1. The stock and scion must be sufficiently closely related taxonomically (i.e. genetically) to form a functional graft union.
2. The opposite of compatibility is incompatibility, which is the failure to form a functional graft union. Incompatibility may develop either rapidly or slowly. The latter is called delayed incompatibility.
3. The degree of "relatedness" necessary for compatibility (i.e. the Limits of compatibility) varies with different taxa (species, genera, families).
4. Compatibility/Incompatibility are discussed in more detail in a separate section.
B. Cambial "contact" (alignment)
1. The secondary meristem known as the vascular cambium (vc) lies between the outer wood (secondary xylem) and the inner "bark" (secondary phloem) of woody plants.
Cell division of the vc and subsequent differentiation results in deposition of new xylem and phloem during the normal course of (ungrafted) plant development. So, it is not surprising that vc formation across a graft union is essential for the production of new interconnecting xylem and phloem.
2. For a new vc to form across a graft union it is essential that the vc of the scion and that of the stock be fairly closely aligned (so called cambial "contact"), although not necessarily in direct contact at all points.
3. Proper cambial alignment is largely a matter of two things:
"Carpentry," i.e. grafter's skill with respect to cutting, and joining of scion and stock
a. If scion diameter is less than the stock diameter:
For some grafting techniques, such as cleft grafting, the scion can be considerably smaller in diameter than the stock. In such cases, it is important that the scion be placed near the perimeter (outer edge) of the stock so that the vascular cambia on that side are in alignment. The scion should not be centered in the middle of the stock! If the bark of the stock is substantially thicker than the scion, then extra care must be taken to align the cambia, not the outer bark of stock and scion.
(1) Note this illustration of cleft graft published in Hortus III. No doubt Liberty Hyde Bailey and/or the staff of the LH Bailey Hortorium at Cornell University, authors of the original and 1976 revised editions, respectively, ( LH Bailey Hortorium, 1976), knew well the requirements for successful grafting, but apparently the illustrator who sketched this cleft graft did not.
What is wrong with this picture?
Which scion (A, B, or C), is properly aligned with the stock? Why? What type of graft is this?
(2) For many grafting techniques, such as the top wedge graft, whip & tongue graft and the saddle grafts, ideally the scion should be the same diameter as the stock.
(3) Similarly the bud piece (scion) used for chip budding should be as close as possible to the same width as the piece removed from the stock. In such cases cambial alignment requires no special effort. However, due to limited scion material or other reasons, it is sometimes necessary to use a scion that is slightly smaller in diameter than the stock. Once again, the scion should be placed to one side or the other so the cambia will align on at least one side of the stock.
b. Choice of technique - The more cambial contact the faster and stronger the graft union will be. The length or proximity of the zone of cambial contact varies with different grafting techniques.
(1) Whip & tongue grafting and splice grafting are very similar in that both begin with long diagonal cuts of both stock and scion, which exposes an elliptical ring of vascular cambium. If stock and scion are the same size and are properly positioned, cambial alignment occurs all along this ellipse, with either grafting technique. However, in the case of the whip & tongue graft, the interlocking "tongues" created by the secondary cuts made in the stock and scion result in additional length of cambial contact. Hence the total length of cambial contact is almost twice as long with a whip & tongue graft as it is in a comparably sized splice graft. All other factors being equal, this is likely to result in a stronger union for the whip & tongue graft.
Why then, would one ever choose a splice graft over a whip & tongue graft?
(2) Chip vs. T-bud
Chip budding is rapidly replacing T-budding for bud grafting of apple varieties onto clonal rootstocks. Since apple budding is usually done in the late summer / early fall, it is important that bud union formation occur rapidly, before the onset of cold weather. One of several advantages of chip budding is that bud union formation occurs more rapidly than with T-budding. This illustration compares cross sections through a chipbud and a t-bud. In the case of chip budding the vascular cambia of stock and scion bud are directly aligned. In the case of T-budding, however, they are not, and hence more time is required for new cambium (and subsequently xylem and phloem) to develop across the (callus filled) gap between stock and scion.
