Bridge grafting

I have a nice tree that was entirely girgled by voles (I suppose these were culprits, but maybe other rodents could do this) during the winter 2014-15. The bark was entirely eaten all around on a length of about 8 inches. I had to dig 2-3" under the surface to find uneaten wood. The diameter of the trunk is about 4".

I attempted 3 bridge grafts in spring 2015, but they all failed. I also pruned the tree quite heavily. It was still alive at fall even if the grafts had failed. Some nice watersprouts grew from the root.

In spring 2016, I tried to graft 2 of the watersprouts to the trunk, but I can see now these failed also. However, and I don't understand how, but the tree is doing nicely although not very vigorous, and is even having a modest crop!

I normally have quite good record grafting but somehow, I never succeeded making a bridge graft (this tree isn't the first on which I try). Wondering what I could be doing wrong... Anyone has a good trick to share?

Another question - could I attempt again grafting watersprouts in August, or is that a case of "don't bother you don't have a chance"?

Thanks for any tip...
Claude

Jolicoeur Orchard
Zone 4 in Quebec
Author, The New Cider Maker's Handbook

I have done a fair number of in-arch grafts on my own trees, (unappreciated destruction by round-headed apple borer eating all the cambium, following which the bark sloughs off). I am successful about 50% of the time. (Like you I generally surprise myself with how successful my scion grafts are - 100% with a recent batch of multiple pear cultivars)). I am not at all certain, but my sense is that one major factor in success is absolute immobilisation. Grafting a root sprout into a trunk which is bending in the wind poses a problem. And simply tight wrapping with electrical tape etc. does not stop the shearing effect of movement. I have never been able to drive nails in to hold the graft tight and immobile, but recently I have taken to using #5 1inch screws. (We in Canada have Robertson drive screws, which stay solidly on the end of the screwdriver. making it very easy to drive the screw straight. But that is a whole other subject.) This seems to provide the necessary rigid fixation.

Now the question of timing. I was confronted with this very question just yesterday. I don't (yet) have a definitive answer, but my reasoning was as follows: Budding is normally done in late July through August, (at least locally), so things will heal at this time. The bark is still slipping at this time, so getting cambium exposed on the stock is easy. The sap is actively flowing in the root sprout, so the graft union will be kept moist. Why not? I did it yesterday, and will let you know the results eventually. Meanwhile, my suggestion is to try it anyway - you have absolutely nothing to lose. And report back also.

One final point. If you are using an inverted-T cut in the stock bark, I think one gets a better take by paring off the corners of the wedge cut on the sprout, so that there is cambium exposed on both the inner side of the graft and on the outer side where it is in contact with the back of the raised bark flap. But leave a "spine" of bark on the sprout where it is exposed by the bark flap on the stock being parted.

Broomholm Orchard
Zone 5b in Nova Scotia

Quote
David wrote:
I have never been able to drive nails in to hold the graft tight and immobile, but recently I have taken to using #5 1inch screws.

How about using T50 staples. Anyone has done that? Any counter indication?
Claude

Jolicoeur Orchard
Zone 4 in Quebec
Author, The New Cider Maker's Handbook

Tried it. Not rigid enough to stop movement, and doesn't pull the pieces together the way a screw does

Broomholm Orchard
Zone 5b in Nova Scotia