A. Natural Grafting
If you have never observed natural grafting, this section should make you aware that it is more common than you might think. It occurs between shoots and between roots, and it may occur with the same tree (autografting) or between adjacent (compatible) trees.
1. Examples of natural grafting
White oak (Quercus alba)
triphylla 1 -
a shrub which grows in the Rift Valley in East Africa (Kenya), naturally
self-grafts in great profusion. Note how the shrub (1st image) is lying
over with the main trunk nearly horizontal due to persistently windy conditions
and shallow rooting in this dry environment. The self-grafting of the
branches (close-up, 2nd image) tends to stabilize the crown against further
wind damage, and suggests an adaptation of this species to a windy environment.
soulangiana) - natural autograft joining main trunk to a scaffold
limb of this landscape specimen out side the Tompkins County Courthouse
in Ithaca, NY. Note mechanical stabilization of crotch.
Poplar (Populus nigra)- natural root graft from the old dead root system of a tree growing on the shore of Cayuga Lake which had been uprooted, probably by flooding, years ago.
2. Criteria for natural grafting
a. Natural graft union formation requires that the vascular cambia of two branches or roots directly contact one another, or nearly so. There must be steady, continuous pressure between the two members. This requires that the outer bark of each must be worn away, primarily by the pressure created as both members increase in diameter over time, while being held firmly together. Hence, thin-barked species tend to form these natural graft unions more readily than species with thicker bark, and most natural grafting occurs with young branches while bark thickness is minimal.
b. Although branch-to-branch contact is common within and between canopies of tree(s), most are unlikely to become grafted together because of the frequent branch movement, due to wind, which would disrupt the continuous adhesion and pressure necessary for the early stages of graft union (i.e. callus) formation. The sequential stages of graft union formation are explained more fully in the section on Anatomy and Physiology of Graft Union Formation.
c. Natural grafting is only likely to occur between two branches that are held firmly in place. In the case of shoots this may occur when one branch is wedged within a narrow Y-shaped crotch formed by two other branches. In the case of roots, the surrounding soil matrix prevents disruptive movement of two roots which have come into contact, and this would explain why natural grafting of roots is more common than natural grafting of shoots.
d. In his fascinating old paper on natural root grafting, La Rue (1934), took issue with the prevailing view that in order for two roots or two shoots to naturally graft, they had to rub against each other until the bark was worn away. Even in the case of roots, previous authors speculated that wind-induced swaying, back and forth, of a tree's above ground trunk, would translate into sufficient subterranean movement of the root system to result in wearing away of the bark between adjecent roots. La Rue ( La Rue, 1934) however observed situations when he considered it impossible/improbable that any root motion was involved. As an alternative explanation, he suggested that the soil matrix prevented roots from moving, so that any two roots that happen to be in contact, would exert increasing pressure on each other as their diameters increased with age (seconday thickening). He asserted that the gradually developing pressure would be suffcient to disrupt the bark of both roots, eventually bringing their vascular cambia in contact, and precipitating natural grafting. By a similar process, pressure would also be generated between above ground shoots, if one branch is held firmly in the crotch of another branching limb. If you look closely at the red pine natural shoot graft, you will see a very narrow crotch between the large stem and the smaller one (near the upper cut) which is now partially fused. Increasing pressure, as all three limbs increased in diameter, caused bark disruption and eventually cambial contact.
La Rue's explaination for natural root grafting was speculative, based on his field observations. Can you think of an experiment for testing this hypothesis? Alternatively, can you think of another explanation for natural root grafting? If you are interested in reading about one, check out Rao, 1966, Developmental anatomy of Ficus globosa. Austral. Jour. Bot. 14:269-76.
3. Consequences of natural grafting.
a. Shoots. Natural grafting may interlock to some extent and stabilize the canopy of an autografted tree, resulting in a stronger trunk system. This is apparent in the case of Maeura triphylla and the white oak show above.
b. Roots. In the case of natural root grafting between two adjacent trees, root grafts may result in transmission of disease organisms including fungi (e.g. Dutch Elm Disease), or viruses (citrus orchards) (Epstein, 1978).
- Exchange of materials between root systems, need not, however, have only negative consequences. Water and nutrient exchange between adjacent trees could promote the growth of a tree in a poorer microsite at the expense of a tree closer to a source of water or nutrients. Although such exchanges certainly occur, the magnitude of their effect on forest ecology is not well understood. It could be quite substantial.
- Research at the The Luquillo Experimental Forest (LEF), in Puerto Rico: Ecological Consequences of Root Grafting in Tabonuco (Dacryodes excelsa) Trees
- Natural root grafting is responsible, in some cases, for "living stumps" i.e. shoots which resprout yearly from stumps of trees cut down many years ago.
B. (Human) History of Grafting
Although there is no historical record of the first deliberate grafting by early agriculturists, it most likely resembled what is known today as approach grafting. In the case of natural grafting, the "stock" and "scion" may not be well-defined, but neither of the two units that will undergo fusion is detached from its own root system, as is the case with most deliberate grafting techniques like Whip & Tongue, budding, etc.
