Grafting
Grafting is one of the oldest known methods of plant propagation and dates back 2000 years or more. It is defined as the process of joining two plants or plant parts together in such a manner that they will unite and continue their growth as one. The main reason for grafting is to propagate plants that are difficult or impossible by other vegetative methods. Another important reason is to grow plants on roots other than their own for such purposes as size or disease control. It is not the intent of this chapter to discuss in detail the subject of grafting. but to cover the important methods. Several excellent books on grafting and budding include:- RI. Garner, The Grafters Handbook.
Oxford University Press: H. T. Hartmann and D. E. Kester. Plant Propagation: Principles and Practices. Prentice-Hall. Inc..

A graft consists of two parts, the scion and under-stock. The scion consists of a short stem piece with two or more buds and is that part of the graft combination which develops into the top (shoot) of the plant. If the scion is reduced to a single bud with a thin slice of wood, the technique is called bud grafting or budding.

The under-stock. also referred to as rootstock or stock, is the lower part that becomes the root system. Under-stocks can be seedling or clonal in origin. Seedling under-stocks are more widely used; however, they are not genetically uniform. Asexually propagated clonal under-stocks provide genetic uniformity, disease resistance, growth modification and prevent incompatibility problems.

Limitations of Grafting
There are limitations to the successful grafting of two different plants. The botanical relationship between two plants is not a guarantee of success; long term observational experience provide the best guide. When two species cannot be successfully grafted together they are said to be incompatible. As a general rule, the closer two plants are taxonomically related to each other, the greater the chance of forming a successful union. From a commercial nursery point of view, grafting is limited to plants that have a continuous cambium. Among woody plants, no successful long term grafts are reported between different families. In short, it is not possible to successfully graft an oak (Fagaceae) and maple (Aceraceae).

Grafting between genera within the same family is possible, however, the number of cases is limited. Poncirus trifoliata is used as a dwarfing under-stock for Citrus (orange); Pyrus (pear) is grafted on Cydonia (quince) as a dwarfing under-stock; a number of Cotoneaster species have been grafted to Pyrus (pear) arid Crataegus (hawthorn).

Different species within the same genus are compatible in some cases but incompatible in others. As noted by Hartmann and Kester, grafting within Citrus is successful and often practiced. Similarly, Prunus dulcis (almond). P. arnieniaca (apricot), P dornestica (European plum), and P. salicina (lapanese plum) are compatible on P. persica (peach).The complexity of grafting between species within the same genus is further demonstrated by reciprocal grafts. 'Marianna' plum is compatible as a scion on peach, but the reverse graft, peach on 'Marianna' plum is not.

Grafting between clones and seedlings of the same species for all practical purposes is successful; however, incompatibility problems do exist. Pseudotsuga tnenziesii (Douglas fir) clone, Quercus palustris Sovereign, and Acer rubru cultivar graft failures have been reported. The incompatibility problem between A. rubrun cultivars and A. rubruni seedling rootstocks has led to the development of cutting propagation methods for those cultivars.

Incompatibility
Mahlstede and Haher (II) noted that incompatibility may be expressed in a number of ways including:
1. Combinations which never form a successful union,
2. Combinations in which only a small number of unions form.
3. Types in which the union is successful initially, but the plant eventually dies.
4. Combinations that produce deficiency symptoms or nutritional disorders,
5. Combinations that result in dwarf trees,
6. Types that produce differential growth at, or close to the union,
7. Combinations causing degeneration of tissue systems, abnormal distribution of stored food reserves, and premature defoliator. Although the causes of graft incompatibility are not completely understood, genetic differences and diseases (virus and mycoplasma causal agents) are important factors.

Time Measurements
Time measurement studies on the grafting of different plants are presented below.

Time from start to finish of the work. .Ed. However, the times should be used only as average time for grafting. Fagus 'Riversu' - side tongue graft, tied with raffia, Average time for grafting: 1.92 minutes. Hibiscus syriacus cultivars, veneer graft. tied with 2-ply fillis. Average time for grafting: 0.67 minutes. Picea pungens 'Glauca Pendula' - side veneer graft, tied with raffia, removal of needles from base of scion. Average time for grafting: 1.41 minutes. Prunus whip and tongue, root wiped with rag, raffia-tied, graft painted with cold wax, .average time for grafting: 2.12 minutes.

