I have the top species of oxalis, and I haven't seen the discoloration that the picture shows. I don't know if I would call my soil well drained, it's constantly moist. The oxalis seems to shoot roots down through 4 inches of substrate to the false bottom. They are right in the area where I feed the frogs, so they're constantly getting trampled from lunging amphibians, and they're still flowering like mad. I have a CF light fixture, and they seem to like it a lot. Perhaps yours aren't getting enough light?