At this time every year, I embark on a crusade to win new converts to the world of oxalis. The whole oxalis family suffers from the bad habits of just two or three members and it means that many people miss out on the seasonal delights of some of the highly ornamental forms. Purpurea alba is one of the very best. It is not in the least bit invasive and I have no problem at all in recommending it for sunny spots in the garden where it forms a flat mat of slightly hairy, clover-like foliage topped with big white flowers with a yellow throat. Where it excels above most others is in the length of its flowering season. It is one of the first to flower and continues through to pretty much the end of the season in winter.
Oxalis purpurea is a highly variable species. The red leafed form with big pink flowers comes into growth much later and is invasive. Decorative but dangerous so keep it confined to a pot. The green leafed form with big pink flowers shares a long flowering season almost on a par with alba and does not appear to be invasive. There are apparently yellow forms of this species too. Overall, there are large numbers of different oxalis. They occur in both South America and South Africa but it is the African ones which give us most of the showy varieties for garden use. Most come into growth with the autumn rains and they have to be planted in full sun because they don’t open their flowers unless it is sunny. They make a wonderful show in shallow containers on a sunny doorstep and you can always refer to them euphemistically as wood sorrel, if you don’t want to own up to growing oxalis.