Yikes, so they look this bad after only about 25-30º or so? How accurate do you think PlantFiles' rating of 20º is? Would like to plant mine in the ground here in so. Louisiana, zone 9A / 8B. "Likes water and seems to thrive in clay soils" in your review is good news, though.
the plants in that photo look bad because they are dying after flowering... not due to cold.. but yes, plants get pretty badly injured by temps below 28F... one of my plants suffered severe foliar damage after 25F, but recovered. Seriously doubt one of these would survive 20F. So I think that is wishful thinking on whoever put that temp in the plant files.