Wetland Plants of Minnesota: A complete guide to the wetland and aquatic plants of the North Star State
, by Steve Chadde, would probably be an obvious choice. I haven’t read it but I have been impressed by Steve Chadde’s work.
Another thing to consider is posting pictures, with as much detail about place and conditions to site such as this and I am sure there are sites with people in Minnesota who are knowledgeable and enthusiastic about their states wetlands.
Local groups can also provide guidance as to the legalities and propriety of collection.
Collecting, maintaining and propagating locally collected plants is (for me anyway) an exciting part of the hobby.
I always encourage people to understand and obey the laws. Compliance and gaining permission is usually not difficult.
Generally when it is difficult there is a reason and many times “they” only wish to make sure you are serious.
That is among the reasons I suggest following all the rules (even the stupid ones
), document your efforts, trust me when there is a close call on an issue being able to demonstrate your serious intent can make all the difference.