I read an article recently about a man in Mexico that built his own island using plastic bottles in nets. And I have seen a few floating islands built by folks on this forum using foam noodles and such.
Here's what I came up with:
I took stainless steel wire and wired a bunch of bottles together, usually starting with a larger bottle in the center and securing smaller soda bottles around it. Then I tied them together and adding filler bottles where needed, followed by a squirt of Great Stuff expanding foam in the gaps between...
This made for a very stable mass. I cut out the bottoms of several bottles and took the caps off or drilled holes in them.
I planted Louisiana Bog Iris in three or four holes, and FrogFruit in seven or eight. I also planted a couple with wort, but I think it was in too rough a shape to survive. Then I piled our rather clayish topsoil on top of it (maybe 50 pounds) and used it to anchor the landscaping cloth around the sides.
this is a horribly blurry picture, but you'll get the idea.
Also taking from someone's idea here on PondBoss, I drilled holes through 1/2" PVC pipe and added a bunch of weedeater twine. This was installed on the anchor cable. It should float right up under the bottles.
Again sorry for the blurred picture. I'll go out today and get a picture of it in the pond...we got a nice rain last night, it'll be intersting to see how it survived.
