many different regions outside its native range, such as the Atlantic coast of Europe, the Mediterranean Sea, and the Argentinean coast. By attaching itself to vessel hulls it can hitchhike across the globe in search of new suitable habitats. This kelp can also enter ecosystems via imported oysters, and some people deliberately or accidently introduce U. pinnatifida to local ecosystems by cultivating it for cooking purposes.

• When U. pinnatifida spread to ecosystems not used to its presence, it can grow uncontrolled and prevent native kelp species from getting any sunlight. This can disturb the entire ecosystem.

• U. pinnatifida has been nominated to the list “100 worst invasive species in the world”.