I'm not sure why the North American Sennas are not better known in the UK. I know two of them reasonably well - hebecarpa and marilandica (both sometimes included in Cassia.) The former seems to be the better of the two. They're big exotic looking plants but completely hardy given their distribution as far north as the Great Lakes.
The foliage is lush and the flowers give a good show in the latter part of the summer.
Ideal for prairie type plantings with other native Americans (so to speak) they grow well in moist rich soils in full sun but should tolerate some shade and drought.
3L pots ~ £10