I put a D. venusta outdoors a few years ago, and it seemed to pretty much hate it. It stays out year-round, and in the winter it always struggles. I thought it died the first year, actually, but it managed to flower first and spread some seed around. Of the surviving seedlings, cold-weather tolerance has greatly increased. Here is what it looks like today (I haven't changed the peat, as the excessive grasses and weeds show):
As long as I was out there, I figured I'd snap a pic of my awakening D. dichotoma "giant"