Cistanthe tweedyi ( syns. Lewisia tweedyi, Calandrinia tweedyi, Lewisiopsis tweedyi ) sure has been renamed often through the years. I have my plants planted were they get light high shade in the heat of the day and twice a week irrigation. The substrate is a very well drained six inches deep layer of decomposed granite. The deeper layer is a clay/loam that is low in humus. They seem to do well for me but I have very low humidity and cool evening tempatures in the summer.
http://science.halleyhosting.com/nature/basin/5petal/purslane/lewisia/tw...
http://plants.usda.gov/java/profile?symbol=CITW2
http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242415723
http://biology.burke.washington.edu/herbarium/imagecollection.php?ID=3551
