Good question, John… for the ‘warmest winter’ in US history, it’s been unusually cool around here for the last month. I usually figure ‘the middle of April’ as safe to start making bark grafts. But it makes an ugly wound if it only tears…
I’d sacrifice the tips of some longer shoots attempting to judge how well, if at all, the bark’s slipping from the ‘wood.’ Sometimes I can find a branch I should have removed when pruning that’s big enough to give a better indication if the juice is flowing – then test and eventually sacrifice it.
Last year I discovered the sap flowing decent on the southern side - yet still tearing on the north side of some 30 year old apple trees. It took several days for it to ‘catch up’ on the north side! South would slip and except the scion just fine; North would tear… as I reluctantly ‘slid’ (slowly jammed) one in there anyway
And, Altitude counts! ‘Up here,’ we can be a week behind the lowlands… so I’ve got to be doubly careful when bark grafting. Did some work a few years back for a guy at around a thousand feet outside of Estacada. He assured me the bark was slipping …it wasn’t. I ended up doing cleft grafts instead of my preferred bark grafts… Never did get any feedback from that adventure.
When in doubt ~ wait. It’s no big deal, all you’d lose is the tiny smidgen of ‘juice’ that’s moved beyond the point you plan to remove – and that’s if you’re doing a ‘crown veneer’ graft, and lopping off quite a bit of tree beyond the graft. If all you’re doing is plugging in some side grafts under the bark … give it a couple more weeks. Only problem I noticed was if we got/get some very hot weather shortly after the scions have put on new growth they’d wilt, yet recover in time. Yah, good question