Planting dahlias in a garden with a plastic liner

I'm wondering if anyone has done this before and has had any success. I am setting up a dahlia garden 8 x 16 in a spot which used to have grass. I am digging out the grass and going down about two feet. In order to retain water I was going to line the bottom with plastic, cut holes for drainage and then fill up the area with dirt. My two concerns are, is 2 feet deep enough for a dahlia and even with holes in the bottom do I run the risk of having the dahlia tubers rot?
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I'd go a lot deeper, those dahlia tubers can get good sized and if the drainage isn't real good, they'll rot real good.
http://groups.yahoo.com/group/dahlia-net
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No and yes. Why do you need to line the area at all? Simply add some good quality soil to the planting area to add fertility and moisture retention and plant them directly in the ground.
pam - gardengal
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