Tree grafting involves joining a piece of one plant, the scion, onto the root system or stem of another, the rootstock. The goal is to encourage the two parts to grow together as a single plant, combining the desirable traits of both. Achieving a successful union depends entirely on performing this procedure at the right moment. This specific timing ensures the biological conditions required for the tissues to fuse are met. This article details the narrow windows necessary for various grafting methods to achieve a successful connection.
The Biological Necessity of Timing
The strict requirement for timing is rooted in the plant’s internal biology. Beneath the bark of both the scion and the rootstock lies a thin, actively growing layer of tissue called the cambium. For the graft to succeed, the cambium layers must be aligned perfectly and stimulated to grow together.
The union forms when these cambial cells begin rapidly dividing to create an undifferentiated mass of cells known as the callus. This callus tissue acts as the biological glue, bridging the gap between the two pieces. Successful grafting requires environmental conditions, primarily temperature and moisture, to be perfect for this cellular activity.
If the cambium is not active, or if conditions are too harsh, the cells will not divide, and the scion will dry out and die. The correct timing ensures the plant’s mechanisms are optimized to heal the wound and establish vascular connections.
Optimal Timing for Dormant Scion Grafts
The most common and successful timing uses dormant scionwood on a rootstock just emerging from winter sleep. This window typically occurs in late winter or very early spring, before the rootstock’s buds have fully broken open. The process works best when the rootstock shows signs of swelling buds, indicating that sap is moving and the cambial layer is becoming active.
Techniques like the Whip-and-tongue, Cleft, and Bark grafts are ideally suited for this period. Using dormant wood prevents the scion’s buds from opening prematurely before vascular connections are established. The moisture and stored energy within the dormant scion are preserved until the rootstock can supply water and nutrients.
The ideal period often correlates with sustained daytime temperatures rising above 40°F (4°C). This temperature range facilitates the cell division required for callus formation without immediately pushing the dormant buds into rapid growth. The procedure must be completed before the tree enters a rapid growth phase or before the weather becomes consistently hot. Grafting too late, when the rootstock is fully leafed out, can lead to the scion drying out before the union forms.
Optimal Timing for Budding and Active Season Grafts
Different methods are employed during the active growing season, particularly those relying on “slipping bark.” Slipping bark refers to the ease with which the bark can be peeled away from the wood. This occurs when the cambium layer is rapidly dividing and well-lubricated by sap flow, typically from late spring through late summer.
Budding techniques, such as T-budding or chip budding, are the primary methods used during this active season. These methods involve inserting a single bud, rather than an entire scion stick, into the rootstock. The timing is perfect when the rootstock is in full vegetative growth and the collected budwood is mature, meaning the current season’s growth has hardened slightly.
The actively growing rootstock rapidly seals the inserted bud, and the abundance of sap helps prevent the small tissue piece from desiccating. This summer timing is frequently used for stone fruits like peaches and cherries, often providing a higher success rate than spring grafts.
The Separate Timing for Scionwood Collection
The timing for collecting scionwood is distinct from the timing of the grafting procedure itself. Scions must be harvested when the parent tree is fully dormant, typically between mid-December and late February in temperate climates. Collecting the wood during deep winter dormancy ensures the buds have received sufficient chill hours and will not break open prematurely.
If the buds on the scion begin to grow before the graft union has formed, the scion will quickly exhaust its stored energy and fail. Once collected, the scionwood must be immediately stored under cold and moist conditions, usually in refrigeration, until the rootstock is biologically ready for the spring procedure. This separation is necessary to synchronize the dormant scion with the active rootstock.