1. Sufficient pressure between stock and scion is required to prevent them from moving independently of each other for three reasons:
a. Movement against each other at the point of contact would prevent the intermingling of callus cells from stock and scion. This is necessary for a continuous bridge of callus to form, across which a new vascular cambium and then xylem and phloem can form.
b. If stock and scion are not held together tightly the entire graft may fall apart before callus or vascular tissue (especially wood) formation can occur.
c. Pressure is necessary to orient the plane of cell division ( Barnett & Asante, 2000) to give rise to spatialy organized tissues (vascular cambium, xylem, phloem) rather than jumbled masses of cells.
2. Sufficient pressure is generated in two ways
a. Carpentry- some grafting methods involve wedging a tapered scion into a vertical split in the understock. The natural elasticity (springiness) of the wood creates compression even without tying.
(1) Whip & tongue (compression) vs. splice graft (no compression)
(2) Cleft grafting and top wedge grafting
b. Tying or wrapping-
(1) Latex rubber budding band - stretched tightly to apply pressure to stock and scion (bud). It photodegrades (breaks down) naturally after several weeks and falls off, eliminating the need for manual removal to prevent girdling.
bands ("budding rubber") used for field budding of apple
rubber patch used for field budding of roses
(2) Waxed string, or raffia (palm leaf fiber) or a polyethylene (plastic) strip - must be removed manually after graft union formation to prevent girdling
(3) Grafting tape
D. Avoid Desiccation
Freshly cut surfaces lose water rapidly, at least until necrotic plate formation occurs (a layer of dead cells that act as a moisture barrier; this process of graft union formation is described in Anatomy and Physiology of Graft Union Formation). Transpiration from foliage or other succulent growth on the scion can cause rapid desiccation. Drought stress resulting from either can result in graft union failure. To avoid desiccation mechanical barriers to water vapor loss in the form of tying materials are important, but at least as important in many cases are seasonal considerations, i.e. grafting at a time of year when transpiration is minimized.
1. Mechanical barriers to water vapor loss
a. Collection of scion (bud) wood
Scion / bud wood is usually collected hours, days, or even weeks before it is used for grafting. Like any "cutting," it must be protected from desiccation to avoid unnecessary water loss. Some of the practices used to minimize desiccation of scion wood include collection in the cool of the morning, avoiding sunlight after collection, removing leaves to minimize transpiration, and storing budsticks in a cool, high humidity environment.
- In this picture a moist gunny sack containing budsticks is stored in the shade beneath the budder's stool during field budding of garden roses onto Rosa multiflora seedling rootstocks at a Canadian nursery.
Note: A "gunny sack" (burlap bag) is preferable to a plastic bag because the evaporation of moisture from the breathable cloth bag maintains high humidity around the scion wood stored in the bag, and evaporative cooling prevents overheating. Scion wood, stored in either type of bag, should be kept out of direct sunlight.
- Scion wood collection recommendations from Texas A&M University.
b. Pressure between stock and scion (see section C. above)
- Tying or wrapping the graft union tightly, to eliminate gaps, retards moisture loss from cut surfaces.
- Good carpentry, i.e. straight, well matched (complementary) cuts of stock and scion to eliminate gaps. Practice makes perfect!
c. Use of wrapping materials impervious to water vapor
(1) Grafting tape - used for pressure and moisture retention, but accomplishes neither as effectively as plastic or rubber budding strips.
(2) Parafilm (tm) - a stretchable, inelastic wax-impregnated plastic that forms a watertight seal when stretched and wrapped around a graft union. Because of its inelasticity, it does not create much pressure.
(3) Polyethylene - in this side veneer graft of a Colorado blue spruce (Picea pungens) cultivar onto a P. pungens seedling, the graft is first wrapped with a budding rubber (red) to apply pressure, and then wrapped with a polyethylene strip to prevent moisture loss.
(4) Latex budding rubbers not only generate pressure, but also act as moisture barriers, to a limited extent.
(5) Grafting wax - a low-melting-point mixture of beeswax and paraffin that is melted and applied with a paint brush to seal the split stock of a cleft graft. The "cleft" would be difficult to seal by other means because of its large diameter.
(6) Polyethylene "tent" - desiccation of a leafy or succulent, non-dormant scion is often avoided by grafting during early spring or late summer/fall in temperate species (see dormant scion grafting, below). When this is not possible, especially in the tropics, the scion can be enclosed in a polyethylene tent, or the entire grafted plant can be placed in a high humidity environment. This is sometimes referred to as "greenwood" grafting. Greenwood grafting of fruit and nut trees is described at a Texas A&M Extension Web Site.
