Poison Sumac is not very common and it grows, according to everything I have ever read about it, only in very wet areas. Perhaps the link will help.

Here is a link that might be useful: Poison Sumac

I'd like to know if this garden weed is poisonous or not. Any help will be greatly appreciated.

Poisonpus in what way? Contact or by ingestion?
The picture is not clear enough to see what you have and lack of information about location would make it difficult at best to make a wild guess. Take a sample in a sealable plastic bag to your state universities local Cooperative Extension Service office where they will probalby be better able to help you.