I did a google with the term:* Hydrangea 'Sweet Dreams' and it appears this is a lace cap type and a florist hydrangea.* You can see it listed here.
Most florist hydrangeas are grown as greenhouse plants and generally are treated as indoor plants and/or a throw away after bloom.* Here's more info on florist hydrangeas.
There are many hydrangeas that will survive in your zone.* Here's their main site with loads of info on hydrangeas.
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.