Budding time will depend on where you live and the type of budding technique you use. Traditional timing in the northern hemisphere above 36 degrees latitude, roughly the Virginia-North Carolina border, would be to bud trees in late August to early September, when the bark readily “slips.” Slipping refers to the ability of the bark to be easily and smoothly pulled away from the heartwood of the tree. When this occurs, “T-budding” or “chip budding” can be performed. If you live below 36 degrees latitude, there is also the possibility of “June budding.” In warmer climates, there may be sufficient early-season growth that T-budding can be performed in June. Finally, dormant chip budding can be performed in mid- to late March. This type of budding must be done before there is any physiological growth of either the rootstock or the fruit buds.
For more information, read this article on Apple Tree Propagation: Budding.
Answer provided by Dr. Robert Crassweller, Pennsylvania State University.