I have several Hybrid tree dahlias growing well here in South Wales.
The first few have just started to flower' see pics in
http://s240.photobucket.com/albums/ff82/taffy4u2/
Some are up to around 8ft at present.
The oldest has been in the ground for 3 years now, othe largest blooms
are around 9 inches wide. they flower later than the other dahlias but
I have flowering going on till just before Xmas, and that is outside.
They take light frost OK.
If anyone has seed of any tree dahlias they would like to swap
David Hill
Abacus Nurseries