Hi Cristoph,
I keep mine under lights and they do best around 45 - 55 with 8 hours of
light. I find that they rot if they are kept in standing water at this time
of year, so I keep my plants on the dry side. To answer your question about
soil, I have found that they need a VERY sand mix, say about 60%, with the
rest being peat. P. ionantha and P. planifolia can be treated the same way
(And for those who think P. pumila is an annual, it isn't. same for them
too!) Feel free to contact me personally if you want more details.
Happy New Year and good growing to all.
-Tom- & Krissy