I wonder if I can do it on cleft grafts after grafting.
You can but its a pain and highly likely to dislodge alignment.
If you cover the entire scion and graft union with foil, how can you tell when it starts leafing out?
After 2-3 weeks - but depends on the temperature, sap flow, scion dormancy.
What if the scion doesn’t get one foot of growth in the first year?
Just do it a 2-3 months after grafting, experience will tell
You mention temperature. […]
When people talk temperature they usually refer to daily highs, and you would want several consecutive days at that temperature. See here:
I’ve lately changed my thinking on temperature, but I then get a lot of failures, and I think climate gets overlooked a lot. We’re barely zone 3, and we get cold dry winds. Plums aren’t common here, especially not grafted ones, so I pretty much stick to apples. This year I collected scions as late as I could before the buds started to swell (so I didn’t have to store them too long), and waited until the sap was flowing well, as in starting to leaf out, before grafting. Did I screw up?
Scion can store just fine for several months if properly packaged and properly refrigerated. Grafting when things start to leaf out works well for apples, pears, even plums. Harder to graft things like peaches need the correct temperature.
You will know if you screwed up soon enough. I had a 2/6 success rate with stonefruit my first year, don’t sweat it.