It all hinges on temperature.
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However either side of the optimum temperature you can still get good results, but the % success rate reduces and length of time for callusing increases.
Take apples, the optimum callusing temperature is approx. 15 deg C and callusing would only take 2 weeks with say a 95% success rate.
However if temperature was reduced to say 10 deg C callusing would take 3 to 4 weeks with a success rate of 85%.
If the temperature was say 5 deg C callusing would take 6 to 8 weeks with a success rate of say 50%.
If the temperature went below 5 deg C no callusing would occur.
So if you were going to graft in the field it would be best to wait until very near bud break, say end March, as the temperature should be warmer. In that case the scions must be stored at cool till required, ie salad tray of fridge, or north side of an outside wall, or even buried in sand.
If however you were in control of the temperature, as you may have a greenhouse or a conservatory, then grafting could take place earlier in the year.
I have in the past had good results by growing rootstock in pots then grafting beginning of Jan. But keeping the pots on top of our kitchen fridge freezer till the spring. As I have found the temperature to be a constant 18 deg C.