hi, 

Dahlia are not as picky about depth like peonies are. If you want to try to overwinter them, try planting them even a little deeper, in the 3-5 inch depth. You could also go a little more shallow, like you said but then cover with compost and leaves  (or mulch) before winter and possibly a row cover or upside down pot (or frost blanket) to help insulate them. 

More so you want to make sure they are in well drained soil as if it is too wet or consists of too much clay, they can rot. 

Let us know if you have more questions with this or other gardening inquiries!

Emily 

You probably don't want to use the plastic since it will cause the soil to not breath and also block water all together. Using something like mulch or compost to insulate will allow them to breath and get any rain water that they may need so they don't dry out. 

In late March you could pull back some of the mulch to help them not be as deep and push though dormancy. 



you're welcome!