I knew that this plant, known as vinagrella in Mallorquin, was not a native but an introduction from South Africa, but I did not realise that in California and Australia it is regarded as one of the most invasive and problematic weeds. I was also labouring under the illusion that it was a nitrogen-fixing legume, which it is not. In Majorca it is often seen forming a dense covering under citrus trees, and seems to be encouraged by autumn ploughing. It grows on my lowest terrace, and appears wherever the ground has been disturbed, particularly in the damper, shadier spots.It is regarded by some as a good thing in orchards, presumably because it competes with invasive grasses. Now that I realise it's not a legume, clover would be preferable, but probably very difficult to establish once Oxalis is there. According to this paper, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2802929/ , based on field research in Menorca, Oxalis pes-caprae can increase the availability of phosphorous, which certainly sounds like a good thing. It also appears to be a poor competitor of rye grass. So, in the case of my land, these are the conclusions:
1. I probably can't get rid of it from where it is already established, except by solarising or sheet mulching, so just accept it where it is growing for the moment.
2. Don't encourage it to spread to other areas, rather work on establishment of leguminous ground covers such as Medicago truncatula (already present in places) and white clover.
3. Don't do autumn ploughing. Actually, I don't do any ploughing anyway and don't intend to.
