In researching this subject, I recalled the fury of a fellow Master Gardener on this subject and ran across a few articles that echoed this sentiment.  Phrases like “fast-growing”, “aggressive”, “menace”, and “winning the war” were used to describe the struggle to control this pervasive weed.

Oxalis, also known as sour grass, wood sorrel, and buttercup oxalis or Bermuda buttercup, is a perennial weedy groundcover.  It looks a bit like clover and the type that tends to grow in warm coastal areas sports long stems with yellow flowers. I have noticed that it pops up quickly in a fallow winter garden bed.  Where does it come from?  Hang on to your trowel:

• Seeds-tiny seed pods that resemble okra
• Stem fragments
• Bulbils
• Rhizomes-a continuously growing horizontal underground stem that puts out lateral shoots and adventitious roots at intervals

The bad news is that oxalis can bloom and set seed at any time of the year. Here near the water in Vallejo, it seems to pop out in late winter to early spring. Another piece of bad news is that “control” is more likely and eradication less likely due to the extensive ways this plant reproduces.

To control oxalis outbreaks in the lawn it is important to keep the lawn healthy and thick so that the sprouting seeds are unable to germinate due to lack of light and soil.  If desired, herbicides exist to kill oxalis, but most gardeners are mindful of the negative effect of herbicides on pollinators and possibly on human health so they prefer not to employ this method.  Applying chelated iron is said to be one way to kill oxalis growing in a lawn, which won't hurt the grass. 

In flowerbeds digging up the entire plant is the best way to prevent new growth.  This is presented as a very labor-intensive and time-consuming method. It is difficult to get all the fragments of the plant out of the soil.  For a measure of control, using at least two inches of mulch is recommended.  Using mulch will starve the seeds of sunlight so they can't germinate.  Covering with black plastic (solarizing) might work, but isn't always practical or good for the soil.  Picking the tops of the oxalis can be satisfying, but is hardly worth the trouble because as mentioned, any pieces of stem or rhizomes will just regenerate. 

Other amusing suggestions I noted during research for controlling oxalis

Chickens scratch up and eat bulbils. Maybe.  But not everyone has chickens and what a mess that would make. 

Eat it! I remember eating “sour grass” as a kid. Another suggestion I ran across was to make pesto or add to salad. (There is low toxicity if eaten by humans but I don't recommend eating weeds from your garden unless you are sure they are safe, and that they haven't been sprayed with pesticides). There seems to be a debate about toxicity to pets, so it seems prudent to prevent your dogs and cats from eating oxalis.

And my favorite: tolerate it, give in, let it die back until next spring, and spend your time in the garden on things that will be more satisfying.