Physiology of Grafting
Aeration, temperature, humidity, inherent capacity to form callus et-al, affect the success of the graft. Temperature, aeration and moisture, are primarily responsible for success or failure in most grafted plants. Temperature has the strongest effect. Warm temperatures promote callus formation: cool temperatures inhibit or reduce it, while extremely high temperatures are detrimental. In a comparison of temperature effects on apple and walnut, apple was found to be less temperature sensitive. Apple grafts showed little callus growth below 50°F or above 95°F. In contrast, little callus growth occurred below 70°F or above 100°F with walnut. Callus growth increased linearly between 50 and 75°F for apple and 70 and 85°F for walnut. Walnut therefore, has a narrower temperature range for success. High humidity is essential for callus formation and that is the reason grafts are waxed and/or tied, and often placed in high humidity chambers.

Tools and Accessories
1. Knives
Of the tools employed for grafting, knives are the most important. A large number of excellent knives are available; however, the success of the grafting operation depends more on the sharpness of the knife than on the type. The best under-stock, scion wood and equipment cannot compensate for dull knives that produce ragged wounds. For propagation work, the two general types of knives used are the budding knife and grafting knife (Fig. 1).The knives have either a fixed or folding blade, with the fixed blade stronger and longer lasting. A grafting knife should be of good quality and have a straight-edged blade flat on one side and sloped on the other. The purpose of sharpening the blade on one side only is to provide a flat backing on the blade, so that it will make a flat cut into the plant. With the budding knife, a cutting edge that curves away to the tip is usually preferred, because it makes it easier to separate the bark flaps during T-budding, Also, a sharp pointed knife will tend to enter the wood of the rootstock.

Figure 1 - Budding (a) and grafting (b) knives.
Grafting knifes have a straight edge.

2. Tying and wrapping materials
Tying and wrapping materials hold the scion and stock closely together, and prevent the callus from forcing the pieces apart. Control of moisture loss also occurs with some materials, such as parafilm (American Can Co.) Almost any tying material can be used to hold the scion and under-stock until union formation occurs, but some are better than others. Widely used materials include: adhesive tape, plastic, polyethylene tapes, raffia, rubber budding strips, and twine. (waxed or non waxed).

Grafting Methods
Numerous grafting methods exist, however, only a few are of major importance to nurserymen. Techniques such as approach grafting, matching, and bridge grafting, are seldom used in propagation nurseries, When the terms bench, or pot grafting appear, they refer to indoor (winter) grafting activity using potted (established) and, in certain cases. barefoot under-stock.

1. Splice graft
This is one of the simplest grafts. Long slanting cuts are made on the scion and rootstock (Fig. 2a). The cut surfaces are joined together so that the cambial zones are in contact and tied (Fig. 2b and c). This graft is limited to plants that heal rapidly, because the method of joining adds no strength to the union, except for the tying material. This method is generally limited to indoor bench grafting.

Figure 2 - Making the splice graft: (a) single slanting cuts are made on scion and understock: (b) the parts are fitted to match cambium layers: and (c) the graft is tied

2. Whip-and-tongue graft
The whip-and-tongue graft, which is a modified splice graft, does not suffer from the splice graft limitations. The whip-and-tongue graft (Fig. 3) is widely used for joining together comparatively small scion and rootstock parts, usually not more than one inch in diameter. Best results are obtained when the stock and scion are the same size. The graft is easy to make, because the interlocking edges form a strong union before tying. The initial long slanting cuts, about 1 to 1½” . are made like the splice graft. On each of the cut surfaces a second cut or tongue is made (Fig. 3a and b). It is started downward at a point about one half the distance between the pith and the tip of the outer edge of the bark. The second cut is one half the length of the initial slanting cut. The scion and rootstock are then inserted into each other (Fig. 3c) with the tongue interlocking and the cambial zones in contact, and tied (Fig. 3d). If the union is to be waxed, grafting twine often is used.

Figure 3 - Making the whip and tongue graft: (a) prepared scion: (b) understock: (c) match graft: (d) graft tied.

