Well, first of, just to make it clear, I am NOT worried about any plants out in the wild, I just think it's really important to distinguish the difference between a plant on your back porch in a pot, and one 100 yards away in a marsh...
The marsh is a bigger system than the pot, therefor it will by it's very nature not freeze as fast as a potted plant, and most likely will not endure temperatures as LOW as a potted plant, which above ground is subjected to winds and elements that hundreds of tons of surrounding soil, water, and plants protect, that a native plant does NOT have to endure as hard...
It may seem strange... but I think there IS a difference...