The first grafting I ever did with plums was with Euros and waiting until they were fully leafed out seemed to increase the percentage of takes. However, last year, my percentage of J. plums takes was very low so this year I followed Mamaungs advice and grafted at very first growth. The results seem to be quite an improvement over waiting, as most of the grafts are already growing vigorously and will likely be decent sized little branches by seasons end.
Right now, my opinion is that it’s best to hold back a bit with E. plums and graft the J’s the same time as I do my apples and pears. Essentially at the first signs of pome growth, when J’s are a few days further along.
I’ve had the same experience with Japanese plums. Grafts made after leaf out seem to get outcompeted by growth on established branches (when multi-grafting a tree) and killed off. Grafts made at least a couple weeks prior to bud break have done much better. Same for peaches and apricots.
Yes, I’m sure that’s true. My sense is that the problem is about the relative timing of when pre-existing buds start pushing relative to newly grafted buds. When there’s already active growth on the rootstock, newly grafted buds seem to get ignored by the plant, regardless of time of year.