Oxalis pes-capre

Sour grass or Bermuda Buttercup

Oxalis per caprae is a native of South Africa that escaped the garden and is now a common weed in home gardens, pastures and public lands throughout California. including Portola Valley.

It has yellow flowers and clover-like leaves.  It spreads by seed and through the formation of little bulb-lets deep in the soil, which will break off even when you try to get all the roots.  Its dense foliage effectively crowds out any native seedlings that try to emerge and once it is established it can be very hard to remove.

Small scale infestations should be taken care of by hand or through “hula-hoeing”, although it will take a few years to get them all.

https://youtu.be/qnb31wpnD1Q?si=QUGfWsUfEzbL8nMJ

Remove as much of the plant as possible as the flower stalks are beginning to emerge, the bulb-lets that are left behind become depleted of energy and are unable to regrow as robustly the following year.  Over time this starves the bulbs of energy and they will die.

Large-scale infestations are very difficult to manage.  The oxalates in Oxalis can be poisonous to livestock so grazing isn’t an option. Soil solarization or sheet-mulching may provide some control but in practice, chemical control may be required.

For more information, see the following:

https://wric.ucdavis.edu/information/natural%20areas/wr_O/Oxalis.pdf

http://ipm.ucanr.edu/PMG/PESTNOTES/pn7444.html