Recall something about Dahlia species... they are tropical montane, cloud-forest and/or alpine plants from Mexican mountains. They normally experience summer rain and dry winters. Summer daytime high temperatures are only rarely as high as 90 F / 32C, often much lower, and summer nights may be in the 40s F / 5-6C. Winter days and nights are almost exactly the same, but no rain. They are quite wet in the cool summer. If you travel in their Mexican habitats when there has been rain you will find them in ditches beside the road. They grow alongside plants such as Salvia elegans, Lobelia species, and Calceolaria species. If we can get Dahlia hybrid tubers in the fall they do fine here in Phoenix winters protected from frost, but they're a lot of trouble so I don't bother. They grow fantastically well in the display bed outside the Victorian conservatory in Golden Gate Park in San Francisco, California. I would expect the English, Scots and Irish grow them very well also. Leo Martin Phoenix Arizona USA