A natural history of that little yellow flower that’s everywhere right now

Chelsea Leu, BAY NATURE
Oxalis pes-caprae is an attractive little flower with five yellow petals and leaves that are cloven in a way that apparently reminded Linnaeus—who described the species in 1753—of a goat’s foot. Commonly known as sourgrass or Bermuda buttercup, it flowers from November to April, and in the last few months oxalis has come out in full force in the Bay Area, encouraged by December and February rains. The flowers dot hillsides, parks, and highway medians like the mottled points of light in a Monet, delighting many observers.
They do not delight Jake Sigg.
“I’ve just been frantic about it,” says Sigg, a retired Golden Gate Park gardener and the Bay’s most outspoken opponent of yellow oxalis. “It’s our most troublesome plant.”
Oxalis pes-caprae is invasive, a weed native to South Africa that was transplanted to California early in the 1900s, probably to be grown as a demure ornamental plant. By the late 1980s, the Los Angeles Times reported in 1988, it was a frustratingly persistent nuisance in home gardens. Now, Sigg has watched in horror as oxalis has taken over the coastal grasslands he tends. He first noticed a small patch of them in San Francisco’s Grandview Park back in the 1980s. By 2003, Sigg says, it was all over the hill. “In the last 10 years it really got going fast,” he says. “It’s just a blitzkrieg.” And the onslaught will probably continue. “If we did nothing, in X many years Twin Peaks would just be one solid mass of yellow, and there wouldn’t be any other plants there,” Sigg says. “It’s destroying our grasslands.”
Oxalis crowds out native wildflowers for light and space, and prevents other plants from gaining a foothold in the land. “Oxalis is terminal,” Sigg says. Once it takes over, the wildlife that depends on native flowers moves on, leaving nothing but oxalis in its wake (and bare ground during the six months of the year oxalis doesn’t flower). An oxalis-dominated landscape drives away coyotes, hawks and owls that feed on grassland foragers, and the situation is especially dire for endangered Mission blue butterflies, which depend heavily on native wildflowers. Whole hillsides are now “marching towards monoculture,” Sigg says. But we haven’t raised the alarm because, as he wrote in 2003, “we hardly notice [the spread] because it occurs slowly, subtly, surreptitiously.”
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