The reason for covering is to preserve the limited stored energy in the scion, remember it got its energy stored as sugars doing photosynthesis! It cannot replace any energy until callousing is complete and the xylem/ Phloem plant cells are connected thru the graft union. When callousing is complete the scion can efficiently replenish its energy need to push buds to growth because fluids can now flow freely back and forth from rootstock to the scion. It usually takes most fruit types about 3 weeks at optimum temps to fully callous, thus denying sunlight just keeps the buds calm until you are ready to initiate bud break. This way the grafter controls when it happens.
Callusing temperatures of Fruit and Nut trees
Try it!
Dennis
Posted on May 21, 2013 by qwertyqweryt61
People ask me what are optimum callusing temperatures to ensure a good percentage of viable grafts.
Nectarines/Peaches – 18-26 deg C. ( 64.4 to 78.8F)
Apricots/Cherries – 20 deg C. ( 68F)
Plums – 16 deg C. ( 60.8 F)
Apples/Pears – 13-18 deg C. ( 55.4 to 64.4F)
Walnuts – 27 deg C. (80.6 F)
Grapes – 21-24 deg C. ( 69.8 to 75.2 F).
Figs - 23.9- 29.4 deg C. ( 75-85 F).
Do not forget tissue damage for most temperate fruit will occur at temperatures over 30 deg C. (86 F). So if you use electrical tape or any dark materials to graft, cover them with aluminum foil to prevent the sun from killing healing tissues.
Temperatures either side of the optimum will also work, but the percentage take will be reduced. See graph below for walnuts.
