By Bruce Cowan
There are several species of oxalis occuring locally, including native redwood sorrel (Oxalis oregana) with white flowers growing under redwood trees in Big Sur, and several others–mostly small non-native weeds. The most invasive is the pretty yellow one often referred to as Bermuda buttercup (Oxalis pes-caprae) shown in last week’s front page photo. This species is from Africa, not Bermuda, and was probably introduced as an ornamental. Sourgrass is another name because it has oxalic acid to give it a sour taste.
Bermuda buttercup can be very hard or impossible to control. It spreads rapidly from bulbs forming on underground stems, not from seeds. Pulling it eliminates it only temporarily, unless every bulb is extracted–which is very difficult. Birds, and possibly squirrels and gophers, plant bulbs in new areas. This year, with the early winter rains, it seems especially abundant.
Herbicides such as Roundup (Glyphosate) can kill it, but spraying is useful only in weedy areas, or where other vegetation, including native plants, are largely absent.
Scattered individuals or small clusters invading new areas can be killed by squirting a few drops of Roundup on each leaf. I have used this method at the PG Museum Native Plant Garden to preserve the habitat for native plants, and in in my own yard.
Where a lot of yellow oxalis is mixed in with other plants, including in PG’s Magic Carpet ice plant, pulling it is the only option, even if control is only temporary. By late spring this oxalis usually becomes dormant and disappears from view–until the following winter. I sometimes refer to it as Pacific Grove’s “winter wildflower”.