I am growing tree dahlia (dahlia imperialis). They got up to 1 metre/3ft this year and have some thick rhizomes. I heard that they are hardy enough to leave in the ground (I have some raised beds that have great drainage). Anyone else tried leaving them out? Also year 2 - are they liable to flower supposing no frost till Dec?
Some people manage to over winter dahlia tubers others dont. The tree dahlias are no hardier than other dahlias.
It is rare for tree dahlias to flower in the UK unless we get a particularly hot summer.
The foliage is impressive by itself but would be nice to get some flowers and seed. From looking on google tree dahlis has flowered in London and the SE and in Cumbria, though rarely.