Pruning Tree Dahlias

I have been out pruning the Tree Dahlia. This is something I do about two or three times, as it is growing. I just take the tip out (you can see it best on the righthand side), and it branches out. Take it out again, and again. It doubles the number of flowers each time, and makes sure the plant is not just long, spindly, and blown over in the first wind. They make a lovely, dense tree canopy.

Trouble is, Tree Dahlias coming out in flower at my place mean the first frost is due. It usually kills them off just as they come out.