I've been getting a number of letters about Cape oxalis (Oxalis pes-caprae), a weed that is now in full bloom in San Francisco. Also known as Bermuda buttercup, this South African native wildflower is has pretty yellow flowers that are cheery to see blooming on weedy road verges, but in a garden, it is a terrible pest. It grows in fall, blooms about now, and then dies back in later spring.
Cape oxalis grows from small tear-drop shaped bulbs that are dormant in summer, so many an unsuspecting gardener has planted in summer without realizing it was there. When it does appear, some think it is a clover, since it has trefoil leaves like a clover, and hope it will add nitrogen to soil. It won't. I have written my February 21st S.F. Chronicle column on this weed and how to (attempt to) get rid of it, so check out sfgate.com on that day.
Oxalis gets its sour flavor from oxalic acid, a substance that is also found in French sorrel, and, to a lesser extent in chard, beet greens, and spinach. We shouldn't eat great quantities of a plant that contains as much oxalic acid as Oxalis, but it is a nice nibble or could be added to salad in small amounts. In fact, I just read an article by a Greek writer about using Cape oxalis with French sorrel and other wild greens in soup or savory pie. I shall try to get recipes!