I think esshup has the right idea. I have planted eel grass, water lilies, etc. using this method. I put the plant and some topsoil in a bag that has been shortened to about 9", covered the soil and roots with sand or fine gravel, stapled the bag shut except where the plant sticks out. Then place it where it was wanted, even in 3' of water. For some, I doubled the bag to make it sturdier when wet. Seems to work.
I talked to two customers where we did the paper bag/garden soil/sand mix method and just tossed them out in the pond. We did that 2 and 4 years ago. Both reported that the plants are established now and spreading like they wanted.
On one pond the customer made PVC cages 4' wide, 8' long and 2' high. I went swimming, put the paper bags in one area, covered them with the PVC cage that had chicken wire on the outside (open bottom) to keep turtles from the plants. Repeated that in another area and we then scattered the rest of the plants in the paper bags around the pond. He said those 2 clumps are the densest areas of American Pondweed in the pond. Both do NOT have TGC in their ponds.
