Grafting is the horticultural practice of joining a piece of one plant, the scion, onto another plant, the rootstock, so they grow together as a single tree. This technique propagates specific apple varieties, ensuring the new tree produces the desired fruit. While the physical act of joining the two parts is important, precise timing is the most significant factor determining the success of the graft union. Understanding the physiological state of both the scion and the rootstock relative to the season is crucial for achieving a successful vascular connection and subsequent growth.
Timing the Collection and Storage of Scion Wood
Successful spring grafting requires that the scion wood be collected at a different time from when the grafting is performed. The scion, which is the shoot of the desired apple variety, must be harvested while the parent tree is in its deepest state of dormancy. This period generally occurs during the deep winter months, typically from December through February, after the leaves have dropped and before any sign of bud swell. Harvesting the scion wood while it is fully dormant ensures that its growth buds remain tightly closed and metabolically inactive.
Scions, usually one-year-old wood about the thickness of a pencil, must be kept dormant until the rootstock is ready for grafting weeks later. This temporary holding period prevents the scion from breaking dormancy prematurely, which would cause it to desiccate and die before the graft union has healed. Scion wood is best stored in a cold, moist environment, such as a refrigerator, sealed within a plastic bag with slightly dampened material like paper towels or sawdust. The temperature should be kept consistently near freezing, between 33°F and 37°F (1°C to 3°C), preventing both freezing damage and premature growth activation.
The Optimal Spring Grafting Window
The most common method for grafting apple trees uses a stored, dormant scion on an awakening rootstock. The optimal timing for this procedure is dictated by the rootstock’s physiological shift from winter dormancy to active growth. This transition is signaled by the beginning of sap flow, often referred to as the bark “slipping,” which is necessary for the cambium layers of the scion and rootstock to successfully merge. Waiting until the rootstock is actively pumping nutrients and water ensures that the new graft receives the immediate support it needs to heal.
In most temperate climates, this window opens in late March and lasts through April, though the exact timing depends on local weather conditions. The critical phase is when the rootstock is actively breaking dormancy, indicated by slightly swelling buds, but before the buds on the stored scion wood begin to break. Grafting too early, before the rootstock’s cambium is active, or too late, results in failure. Successful grafting requires consistent temperatures above the cambial activation threshold of 40°F to 45°F (4°C to 7°C).
The successful formation of callus tissue occurs most rapidly within an optimal temperature range of 55°F to 75°F (13°C to 24°C). Grafting during periods with consistent daytime temperatures in this range maximizes the speed of healing and minimizes the risk of desiccation or infection. Avoiding late-season frosts immediately after grafting is important, as sudden temperature drops can damage the newly formed connection tissue and halt the healing process. The spring sap flow provides the necessary moisture and energy to “push” the dormant scion into growth once the vascular union is established.
Late Season Budding: An Alternative Timing
While spring grafting uses a dormant scion, an alternative method called budding is performed during the period of active summer growth. This technique utilizes a single, mature bud from the current season’s growth, rather than a whole scion stick. The timing for budding is mid-to-late summer, often extending from July through August, when both the rootstock and the bud-source wood are actively growing.
The primary indicator for the correct summer budding time is the rootstock’s heavy sap flow, which allows the bark to “slip” or peel easily from the underlying wood. This flexibility is necessary to insert the bud shield cleanly beneath the loosened bark of the rootstock. In contrast to spring grafting, the bud inserted during the late summer is intended to form a union but remain dormant.
The bud heals into the rootstock over the late summer and fall, but it will not begin to sprout until the following spring. This timing is advantageous because it allows the rootstock to fully recover during the rest of the growing season. The late-season timing also offers an opportunity to propagate varieties or fix failures from the earlier spring grafting window.