If you mean dahlia tubers, yes, unless you're going to keep them indoors or in a greenhouse as plants all winter. If you mean dahlia plants, you can put them in now, but they'll need lifting in autumn if you want to keep them over winter.

if you have tubers that you didn't plant then it is better to plant them and look after them properly. or as Bamboo says you can by them as pot grown plants now.

My Dahlias have started to flower and they are huge. I agree if you are buying new ones fair enough, if you are just planting them, don't expect too much this year.

Yes, too late to start dahlia tubers. Mine grow so slowly. I planted them in May and just now starting to flower. Plant dahlias indoors? Would they do well? Mine like the outdoors, full sun, wind, rain. They are loaded with flower buds. As indoor plants, you'll have to worry about spider mites, downy mildew, chlorosis (yellowing of the leaves) and you might get a flower or two. They prefer to be outdoors.

I agree with Seaburngirl, if you forgot to plant them you might as well put them in - rather that than just let them die. I have some in pots which I forgot about (covered in weeds and decimated by slugs) but they will go in and hopefully will make up enough to store over winter.

They put on a spectacular show - covered in blooms all summer and make a nice indoor arrangement.