3. Veneer and side grafts
The veneer (Fig. 4) and side graft are similar and widely used to propagate shrubs and conifers. The under-stock stem is cleared of leaves in the region of the graft and preferably cut first. This sequence eliminates laying the scion down and possible contamination. A cut about I 1/4” long is made on a straight portion of the stock that is free of side branches. and as close to the soil as possible. (Fig. 4a) The cut is made through the bark and slightly into the wood. At the bottom of the first cut, a second cut is made downward and inward about 3/16 to 1/4" and through the flap. This will leave a short lip of bark and wood at the base. The scion is prepared by making a cut of equal length from top to bottom on the straightest side and deep enough to expose the wood. A second cut is made across the base from top to bottom, and slanting downward at the same angle as the cut lip on the under-stock (Fig 4b). The scion and under-stock are joined, the cambium layers matched and tied (Fig. 4c). If the stock and scion are not the same size, then the cambium layers should be matched along at least one side.

Figure 4 - Steps in making the veneer graft with a narrow-leaf (needle) evergreen: (a) position of the cut on the understock: (b) the scion prepared for insertion: (c) the completed graft.

The side graft is similar to the veneer, except that the second cut is not made to the under-stock, thus leaving a long flap. The scion is given three cuts, with the first about 2" long, the second not quite as long on the opposite side and the third slanting across the base. The scion is then fitted to the under-stock with the longer side against the stock, the flap brought up and against the shorter cut side and tied. Plants listed as being grafted with the veneer type include Abies taxa, Cedrus atlantica, Cupressus taxa. and Rhododendron indica. The side graft is mentioned as being used with Acer palmatum and A. japonicum, Fagus taxa, Ginkgo, Hansanielis neollis, Ilex opaca, Magnolia grandiflora, Rhododendron taxa, and Viburnum carlesii. Plants using either type include: Cryptomeria japonica. luniperus taxa, Picea pungens and Thuja taxa.

4. Saddle graft
The saddle graft (Fig. 5) is not used as often as some other types because it is more time consuming to make. However, it is valuable in certain situations for rhododendron grafting and plants with fairly large stems. The scion and rootstock should be the same size. The top of the rootstock is removed close to the soil (I to 1 1/2”) and trimmed on two opposite sides to form a wedge (Fig. 5a). The base of the scion is cut to remove a wedge (Fig 5b). Be sure the bark at the base of the scion does not separate from the wood, or the scion will not take. Rootstock and scion are joined together and securely tied (Fig. 5c). All cut surfaces are waxed if the grafts are not placed in a grafting case

Figure 5. Making the saddle graft: (a) two slanting cuts on opposite sides of the understock to form a blunt point: (b) a section at the base of the scion is removed to match the understock: (c) the graft is tied after the cambium layers are matched.

5. Cleft graft
The cleft graft (Fig. 6) is often used for field grafting. especially when the stock is larger than the scion. Branches or tree trunks from I to 3" in diameter are best. Preferred grafting time is late winter or early spring when the buds of the rootstock are beginning to swell, but not actively growing. The under-stock is cut off at a right angle to the main axis where the union is desired. A cleft (split) is then made in the end of the stub with special cleft grafting tool (Fig. 6a) or large knife, such as a butcher knife. The cleft is held open by the wedge-shaped prong on the cleft grafting tool or by the use of a wooden or metal wedge (Fig. 6b). The scion is made from the previous season's growth and includes 2 to 3 buds (Fig. 6c). The lower end is cut to a wedge shape about 1 to 1 1/4 "long with the outer edge slightly thicker for better cambium contact. Larger stock (tree trunk) receives 2 scions while smaller stock receives one scion (Fig. 6d and e).

Figure 6 - Making the cleft graft: (a) using the cleft iron to make the cut: (b) spreading the cut to insert the scions: c) the basal slant cuts on the scions: (d) placement of scions in cleft: (e) cambium region of scion in contact with cambium zone of cleft graft: (f) graft waxed.

If both scions grow, one is removed after the first year. Generally the union is not tied, the pressure of the under-stock stub being strong enough to hold the scion in close contact. After setting the scion, all the cut surfaces including the tip of the scion are covered with grafting wax (Fig 6f).

6. Cutting grafts
A cutting graft (also called twig graft) is made by grafting a leafy scion to an unrooted leafy cutting. This technique. although not widely practiced, has been utilized with citrus and rose The method presented is that used with citrus. Scion and rootstock cuttings are taken from spring growth that has hardened off. The scion and rootstock are splice grafted. A veneer graft is also used with this technique. After tying, the graft is handled as a cutting. The cuttings are treated with 8000 ppm IBA. inserted into the rooting medium, and placed in a fog chamber that maintains high humidity. High humidity is required because the scion has no contact with the rooting medium. Bottom heat (70 to 72°F) is supplied to speed rooting. After rooting and union healing, the grafts are hardened off.

