Answer: Trachelium is grown as a perennial in some parts of the country such as California; it is only hardy to about 20 degrees. The strikingly dark purple foliage variety would probably be a named cultivar such as Trachelium caeruleum
'Hamer Pandora' perhaps, normally it has green foliage with a purple tinge. In some places it is grown as an annual particularly for the cutflower industry where they are sometimes grown in greenhouses. I tried to grow it in the midwest and in Pennsylvania but it did not tolerate the high humidity and hot summer -- just as it came into bloom with the first daylilies (it needs long days or 16 hours to begin blooming), it basically collapsed. This makes sense because it is originally from the Mediterranean. Some specialty seed houses offer seeds if you want to experiment, such as Chilterns in the UK.
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