Hi Alexmom,
You probably have your answer to your question, but just in case:
Dahlia, canna and begonia need to be planted after all danger of frost.* You can pot them up and start them indoors if you like.
http://www.dahlias.net/dahwebpg/Cult...ex/Culture.htm
http://www.extension.umn.edu/project...cannacare.html
http://www.cannas.net/faq.asp
I don't know which begonias you have, but here is tuberous begonia care.
http://www.extension.umn.edu/yardand...9begonias.html
The containers that the plants came in should have planting instructions.* Read number 4 here for handling bare root plants.
http://acreage.unl.edu/Newsletter/NLS/April2004.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.