Many people ask me what are optimum callusing temperatures to ensure a good percentage of viable grafts.
Nectarines/Peaches – 18-26 deg C
Apricots/Cherries – 20 deg C
Plums – 16 deg C
Apples/Pears – 13-18 deg C
Walnuts – 27 deg C
Grapes – 21-24 deg C
Olive – 25-35 deg C
Spruce – 20-25 deg C
Maples, Pines, and Fir – 21 deg C
Do not forget tissue damage for most temperate fruit will occur at temperatures over 30 deg C.
Temperatures either side of the optimum will also work, but the percentage take will be reduced. See graph below for walnuts.
Callus formation is required at the graft, therefore temperatures are only needed at the graft. The rootstock and scion can be held at lower temperatures to avoid breaking the dormancy.
If you are budding in the field and temperatures are going to be excessive, then do the budding on the north side of the rootstock, so the bud is in the shade.