If you have driven around California, especially Northern California, you have seen the “wild” mustard (Sinapis arvensis). That is a weed too. Formerly, it was the biggest villain on open fields and gardens. No, we have not cut back on the crop, besides places where the land was just built or paved over, but a new, WORSE enemy has moved in to the neighborhood.
Pictured above, is a quaint example of Oxalis (corniculata, pes-caprae) with several common names such as Woodsorrel, Buttercup Woodsorrel, etc. It is a really cool looking plant (and probably is, in its native environment), but in California, look as it might, it has used its aggressive reproduction techniques to spread wildly, and is quite invasive, choking the land, disallowing natives from growing.
In the garden, it has the same effect, disallowing not only natives, but your ground and potted plants.
Dealing with Oxalis
You have to get to it! Find those clover like leaves, and get rid of these plants, hopefully before they flower. They are sneaky, go from seed to flower to seed to germination at what, to the untrained eye would be the blink of an eye.
Pull them out by hand, and pull them by the root. If it has just rained or if you have soaked your garden well, it will be wet enough to pull. If you leave the roots behind, fully expect the determined weed to come back (some particularly disturbing species have bulbs).
I do not use pesticides and do not recommend it. It will do more harm than good. Pulling by hand makes sure you only remove what you ought to, and find other germinating surprises (good and bad!), and maybe, a good workout!
If you reuse soil that has had Oxalis infestation, be sure to remove any young plants, bulbs, roots, etc. completely.
I will post more such updates on weeds. If you have your own weeding tricks, you are welcome to enlighten me and other readers here!