Hi Uli , Thanks for the introduction. I am very fond of "Tree Dahlias" which to me means the single pink form of Dahlia imperialis which is grown commonly in gardens where I live. It grows to about 3 metres and flowers in early winter before heavy frost and tends to be quite an open leggy plant. I also grow a semi double white form which is not so tall - about 2 metres and a much more dense clump - to my eye a much better garden plant . Unfortunately it flowers several weeks later than the single pink and therefore is not often enjoyed. This year our winter was quite mild until June 21 (our shortest day) and I had a nice display of flowers almost ready to photograph. We then had -2C and -1C two nights in a row . Next weekend I cut them all down (saving plenty of cuttings.) I grow them in full sun and wind in our new garden hoping that in years to come as trees and shrubs grow they will get more protection from frost. I also grow Dahlia coccinea var palmeri thanks to Uli and find it flowers almost continuously from summer to frost. For me thus far it has only reached 1 metre tall and is also a wonderful garden plant. I have raised 4 seedlings of Dahlia merkii this year so should be able to share my experiences with this next summer. Cheers, Rob in Tasmania Zone 9