The spread of Oxalis that do not set seeds can be put to carelessness. With regard to O. pes-caprae, one could say that most of California has been horticulturally careless, even though this is a beautiful and unique species with a distinct floral fragrance. Contained in pots there should be no problems controlling their spread. I recommend an austere regimen of mineral soil (mostly sand), very little fertilizer and full sun. From my experience growing many dozens of accessions of Oxalis, I found there are two basic requirements for seed production: pollinators and multiple clones (or species that will cross). Most species seem to be self-incompatible. Oxalis that set seed are usually obvious because the flower stalk is firm and even a little taller after the flower wilts. Unfertilized flowers wither quickly. The young fruits can be pinched off before they distribute their seeds but it is easy to miss a few and end up with volunteer seedlings. This can be a nightmare but also can result in some very nice chance hybrids. If you are growing more than one form of, say O. obtusa, then it can be a challenge to keep them "pure" over time. Oxalis is an extraordinarily diverse genus with bulbous, tuberous and fibrous-rooted species. Off-hand I cannot think of another genus of geophytes that exhibits both bulbs and tubers. Some produce rosettes of beautiful foliage for many years before they offset (if they ever do) and never set seed. Others that make rice grain bulblets can be a real menace and should not be promoted. It might be worthwhile to designate on the Wiki those species that are known to be weedy. Dylan