Hi Bratty,
Dahlia are tender plants that will not survive your winter. You will need to dig up the swollen root system, called a tuber, in fall after the first killing frost and store in a cool place. You can replant it after all danger of frost next spring. These sites will tell you how to care for them.
http://www.dahlia.com/dahliatips.html
http://www.extension.umn.edu/distrib...re/DG1115.html
This site give good instructions on how to store the tubers if you decide to.
http://www.colostate.edu/Depts/CoopE...bs/dahltub.htm
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.