Maybe Emerson was correct that a weed is merely “a plant whose virtues have never been discovered,” but I am hard pressed after a day of wrestling with the indomitable oxalis (aka buttercup oxalis, cape sorrel or sourgrass), to see its virtues. I diligently pull it up each Spring as recommended in a recent article by a fellow Master Gardener, Chantal Guillemin, “Taming Oxalis.” However, solarization seems to be the best means of eradication. A planting area layered with cardboard and a thick layer of mulch last Fall is oxalis free -- at present.
FYI: my go-to resource for identifying & battling plant diseases and pests is the Integrated Pest Management website of the University of California at Davis. Bay Area gardeners can also visit the Sick Plant clinic at the UC Botanical Garden in Berkeley. The Clinic is held on the first Saturday of the month from 9-12 pm. Entomologists and Master Gardeners are available to identify what ails your plants.