We have slender spikerush problems in some of the resedential lagoons that I manage. From what you described, most of them are similar to yours in that they all have littoral shelves around the edges that extend out well into the pond...and they all have pickerelweed and duck-potatoe planted around the edges of them, so we are some-what limited in what chemicals we can spray. The best(long-term) control for us has been with grass carp. We do spray Diquat/Chelated copper mixes and they work well, but we can't spray too often or too close to the shelves or we will burn back some of the other plants. If you don't have grass carp, look into them....they will do wonders on slender spikerush mats like you have.
