By Bowtow
United Kingdom 
the tubers i purchased from dobbies i have found out since the last post they are treated by the company they buy from so that is not possible to take seeds.
16 Mar, 2020
Answers
The tubers don't produce the seed anyway Bowtow. If the flowers are open to insects then you may get seeds in the late summer.
16 Mar, 2020
But as Bathgate says, we usually split the tubers anyway, or take root cuttings in late spring. So seed wouldn't be necessary. Much quicker to get good results by root cuttings Bowtow. If you google it there will be loads of videos showing you how to do it. Just google 'Dahlia Root Cuttings'
16 Mar, 2020
That's really not an issue at all. Dahlias most successfully reproduce by division of the underground tubers. Why are they treated? So that the energy is diverted to tuber production, not setting seed. There is nothing wrong with your tubers. Plant them according to instructions and enjoy.
16 Mar, 2020