According to the book
Grey Wolf, Grey Sea (E.B. Gasaway), the crew of U-124 decided to adopt the Edelweiss as their insignia because the German troops in Norway who helped rescue them from the sinking of their first sub (U-64, I think) had it as their insignia. It is a type of mountain flower which grew in the region, apparently.
When Schultz (U-124's first commander) stopped to help the crew of the Lake Ship SS Trevisa, her captain used the Edelweiss insignia to locate Schultz to meet with and thank him after the war.
EDIT - Didn't see that last post Ducimus. Sorry.
