I think I am zone 6, but maybe 7. I'm in South-western Germany.
I was planning on planting all my Dahlias and other plants directly in the ground but my landlord decided to rip my front garden area out and replace it with stone. The last tenant just let the patch grow wild and never tended to it and he was tired of the eye sore. So I decided to plant everything in pots. Oh boy did I spend a fortune in pots, stones, soil, compost and fertilizer.
But it was worth it! I have 14 Dahlias(two smaller varieties sharing larger pots) and about 30 other pots of odds and ends- lavender, glads, snapdragons, stock flower, hollyhock, a peony, etc. It's a beautiful garden.
I fertilize every 3 weeks with a 24-10-16. The downside to living in a country where you don't speak the language...yet. This was pretty much the only one I could find that had anything higher than 4 for phosphorus. Chemicals are very regulated here and I am constantly watching for a more suitable fertilizer.
Since everything is potted, I will have to dig/store them or bring them inside in their pots. I understand that part of digging and storing is to divide tubers if needed and to keep them dry.

So here are my questions:
If kept potted and dry this winter, would dividing them next fall be sufficient to prevent "harm" to the 2015 spring blooms? Some didn't even bloom this year as we had a really cold/wet/long spring and I didn't want to harm them by planting too soon.

Since they are potted should I cut them down before the first frost? They don't have as much insulation as in-ground plants and I don't want the tubers to be harmed during the "curing" process.

I am hoping to cover my balcony in thick plastic so I can start them earlier this year, since our summer is sometimes so short.

I've [hopefully] attached a picture of my favorite bloom this year!

They do have "high P fertilizer",
Perhaps that helps,
Keep going with your great container garden,

Bye, Lin

I had a small window box with a short Dahlia sitting around for a while completely dry, around October, I meant to chuck it all into the bin. When finally getting around to emptying the window box I found a nice firm tuber, the dry spell seemed to have helped.

about dividing, not sure whether I understood you right:
did you plan to just keep them in the container and grow in the same pott for the next season?

You could probably store them in the pots, but I would always repott, making sure there are no critters or funny bugs eating away at your tubers.
And often parts of the tuber, the "old" original bulb, are spent and rot.

It would probably work to grow without dividing fot 2 years, but in general Dahlias tend to be more free flowering if planted as divisions, really old large clumps produce less flowers.

(BTW what kind of potting medium did you use?)

bye, Lin

They're in about 60% sifted potting soil, 10% sand, 10% peat and 20% horse poo and shavings. I am planning on retaining most the potting medium and adding more poo and keeping it in larger tubs in my basement as well, so that it's even more broken down and "ready" for potting next spring.

When trying to overwinter them in ground it is important to keep the clumps dry and insulated. I've heard of tar paper over the ground working, as well as tarps with leaf mulch on top. Typically the random success stories in the colder zones have the clumps near a foundation, which protects them, and provides drainage that was put in place for the building (ie, French drains, or graded ground away from foundation).

Personally, I wouldn't leave any cherished variety in the ground overwinter, but if you are okay with losses while you experiment, that's perfect.

Mine wrinkle a bit every year (I just checked, they're wrinkled a bit) and make it through fine--but most of my varieties are tough as nails to begin with.