At one end of my 5 acre lake we have a 4ft deep section that might make up 3/4 of an acre. It is consistently 4ft deep throughout this section. I am trying to determine if I could plant Lily pads and the like without having them spread and become a nuisance. My idea is to lay out a 20x30 ft tarp and use cinder blocks to hold the edges and have pea-gravel everywhere else. In the middle I would like to cut a 7-8 ft circle and have these plants grow up through the hole but be limited to this area only. I think this will work in the short term and provide great habitat but I am afraid after a few years they might spread. Is my concern valid? Will they grow and survive in the pea-gravel without any soil in it? This tarp would be 20 feet from the shore on both sides so silt shouldn't be a factor for some time. If this doesn't work will another plant be less invasive but still give us some natural cover? Any advice would be appreciated.