I would go for the attic, 35-45 degrees being ideal for storage.

Thanks Poochella. I have the boxes lined up on my kitchen counter. They are on their way up to the attic.

I'm having the same problem with my canna rhizomes, but my dahlia tubers always do well in a cheap little dormitory refrigerator set at 45 degrees.

That's about the same situation I have. They do well in the attic. I just have to check them earlier in the spring since the attic starts to warm up then.

ah.....but cannas don't like it quite as cold. Try to keep them about 50 degrees, as if in the cold, they often don't survive. Practical experience speaking here.

Move SOUTH !, no problems storing tender bulbs down here. Only problem is, tulips and daffodils need to be chilled in the refrigerator in order to bloom.

Raul, do I remember correctly that you are at some altitude and can indeed have cooler temps in winter? OR does it stay enviably warm in which case, my curious side wants to know at what temperature and how you store your dahlia tubers?

Temps can drop but very seldomly below 32°F, even in the coldest days of "winter" the temp stays above 40°, winter here lasts ( in the worst case ) about a month, from the second week of Dec to the second week of Jan.

Never store my dahlia tubers, they are left in the ground, I only lift them when I want to divide. Dahlias are native to Mexico specifically in this area called "El Altiplano" ( the literal translation would be the flat highlands, an area that covers parts of the states of Queretaro, Hidalgo, Guanajuato, Michoacan and Jalisco ), they used to grow wild along roads and in abandoned farmlands.

Raul,
Do you see any still growing wild. Dahlia in captivity seem to like a lot of water and food I assuming the native ones are much smaller. Are cosmos native to Mexico or SW US? It would be hard to imagine them growing side by side.

There are several types of wild dahlias, from small ones not bigger than a Sweet William to those towering monsters 9 ft tall. Unfortunately you don´t see them oftenly now growing wild, wild growing dahlias are a memory of my youth when I saw them growing along the road sides and in places where cattle used to roam right here in my grandfather´s ranch and I´m not THAT old. The reason why they like lots of water is because they grow during the rainy season from June to September, that´s when the Altiplano gets most of it´s rainfall. The way you can differentiate a wild one from a cultivated one is by the flower type, wild dahlias are single petal blooms with simple colors and are very easy to propagate by seeds, bees love them because they produce huge ammounts of polen.