The Bench Grafter's Handbook

Principles & Practice

Brian E. Humphrey

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781351694933
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

Containing 500 full color photographs and illustrations, The Bench Grafter's Handbook: Principles and Practice presents exhaustive information on all aspects of bench grafting. It details requirements of more than 200 temperate woody plant genera, covering over 2, 000 species and cultivars including important ornamental, temperate fruit, and nut crops. The book explains the principles and practices of bench grafting, new procedures to enhance grafting success, and recommendations for further scientific investigation.

Practical issues to aid professionals and the beginner, include detailed accounts, supported by pictures and diagrams, of the main grafting methods, knifesmanship techniques, and methods of training. Provision and design, now and for the future, of suitable structures, grafting facilities, and equipment, to provide ideal controlled environments for grafts, are described.

The book describes major grafting systems, sub-cold, cold, warm, supported warm, hot-pipe, and other grafting strategies. It provides details of health and safety issues; work stations, seat design, lighting levels; recorded output figures for various types of graft; grafting knives and tools; and methods of sharpening by hand and machine.

Features:

• Comprehensive description, pictures, and diagrams of how to learn and utilize important grafting methods.
• Detailed information and scientific principles behind the selection, specification, and choice of the main graft components – the rootstock and scion.
• Scientific principles and practicalities of providing optimal plant material, equipment, facilities and environmental conditions for graft union development including addressing the problems of graft incompatibility.
• Discussion of the actual and potential role of bench grafting in woody plant conservation with suggestions for new initiatives.

This book is intended for use by nurserymen; those involved in the upkeep of extensive plant collections; conservationists; plant scientists; lecturers in horticulture; horticultural students; and amateurs with an interest in grafting.

Part One

Bench Grafting in Practice

1	Grafting in Nature and in the Hands of the Grafter

Grafting is based entirely on natural processes. Common examples of natural grafts are seen in the interwoven stems of Ivy (Hedera helix) or interlacing branches of a beech hedge (Fagus sylvatica), but given appropriate conditions probably all dicotyledonous woody plants can unite. When in close contact and subjected to increasing pressure as growth precedes, the bark between the branches becomes squeezed and thinned to a point where they are able to bond together to form a natural union (Figure 1.1). Equally important is abrasion of adjoining surfaces, while the bark is still young and thin, caused by pressure and movement due to wind or other forces. To ensure a successful union there is a further crucial requirement: the participants must have close genetic linkage, that is, be compatible. In the wild this will invariably be of the same species; interspecific grafts are theoretically possible but rarely, if ever, occur while intergeneric grafts almost certainly never found.

FIGURE 1.1 Natural graft of a main branch and lateral branch on Magnolia ‘Elizabeth’.

Natural grafting of roots is very common because they are often in close contact, and soil structure frequently dictates that pressure is applied as incremental growth squeezes the partners together (Figure 1.2). Stability of groups of trees such as beech (Fagus sylvatica) in very exposed, windswept situations is often dependent upon root grafting, which can provide massive interlocked anchorage.

FIGURE 1.2 Cercidiphyllum japonicum with exposed surface roots. Natural grafts are visible as thickened and interlocked crossing roots.

A pattern of natural graft development is emerging. These grafts are formed from genetically similar partners that are compatible, growing in close proximity, held fast together under pressure and exposed to mutual damage at the point of contact. Grafters mimic these conditions by cutting the surface of each component, placing the cut faces together and holding them tightly in place with a tie or other means.

Natural grafts are inevitably of the ‘approach’ type, as the components are still attached to their parent roots, whereas man-made grafts predominantly use detached scions. To ensure the ultimate plant produced is as shapely as possible, the shoot to be attached (scion) to the rooted portion below (rootstock) is aligned along the same axis as closely as possible. If, by mistake, the scion is grafted upside down, a union may be formed but the plant spends the rest of its life trying to re-orientate, resulting in stunted, deformed growth.

The most significant difference between natural and man-made grafts is in the components. In nature, almost without exception, only the same species unite. Under cultivation inter-specific combinations are normal. Grafts between genera can be successful; examples are seen with Cytisus scion on Laburnum rootstock, Pear (Pyrus communis) on Quince (Cydonia oblonga) and an entirely unexpected combination of the rare and beautiful Melliodendron xylocarpum succeeding on rootstocks of Pterostyrax hispida (Figure 1.3). In these examples the combinations involve genera from the same family Papilionaceae, Rosaceae and Styracaceae, respectively. Reports of successful grafts between woody plants from different families have not been found and it seems almost certain that they could not be successful.

