Hi there,
this could help. I garden in a warmer climate (Z 7, Switzerland), about mid May + 2 weeks is the time to plant tubers around here.

I usually pot up some tubers to give them a head start, helps with slug attacks and really gives earlier flowers. I just use a window sill and a balcony and start them 6 to 4 weeks earlier, moving them outside on sunny days.
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having said that: I reckon it`s most important that the soil is warm enough and not with stagnant water. I got some tubers last years, not sprouted or anything around June. After planting they really took off. A neighbor at the garden planted huge clumps at the same time without dividing them, and they started much later, well go figure.

Perhaps you should look out for early flowering varieties as well.

Have a great Dahlia season, bye, Lin

I start most of my dahlias in pots. I start them in march in 4 inch pots. When they get to big I move them into larger pots. This year I will start over a hundred tubers in the poly house in march. Some will stay in pots all season long. Due to the bad slug problem around here I will never again put tubers direct into the ground. They ( slugs) destroyed over twenty tubers over night in the ground.

I also start all of mine in pots to get a jump on the season. I dont plant them out until the end of May, so I usually pot them up mid to late March. It is still usually the end of July before I start getting regular blooms, but they are worth waiting for.

I start mine in March also. I checked back with my post here and found I had my first dinner plate dahlia on July 3 this year. This is a new record for me due to an early spring. We tend to be a bit warmer than mandolls area.

Thanks all ... so if you start them in March, besides in a poly house ... where do you have them growing? Though now that I think about it, I do have a poly house I could use ... but we're much colder than LINY ...

I have a light set up in the house. I have zinnias started from seeds growing under the lights now. Just to keep my hands in the soil till summer comes.

I have a space that used to be an attached garage , that I converted to a heated studio/workroom. I have shelves with fluorescent lights, though I have had pretty good luck starting the dahlias on southern window sills. Even in March a poly house wouldnt be warm enough where I am.

Like Mr. Viking I have seeds started now of various things to help my psyche get through the winter. It helps a lot.

I am also going to start tubers in cowpots this year. That way I can start with a 3 inch pot and than place it in a 5 inch pot without harming roots.
What I want to know is what do you put in big planters to take up some of the room in the bottem? I don't want to be shoving dirt into ths huge pots?
Viking, Are you sure you don't have voles if the tubers were eaten? The slugs stay on top of the ground.

I used a couple of large planters last year for annuals, and filled the bottom third of them with styrofoam peanuts that I had on hand. It lightened the load considerably. I think I read about that somewhere on GW.

The tubers were not eaten they rotted after the shoots were eaten off. I guess I should have explained better. One of the tubers was in a planter over three feet off the ground. I had sprayed at 11 o'clock at night. When I got up in the morning the shoot was gone.

Thanks all ... seems that you are all starting them inside a warm area, something I'll have to think about.