7. Root grafting
In root grafting. the roots of seedling or clonal rootstocks serve as under-stock (Fig. 7). If the whole root is used, it is termed whole root grafting and if a piece is used it is piece root grafting. A whip- and-tongue graft is the method of choice and grafting is carried out from December to March. The initial long slanting cuts for the scion and under-stock (about 1 to 1 ½" long) are made like the splice graft. On each of the cut surfaces a second cut or tongue are made (Fig. 7a and b). The cut starts downward at a point about one half the distance between the center of the stem or root, and the tip of the outer edge of the bark. The second cut is 1/2 the length of the initial slanting cut. The scion and rootstock are then joined (Fig. 6c) with the tongues interlocking and the cambial zones in contact, and tied (Fig. 7d). After tying, the grafts are stored at about 40°F for callus development. When weather conditions permit, the grafts are planted out.

Figure 7 - Making the piece root graft: (a) prepared root pieces: (b) prepared scion: (c) joined parts:

8. Bare root grafting
This type of grafting is carried out in the winter with fall dug under-stock that is cold stored until used. Plants such as Malus lapplel, Comas (dogwood). Magnolia (magnolia), and Acer (maple) can be grafted by this technique. After tying. the grafts are dipped in wax and boxed in alternating layers of moist material, such as peat moss. After callusing, the grafts can be stored at close to freezing until planted out.

Care of Grafts
1. Closed case
For graft healing, especially with leafy cuttings. the grafts are placed under double glass, a closed case or polytent in a heated greenhouse. Moist peat is placed in the base of the case to provide humidity. Under high humidity conditions. drying of the union is not a problem and the unions are usually just tied. The grafts are not watered until extensive callus formation is visible: at this time airing becomes necessary. Heavy shading should be used to lower temperatures, and prevent scion stress. Once the union is established, the grafts are gradually hardened-off .

2. Open bench
Completed grafts can be placed on an open greenhouse bench with pots plunged in peat, perlite, sand, or any combination at a depth sufficient to cover the union. The purpose of burying the union is to prevent drying out. The time period will depend on the plant type and temperature. Initially, the medium temperature should be kept between 65 and 75°F for 4 to 6 weeks. A heat source under the bench allows the maintenance of a cooler air temperature (50 to 60°F). The tops are supplied with adequate moisture by syringing, shading. or covering with polyethylene. When polyethylene is used, the covering should be lifted daily to dry the grafts for disease control.

After callusing is evident, the grafts are ready for hardening- off. The understock is pruned back about half-way, and the grafts are placed on the medium for an additional 4 to 6 weeks. At the end of this period the remaining top is removed.

3. Poly bag chamber
This technique is useful when space is limited or for grafting specialized forms such as standards. After the graft is made and tied, a ball of wet sphagnum moss, the size of a lemon, is tied to the rootstock one inch below the union. A plastic bag is then inflated and put over the scion and tied with a rubber strip below the sphagnum moss. Grafts are then placed in a greenhouse. No other care, except possibly watering the containerized standards. is needed.

Figure 8 - A cross section of the hot-callusing device with a graft union in place.

4. Hot-callus pipe
The hot-callus pipe is constructed with 2" PVC pipe that contains 1/2 and 5/8" slots cut perpendicular to the length of pipe (Fig. 8). Hot water is circulated through a ½" PVC pipe inside the 2" slotted pipe. A hot water heater, in concert with an electric circulating pump, and an expansion tank, placed 6' above the pump are used. This system performs best between 20 to 30 psi at the pump which is placed on the cool side of the water heater. An exit temperature of 81 to 82°F is maintained. The top of the slots are first covered with 1/8" closed cell foam, and then a layer of 6 mm. black poly. A slit is cut in the center of each slot in the pipe. Moist sawdust between the pipes is used to cover bare roots. The best location for a hot-callus system is where temperatures can be maintained cold, to prevent premature bud break on the scion while the unions are healing. The system has worked well with Acer. Cedrus, Cercidiphyllum, Corylus. Fagus. Malus, Prunus and Sequoia. Spruce union formation is not favored by the high temperature.