Maine et Loire, France

Dahlia imperialis... I've been growing this for three years now and last year's plants came back, especially the ones which I overwintered in pots. These I planted out and they are at least 20 feet tall! The foliage is great but I really want the flowers. I have read here that some people HAVE successfully got them into flower and I wondered if there is any cultural technique that could be used to force them to flower in October or November before the frosts? Are my chances likely to improve if I keep them from year to year?

• 10 Oct, 2012