Is a branch on a bonsai failing? Getting weak? Feel it’s essential and don’t wish to lose it?
Well, you MAY have come to the right place. I say “may” as we’ve only tried this technique once in our yard, but do have other data points that support its use. David DeGroot has used this technique and thinks it helped a branch on another juniper (his story later in this post). Also, I’ve seen the technique used in Japan, as has David.
With that background…
We had an Itoigawa juniper with a weak top. A bunch of branches had died up there and only the tippy top had a few living poofs remaining. The top of the trunk line was, frankly, one of the few things of value about this tree, so we tried this trick.
Taking a strip of terry cloth, about 5 years back we wrapped the weak upper trunk. Every time we watered the plant this cloth was soaked as well.
Dave’s story below details why this is done. But first, more embarrassing details of our own experiment.
Completely negating our experiment, we also trimmed the extensions and thinned the bottom branches for the last several years in hopes of giving the top additional advantage (in the above photo it’s why the lower branches are thin). It’s possible that’s all we needed to do. But, given that it took 5 years before we got any extension on top, it may have needed all the help we gave it, including the cloth wrap.
This week we “wrapped up” this non-experiment by taking the wrap off. Now no one—excepting all of you—will know we had it wrapped up like a dog on an arctic walk with only its nose sticking out. 
The terry cloth snake after 5 years, complete with algae and moss.
Far more solid evidence than what I’ve offered here is this wonderful story supplied by Dave DeGroot, which he gave permission to share with you:
I salvaged a Hollywood Juniper at the Museum after it responded badly to an over-zealous root pruning to reposition it in its container. I desperately tried to think of what to do, and remembered two things:
One was from an old Bonsai Sekai magazine article about creating spiral shari on a trunk. In order to sustain the remaining live phloem tissue, the article recommended laying strands from a string-type floor mop along the fresh edges of the live vein, with one end in a cup of water, to help keep the live vein hydrated.
Secondly, I had recently read in a botany text (I took the Weyerhaeuser botany course for foresters) that woody plants do absorb some moisture through their bark.
With this information, it occurred to me that I could wrap the trunk and primary branches of the tree with a material that would hold water. I immediately went to a drug store and bought a package of disposable diapers that I cut into strips, dunked in water, and wrapped around the trunk and branches. I was shocked the next morning to find a large amount of a clear, jelly-like substance oozing from the diapers and accumulated at the base of the tree. I realized the diapers had a silica gel that had been added to increase absorbency. I switched the wrappings to strips of terry cloth towel.
After one year, I removed the wrappings, and within a day or so the foliage began to wilt and lose color. I immediately rewrapped it, and after another year I again removed the wrappings and found the tree had stabilized. I was very proud of myself, thinking I had discovered a new technique, but on my very next study visit with Mr. Mitsuya at Tokai-en, I saw that he had wrapped a weak branch on a large beech with cloth that he dampened with each watering. While I was happy for the confirmation of the technique, I was disappointed that I could not claim rights as its sole inventor.
-David DeGroot