This is a remarkably futile exercise, in our climate. Perhaps where they live naturally, in Mexico, the seasons are longer, the frosts less damaging to plants.
But still I like them. They are well-named, for my young plant, only in its second year, has twice produced tall growths, well over 3 metres high, then thrown some sprays of flowers. And when walking down my street the other day, I saw some towering over a neighbours shed, probably arching some 5 metres into the air. And yet, like any normal Dahlia, they are a true perennial, the annual growth dying off, leaving only the tubers, from which new growth emerges in Spring.
Of course, in the case of the Tree Dahlia, the tubers are massive. The clump from which my plants came was a division, with long tubers 45 Cm or so in length, and a total root mass which was more than the previous owner could lift, without first dividing it with an axe.
It is interesting to think that this tall plant could, overnight, be reduced to a soft brown slimy mass of pulp, after our first serious frost. And yet, from that mess, will emerge next year, even more spires of these tall flowers, in an endless cycle of trying to outlast the season, to make enough growth to flower, before the inevitable result of being cut back to nothingness. As I said at the beginning,Tree Dahlias are the most optimistic of plants.