>Is there an easy way to tell between P. lutea, pumilla, and ionantha (and the
>others) without flowers?
First, you have obviously seen flowers on some plants, so you should try to
identify these first (BTW: white flowered specimens of _P.lutea_ do
exist!). Do you have Don SCHNELL's book? In it you will find fine
pictures/descriptions which in most cases will allow correct
identification.
Without flowers you might run into considerable trouble because you do not
know if small plants are really _P.pumila_ or just seedlings of one of the
remaining species. There are slight differences in colour but these are of
help only for the rather experienced field worker (sometimes these colours
depend on environmental factors). My tip would be counting chromosomes
(_P.lutea_ has 2n=32, the two other 2n=22), and waiting for flowers. 8-)
In some cases attempts at identification without flowers are rather
hopeless, I fear.
Kind regards
Jan