Last year I was having trouble getting the larger trees to fuse. Several twist ties had snapped and were no longer holding the trees together. A few trees were now approaching an inch in diameter and twist ties were just not strong enough to hold them together. So I tried short brass screws and they worked great. In the pic above you can see that the large tree (right center) fused nicely to the one on it’s left after being screwed together. The only problem now is that because I counter sank the screw and the tree healed over the wound I can not find the screw. I am afraid that when I remove the excess whip down to the trunk I will find the screw and damage my pruners.
Farther down the tree you can see I used several screws, 8 deck screws plus 4 more brass screws from last year. There were several areas that needed to be locked down with pressure to force them to fuse. No pressure, no fusion. Note that the bottom screw actually goes through 4 trees.
My friend Shane Martin ( http://shanemartinbonsai.wordpress.com/2013/01/) used a brad gun to shoot nails into half inch diameter seedlings to attach them to a wooden frame. He said it works great. I will follow that project closely.
There are many ways to attach seedlings together. The important thing is to apply constant pressure. Thin seedlings are easily held together with wire, but as the seedlings get larger they often snap the wire releasing the pressure. If that happens you need to get creative.