It is inevitable that at some point in time you are going to remove a branch on a bonsai tree. Removing it will leave a scar. This post is not about how to make cuts which heal over as best as possible but you can read these previous posts which touch on the topic.
But what do you do if you have a cut which has never healed over, for some or other reason. After all the goal is for the wound to callous over completely and so that after a few years you cannot see a cut was ever made there to begin with. Now we are getting to the purpose for this post.
Image caption. Field grown Hackberry, shohin size, 2011
Image caption. Five years later and the scar is still not healed over.
The Hackberry depicted in the photograph was one of many which I have field grown (more on that here) and have lifted at some stage to begin its development. You can easily see that the large scar resulted from the cut I made when I reduced the trunk the first time, to change the trunk line. I believe the reason it never healed over is that I did not seal it properly and the subsequent growth was not allowed to develop to such an extent that the growth covered the cut.
However all's not lost and we can still with great ease remedy the problem.
What we need to do is quite simply create a surface over which the callous can easily form. Sometimes this is prevented by rotting wood, if the wound is too deep or some other reason.
What you're going to need
You will only need a couple items, the most important of which is some Rockset. Essentially this is a powder which when mixed with water becomes as hard as a rock.
Image caption. Rockset is a product freely available across the country and is found in the hardware department
You could however simply use cement and water if you have some around. Whatever you use it must be pliable to get into the cut and it must be fairly firm so that once you have pressed it into the old wound it will stay there and not run out.
So first off you need to pick the time of year to do this. I believe the best time is when the tree is actively growing and not when it is dormant. The reason for this is as this is when sap is flowing and you need sapflow in order for healing to take place. So where I am this would be best in mid spring (if you do it too early when sapflow is too strong the wound will leak profusely) or late summer/early autumn when we experience a 2nd growth spurt.
If there is any rotten wood inside the wound you need to clean that out first. Kikuwa, a Japanese tool manufacturer has developed an ingenious tool which can be used for many things but in this scenario you can use the sharp end as a gravel to remove the deadwood. If you don't want to invest in this item then you can also use a chisel to carefully carve out the old rotting wood and use a blade or grafting knife to score the bark.
Mix the product you are using to fill the hole according to the packets instructions. Remember, we want a putty or double thick cream like consistency so once placed in the old wound it will remain there and not run out. Don't fill the hole too much, you have to try to think about how the callous will crawl over the surface. If you are working on a Chinese maple then the callous can be quite thick, so you need to leave a concave surface so once the wound has healed over it will be level with the surrounding tissue and not a swelling. If you are working with something which has a very thin bark and heals very slowly, like an azalea, then you can fill the old wound to pretty much level to the surrounding tissue.
Image caption. After filling the wound remove the bark to expose the cambium around the edge of the filler.
Once you have filled the wound then you can once again use your Jin tool. Using the sharp end of the tool remove the bark along the edge of the filler to expose the green cambium below. If you don't have the Jin tool then you could also use a grafting knife. You do this to stimulate callousing again, however this time it's going to make a lot more progress than before.
The importance of sealing
Seal the exposed area well. This is very important. You don't want any air getting to the cambium and drying it out. You want to keep it airtight and moist below the sealer so it can just roll easily over the filler.
Image caption. Sealing the exposed area ensures the live tissue can advance over the now solid surface quickly.
Depending on the size of the wound and species of tree your wound might have healed over in a single season. However for larger wounds and slower growing trees it could take several growing seasons.
Tip! Allow any branches immediately above in the near vicinity to the scar to grow strongly. This will promote sap flow beneath the branches and therefore assist in healing the scar quicker.
It's a good idea to remove the sealer and score the bark again to stimulate faster healing every season, if it does stretch over multiple seasons. However be sure to seal it immediately afterwards once again
And that's all there is to it, and of course this technique can be applied to conifers or deciduous trees alike although in the case of conifers I would recommend you use the conifer cut paste as it has a hormone in it will assists with healing (as these trees don't heal as readily as deciduous trees do).
Image caption. One season later and the scar is healing over well.
At this point the edges of the callous should again be scored and sealed. This stimulates faster healing. Repeat the process as many times as is needed, on a roughly annual basis, until the wound has disappeared.