FIGURE 1.3 Melliodendron xylocarpum ‘Tregye’, a rich pink flowered selection. On the left, two seasons after grafting to Halesia carolina; on the right to Pterostyrax hispida. Both grafts were made on the same day. Halesia subsequently failed to produce a compatible union. Pterostyrax has successfully united and in some seasons produced scion extension growth of 2 metres or more. After 8 years, no sucker growth is present on grafts of various selections which include white and pale pink flowered forms. It would appear the combination is entirely compatible. A good example of an unlikely but valuable inter-generic graft.

Vascular Continuity

Woody hardwoods and conifers have a continuous vascular system stretching from root tips to shoot tips. This comprises an inner core of xylem tissue (wood) separated from an outer core of phloem by a thin layer of cells capable of active division, known correctly as the vascular cambium, invariably shortened to the term cambium. It is this that is responsible for the formation of the woody plant’s vascular system. The xylem cylinder contains vessels that carry water and dissolved mineral nutrients from the roots. The outer cylinder of phloem has vessels responsible for transporting carbohydrate-rich fluids (elaborated sap) from the leaves. Included in these transportation systems are substances vital for growth and development, the plant auxins (hormones) and cytokinins.

Grafting using detached scions interrupts the continuity of this system, as the two components act separately, in which situation the scion has a limited period to survive. The essential feature of successfully united grafts is re-establishment of the vascular system between the rootstock and scion, once again providing vascular continuity and ensuring survival of the scion.

Re-establishment can occur when a number of requirements are met:

• The respective components are compatible.
• The cambium of each component is well aligned, in contact or in close proximity with the other, and held together under pressure.
• Correct conditions are provided for the formation of callus. This is a mass of simple undifferentiated parenchyma cells, which have potential to develop and infill any voids between the cut faces of the rootstock and scion after they are placed together.
• Respective cambia of rootstock and scion are subsequently able to become linked within the callus to form a completed graft union.

The task of the grafter is to make cuts and placement of rootstock and scion accurately enough to guarantee good cambial alignment, thus providing the best chance of success. At the same time, sufficient pressure must be applied to the interface of the components to promote essential cell and tissue differentiation. This is normally achieved by tying the components tightly together. Finally, suitable environmental conditions must be provided to maintain the scion and promote callus development and subsequent union formation.

The structured vascular system of woody plants described above relates to those formerly known as dicotyledons, because of the diffuse vascular structure of monocotyledon stems and roots, cambial matching is almost impossible and results in much less success with this group. A few genera, mostly in woody Liliaceae, e.g. Agave, Cordyline, Dracaena and Yucca, enter into a growth phase where the vascular elements become arranged in a more orderly fashion, not dissimilar to that of dicotyledons, and, at this stage, grafting for some may be feasible.

2	Grafting Strategies: Categorisation of Grafting Methods

Grafting Strategies

Before beginning a graft there is a need to decide between two main strategies: apical grafting and side grafting. Both utilize scions detached from the mother plant and grafted onto separate rootstocks, a process known as detached scion grafting. The choice of strategy influences grafting methods, rootstock pre-preparation, post-graft treatment and associated activities. These will be discussed here, in subsequent chapters and also as related to specific requirements for different genera, covered in Parts Seven and Eight.

A further strategy is to replace or, more often, supplement manual grafting methods by the use of machines or tools, to be discussed at the end of this chapter, as well as in Chapter 5 and Appendix K: Grafting Machines and Grafting Tools.

Somewhat different from other systems is a much less used, separate strategy with scions still attached to the mother plant, known as approach grafting; it is not considered at this stage, but described in Part Five: Chapter 18: Grafting Systems.

Apical Grafting

In apical grafts the scion replaces the top of the rootstock stem, which is cut down to a chosen height, a process known in grafter’s parlance as ‘heading-back’ or ‘heading-down’. Grafts may be close to ground level, where they are said to be bottom-worked, or sometimes higher, where they are known as top-worked. Grafting methods most often produced manually are called by the terms ‘splice’, ‘apical short’ or ‘long tongue veneer’ and ‘wedge’. Those produced by machine or grafting tool include splice and variants of inlay such as the ‘V’ notch and omega.

Apart from side grafting in the field using a single bud (budding), apical grafts constitute the most widely used strategy and huge numbers of apical bench grafts of plantation crops such as grapevines, fruit and nut trees are produced annually, many by grafting machines or gra...