Growing full sized dahlias in pots is challenging in most locations. I believe that pots stay too warm in most climates. There are advantages in using all potting soil rather than mixing in some garden soil. The biggest issue is pathogens in your garden soil. Also, what type of soil you have is an issue. I know of a dahlia grower that has sandy clay soil and it makes excellent potting soil without any additions. When I grew some dahlias in 10 gallon pots, I used all potting soil. I also planted a rooted cutting that was about 12 inches tall in the pots. The plants grew very well and had 2 blooms, then 4. After the four blooms they did not do well. The variety was Cornel, the red ball dahlia. I no longer grow dahlias in pots except for my pot tubers that are in 4 inch pots.

I tried growing three plants in large pots this year, and am about to put them in the ground because the flowers have shrunk. It is just too hot. Flowers grown in smaller pots were fine last year but only because of the cold and rain.

See my comments here: http://forums.gardenweb.com/forums/load/dahlia/msg0716092527347.html?1

I agree with Teddahlia. If the roots get too hot there is a problem. Hence one can either double pot or use very large styrofoam pots.

Interesting feedback. I never thought of heat. So what about plants marketed as "container dahlias"? Are these more likely to be successful because of heat tolerance or do they get that name solely based on size?

And I'm using Miracle Gro garden soil. I don't know if they cook the soil to remove pathogens or what.

"So what about plants marketed as "container dahlias"? Are these more likely to be successful because of heat tolerance or do they get that name solely based on size? "

Yes, smaller plants have smaller root systems. It may be a function of both heat and water with larger varieties in pots. They require an awful lot of water in a pot. Probably because warmer soil allows more evaporation. A partially buried pot seems to work really well; even smaller pots can be used.

I got a Neon Splendor from SI this year, it is in a clay pot and is spectacular!

I also bought Fire Magic (though they apparently delivered Dark Magic instead), it is in the ground and growing slowly, albeit with 2-3 beautiful blooms so far.

I have found advice from SI to be right on. They admit (as do most growers) that it is harder to grow dahlias in containers than the ground and I have found that to be true as well. One of my PAST mistakes though was underwatering. I had read so much about not watering dahlias after planting and having lost several that rotted anyway, that I held water back sparingly. I re-read their catalog this summer and it actually says you can't overwater dahlias (I'd heard that about roses years ago) once they start sprouting and blooming. So now I soak mine to the bottom every 3-4 days instead of the light 'washing' I used to give them every 2 days and they are loving it!

As for what soil I used in the containers? Lord only knows. Parts potting soil, part planting mix and part sand (I live in the dunes of S.F). The sand keeps them draining easily, the potting mix seems to be light enough for them to push through and the planting mix seems to keep it all together.

But other containers of mine are ALL planting mix or all planting soil (exactly what you are NOT supposed to put in pots) and they all are ablaze in blooms this year! In my opinion, the soil won't matter much once you start feeding them the right stuff (little to no nitrogen) and include blooming fertilizer that strengthens the stems and allows for blooms to do their thing!