Are they cuttings or mature plants? The mature plants will have quite large root systems (tubers) so they will require lots of room so they will be best in the ground. Any of this year's cuttings or more mature plants with the least amount of root systems will be OK in large pots. Any moisture retaining composts will do. Regular feeding of the containers will be needed as constant watering will quickly wash any remaining nutrients away. Don't forget to dead head regularly to keep them flowering.

They are tubers bought on-line last autumn which I have potted up. I think they would be happier in the ground so will have to make some tough decisions......

I've tried dahlias in pots. Never again for the exact reasons Jimmy stated. Constant watering & regular feeding. Who wants to do THAT all summer long? Not me! I'm going to the beach! I'm going fishing! I have summer barbecues. I have better things to do. They just don't last as long anyway, then the constant deadheading.

I have a neighbour who has been growing them in large pots successfully for a number of years but I don't know how much care she gives them.. When I tried the snails got there before I could really judge...

I always have some in large pots, ones that I use for toms [3litre- 4litre]. I use multipurpose with slow release fertilizer and they get watered when I remember ie they are starting to wilt oops! or they just get what the weather throws at them and last year that was a lot of rain.

if a space opens in the border I sink the pot and dahlia, makes it easier to lift it in the autumn.

the secret is to dead head regularly.

I have grown mine in pots for years. I change the compost every March and I don't over feed them. I have lots of plants in pots so they are watered with the other plants. I do have to put supports in as they get tall and I have them in large pots. I live in Scotland so I have never considered putting them in the ground.