"Ludwig calls this technique "plant addition." To do it, he uses one-year-old willows that are thin and flexible but at least 10 meters (33 feet) long. Once the willows have matured to full strength, the strands will be able to support the eight-meter (26-foot) tower that Ludwig plans to begin building near Lake Constance in southern Germany at the end of July, as though they were steel beams."It's quite complicated so you really do need to read the original article but this is not all pie in the sky: the group has already built some structures such as the "diagonal support frame", above, for a bird watching station in Bavaria. Do check out their marvellous slideshow of arbo-architecture too.