Anyone growing this S. African native? I'm having a hard time keeping it looking good. None have died, but most of the plant's foliage is getting rather yellow. I have some in very fast draining soil ( almost pure gravel) and some in regular garden soil. The ones in the fast draining soil are allowed to go quite dry before between waterings. The ones in garden soil are getting watered regularly. They both look about the same. Any clues?