Phred,
Thanks for the information. I've been trying to work on making my mixes more airy. The summertime here is brutal, on a par with Charleston, SC, but luckily most Trilliums are dormant by then, otherwise I'm not sure they could survive.
As for the P. rivale, I wonder how cold it can handle even when dormant. I'm guessing it should be OK to the warmer end of "zone 7", but I think it would be marginal in zone 6. I base that on it's limited distribution in southwestern Oregon and bordering counties in California. Probably best to keep it unfrozen in winter, or perhaps under a heavy layer of mulch. Thoughts?