Seasonal changes in the degree of succulence and leafiness of stock and especially scion, has a great effect on potential moisture loss. This is discussed below in section E. Seasonal Considerations.
3. Pre- and Post-grafting management
Before grafting, scion-donor plants and stock plants must be managed to minimize drought stress. This is also important after grafting has taken place.
a. Moisture management
(1) Rainfall or irrigation
- Pre-grafting: Drought stress of either the stock plant, the scion, or both has a deleterious effect on graft union formation, as it does on most plant physiological processes.
- Post-grafting: Lack of adequate soil moisture can have deleterious effects on grafting even before drought stress is extreme. For example, in the case of rind grafting techniques such as T-budding, the bark must be "slipping." This depends on active cell division of the vascular cambium. During times of low soil moisture, cambial cell division of most woody plants stops and the bark ceases to "slip." At such times chip budding or other non-rind grafting techniques may be an alternative if drought stress is not too great.
(2) Humidity Management (Post Grafting).
- Moisture loss through transpiration and evaporation from cut grafted surfaces is reduced at higher ambient humidity levels.
- For all practical purposes humidity management is not practiced for field grafted plants.
- Humidity control for containerized or bare root grafted plants may involve the use of a plastic tent to cover the scion and scion/stock junction in the case of top wedge grafting.
- Grafting a leafy scion is sometimes referred to as "greenwood" grafting. Greenwood grafting of fruit and nut trees is described at this Texas A&M web site.
- Pots of side grafted Taxus, for example, are packed in damp peat, under a glass barrier.
- Appropriate apple interstocks such as EM9 are whip & tongue grafted onto bare root rootstock at the bench (indoors in winter) and stored in a humid, low temperature (50F) storage facility before lining out in spring.
High light levels and the high temperatures associated with them result in increased transpiration from leafy or other non-woody surfaces, at least until the point of incipient drought stress, when stomata close. Shading of grafted plants can be used to avoid excessive temperature and transpiration.(1) Although shading of field grafted plants is not practical in most cases, containerized plant are usually placed under shade, at least during the early stages of graft union formation. This is especially true when actively growing scions are grafted, such as top wedge grafting of hibiscus and other tropical plants.
(2) The problem of overheating due to excessively high light levels is critical when grafted plants or portions of them are grown under light transmitting enclosures due to a "greenhouse" effect. This is true for entire greenhouses, but it is especially true when the scion and scion/stock junction are covered by a plastic bag to increase humidity, as in the case of top wedge grafting. The increased humidity under the plastic bag "tent" tends to reduce transpiration, but under direct sunlight or even high indirect light, the temperature inside the bag can be considerably higher than the ambient temperature outside the bag, and the scion can be killed due to temperature stress.
E. Seasonal considerations
Optimal seasonal timing is largely a balance between temperature and moisture loss. Hence this and the previous requirement for successful grafting (avoid desiccation) overlap to a considerable extent.
1. Factors involved in selecting the appropriate time of year for grafting
The optimal time of year for grafting or budding depends on both species and grafting method. Even within a species such as apple, different grafting techniques are performed at different times of year.
The optimal time of year for any given combination of species and grafting method is dependent on temperature, potential for moisture loss, and other environmental factors affecting phenology and physiological activity of the stock and scion.
a. Sufficiently high temperature to support growth and development including:
Callus formation (parenchyma cell division and growth), and vascular cambial activity and cell differentiation necessary for xylem and phloem formation.
b. Sufficient level of dormancy to avoid excessive transpiration
(1) As discussed above (in the section on avoidance of desiccation) water loss from newly cut surfaces of a graft and the detached scion itself can be accomplished to some extent by wrapping materials, bagging, etc., but that is only part of the solution. The season and phenology of the plant can also have a big effect on transpirational water loss.
Grafting when scions are leafless minimizes scion transpiration.
(2). Minimizing transpiration of the scion is critical for successful grafting since a detached scion is extremely vulnerable to desiccation and drought stress prior to graft union formation, due to the absence of its own (original) root system and the lack of vascular connections (xylem/water transport) with the new stock. Leafing out of buds (shoot growth), as the growing season progresses, is associated with high potential for transpirational water loss from leaf stomates and the thin cuticle of new leafy shoots. The succulence (thin cuticle, etc. prior to lignification) associated with active shoot growth is especially prone to transpirational water loss. Hence, grafting when scions are leafless minimizes scion transpiration.
