How large were the tubers? Are they growing in shade? I start mine indoors under grow lights sometime in April...hardened them off and planted into the garden (or in larger pots on the deck) sometime in late May. Some of mine are at least two feet tall and blooming. Depending on the variety, you can have short growing dahlias too.

The clump of tubers were about 8 inches wide. They're supposed to get 2-3 feet tall. Mine are 3 inches tall now. They get sun almost all day. Shades in about 330 in the afternoon.

Are they ALL growing that slowly? There are quite a number of tubers in that bag (from Costco, correct?)...I bought a bag too this Spring. Did you plant them out separately? What kind of watering program do you have? I'm wondering if some of them didn't rot. You also have to make sure they have an "eye" because that is where the growing/flowering stem emerges (otherwise you'd have "blind" tubers that won't grow).

I have 2 in pots that are growing the same. The ones I put in the ground never came up at all. Yes I did buy at Costco. It said for my area April to June so I planted mid may as frost was still an issue. I don't over water so it shouldn't be rot. They had eyes. You could see where the stalk was. They looked like the picture on the bag. Maybe I should've started them indoors earlier.

My dahlias sat for months without growing, although they seemed to do okay in pots. I finally did a soil test and found my soil was very low in nitrogen. I gave them a bit of nitrogen and they started growing and blooming quickly. Makes sense, after all the potting mix had balanced fertilizer. And I always thought to avoid nitrogen. Looks like you are growing in potting mix though.