Go to the thread at the bottom to see how I and others built our latest islands. The flowers I used were petunias. So I know they worked with the soil being damp all of the time. If I had thought about it sooner I might have made a large hole in my new island so a basket of new flowers could be set in the hole each year. The foam and rug can be cut using a hot soldering iron. The hole would have to be cut just right so perhaps an inch or less of the basket would be in the water and the flowers would be self watering all summer.