2. Species generalizations regarding optimal timing
a. Temperate deciduous species (e.g. apple, Norway maple)
Because of the phenological considerations, discussed above, the optimal times for grafting temperate deciduous species is either before bud break in the spring, or after dormant buds have been set in late summer:
(1) Winter / early spring, when the ambient temperature is high enough to support callus formation and other seasonally appropriate physiological activities including cambial activity , bud swelling, but before buds have leafed out. Before leaf-out the potential for transpiration and desiccation from the scion is low. ( Part A)
- Cleft grafting (field)
- Bridge grafting (field)
- Approach grafting (field)
- Whip and tongue (bench) grafting
(2) . . . but not late spring - early summer, when shoots are easily dessicated from excessive transpiration from succulent, actively growing shoots. Potential for transpiration and dessication from a scion is high. ( Part B)
- Grafting is not usually practiced at this time except for greenwood grafting and top wedge grafting of some tropical plants under protected (shaded) conditions.
(3) Late summer / early fall - when shoots have hardened off (i.e. new growth has become woody) and dormant buds have set due to a shortening photoperiod. If leaf is removed from scion bud shield ( Part C) the potential for scion transpiration is relatively low.
b. Temperate narrow-leaved evergreen (NLG) species
(e.g. junipers, yew (Taxus), spruce, pine , etc.)
Most narrow-leaved evergreens are dormant grafted, during the winter, in a cool greenhouse using a side veneer graft or other similar method. Containerized stock plants may be brought into the greenhouse several weeks before grafting so they will begin to emerge from dormancy. During this stock "wake up" period, scions are kept dormant (refrigerated) until grafting so that bud break and shoot growth, and hence transpiration, will be minimized. Often side-veneer grafted narrow-leaved evergreens, like Taxus bacatta repandens, are placed in a high humidity environment, e.g. pots packed in damp peat, under a glass sash.
c. Tropical species (e.g. hibiscus, avocado, citrus, passion fruit)
(1). Since tropicals do not undergo a winter (low temperature) dormancy, they will grow more or less continuously, and may be grafted anytime, as long as water is not a limiting factor.
(2). If budded with a more or less dormant lateral bud (e.g. citrus), the leaf blade is removed from the scion bud to minimize water loss.
(3). If actively growing, succulent, easily desiccated, multinode scions are often protected by putting a plastic bag tent over the scion as described above for hibiscus.
3. Rind vs. Non-Rind Grafting
a. Rind grafting and budding
- Many grafting or budding techniques, such as T-budding, can only be performed when the "bark is slipping." When actively dividing, the cells of the vascular cambium have thin cell walls that are easily torn, allowing the rind (inner plus outer bark) to peel away ("slip") easily from the underlying wood.
- The scion consists of the rind and associated bud, which is placed against the exposed wood of the stock.
- The bark slips only during periods of active tree growth (growing season), i.e. spring and summer, but not during fall or winter.
- Even during the growing season, slipping is dependent on environmental conditions being conducive to growth. Drought or the natural onset of dormancy will arrest growth, and cause the bark to "harden up" - vc ceases division, and newest xylem cells "harden" (secondary cell walls, and lignification) resulting in tearing of the rind rather than slipping.
- Examples of rind grafting / budding:
b. Non-rind grafting and budding (also referred to as "cleft" grafting ( Garner,1988)) involve cutting right through the cambium of the stock, and into the underlying xylem(wood). Thus, the bark does not have to be slipping (peeled away).
*Note: most other modern writers, including this one, use the term "cleft grafting" to refer to one particular method commonly used for top-working.
- Non-rind grafting does not require that the bark be slipping, and thus can be performed any time of year (within reason; e.g. temp. must be high enough for callus formation, etc)
- Sometimes non-rind grafting is done "at the bench." Bare root stockis bench grafted when both stock and scion are dormant (e.g. whip & tongue grafting and chip budding)
- Greenwood grafting is an example of non-rind grafting performed when scion is actively growing (e.g. top wedge grafting of hibiscus or avocado); with a plastic bag over scion to keep it from desiccating.
Examples of Non-Rind Grafting techniques
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