Here is another idea. "Real" garden designers seem to recommend a narrow path along the back of a flower bed, so you can get in there and reach in to weed. I did that in one garden and it really does help. From the front you don't even notice it. So maybe you could put in a path about 18" wide along the fence? Then you could use a weed eater or something if any grass is encroaching from the retention pond area. Right now there is just bare earth where the fence was placed, this will change rapidly!
I put a concrete "mowing strip" between my lawn and flower beds, this makes it easy to run an edger along it, and grass is shallow rooted, does not sneak underneath. You would not want to use chemicals like Roundup right by a pond, and a flame weeder would catch the fence on fire, so weeding in there is going to be a real problem without some kind of mechanical barrier. A gravel path can be heartbreaking to weed, especially if you put "landscape cloth" under it as is sometimes recommended. The weeds sprout anyway, and the roots grow into the mesh, so you cannot pull them up cleanly.