Approach grafting is an attached scion method, similar to natural grafting in that the scion remains attached to its own rootstock until after the graft union is formed. In both cases, the scion has a continuous supply of water and nutrients, essentially eliminating the potential water stress that is likely to occur after a scion is detached from its root system but before it has reformed vascular (xylem) continuity across the graft union. Hence, approach grafting would require less attention paid to managing the environment and avoiding desiccation, and would thus be more likely to succeed.
In one of the earliest English language commentaries on grafting, Mascall, in 1589, gave a broad overview of the written history of grafting.
a. Ancient Greece
Theophrastus, philosopher and pupil of Aristotle, is sometimes known as the Father of Horticulture. In his treatise on propagation, written ca. 300 BC, he describes grafting in a curious way, as if it was a variation on cutting propagation in which the scion takes root in the stock.
"...the twig uses the stock as a cutting uses the earth. So bud-grafting too is a kind of planting, and not a mere juxtaposition..."
And he attributes the growth of the scion to
"...the generative fluid: the bud possesses this when it is fitted into the stock, and getting its food from the latter produces its own type of sprout."
b. Ancient Roman Empire
(1). Virgil is well know for his epic poem on agriculture called the Georgics, which was written in the first century (29) BC, for the aristocratic (gentrified) farmers of his time. He discusses grafting and other methods of propagation at some length. In the quotation below and in the right frame, he is apparently describing first a technique akin to t-budding (a "rind" (bark) grafting technique in which the bark must be "slipping," as will be discussed below), followed by a non-rind or "cleft" grafting technique in which a scion is inserted well into a cut in the wood of the stock.
"Nor is the method of insterting eyes And grafting one: for where the buds push forth Amidst the bark, and burst the membranes thin, Even on the knot a narrow rift is made, Wherein from some strange tree a germ they pen, And to the moist rind bid it cleave and grow. Or, otherwise, in knotless trunks is hewn A breach, and deep into the solid grain A path with wedges cloven; then fruitful slips Are set herein, and- no long time- behold! To heaven upshot with teeming boughs, the tree Strange leaves admires and fruitage not its own."
-- from the Georgics, by Virgil, ca. 29 B.C.
(2) Early Christianity (Holy Land along the Eastern Mediterranean). The New Testament of the Bible (1st Century AD), was rich in imagery from everyday life. In the Book of Romans, 11:17-24, Christ describes a scion's dependence on the root stock in a parable to illustrate humankind's dependence on God.
(3) Ancient China
In the book, Tsee Ming Yau Su, written in the 3rd century by the Chinese writer Chia Shi-yi ( Westwood, 1993), grafting of pear is described. It is clear from these early writings that the effect of different rootstocks on scion fruit quality was well understood.
c. Middle Ages and Renaissance Europe
The art of grafting probably moved into Europe as crusaders returned from the holy lands in the 13th Century. Clonal selection of apples, at least partially by grafting, was underway in France during the 15th century. The Frenchman, Caylus, wrote a Historie du Rapproachement (History of Grafting) in 1806 which contains some fascinating drawings.
Grafting did not become common in England until well after it was widely practiced in France and other parts of Europe. Perhaps the earliest account of grafting written in the (old) English language was written by Leonard Mascall, in 1592, A book of the arte and manner how to plant and graffe all sorts of trees…; in the style of that time, the full title was 12 lines long. Mascall chided his countrymen for being slower to adopt "planting and graffyng," than other parts of the western world. He goes on to point out the advantages to be gained by adopting this practice in England.
Nearly a century later, in 1671, Robert Sharrock wrote The history of the propagation and improvement of vegetables by the concurrence of art and nature. In the chapter titled "Insitions" (archaic term for grafting), he describes in detail several grafting techniques which are illustrated in the lone figure from the book. These include shoulder grafting (shown in the top right portion of the figure), "grafting in the cleft" (lower right), "ablactation," which we know as approach grafting (lower left), whip grafting (upper right), which we often refer to as splice grafting, and several variations on budding (t-budding, cross-budding [my terminology], and patch budding, all along the main trunk), which Sharrock refers to as "inoculation."
2. What progress has there been in our understanding of the biology of grafting over the centuries?
a. Very little, in some respects
As Bailey pointed out, "Grafting is one of the oldest of the arts of plant craft." In many respects, it is a bit humbling to consider just how little modern scientific understanding of grafting has contributed to improvements in the practical aspects of this art.
(1) Most of the grafting methods (carpentry) practiced today have been practiced with little change for centuries.
(2) Little has changed in our understanding of the importance of scion wood selection from the donor plant. For example, in the 17th Century, Sharrock, in his History of Propagation and Improvement of Vegetables (1672) advised,
"Cyons are best chosen from the fairest, strongest shuits, not from under shoots or suckers, which will be long ere they bear fruit which is contrary to the intention of grafting".
Sharrock's accurate assessment of scion selection is still one of the most important reasons for grafting, i.e. selection from bearing wood (capable of flowering and fruiting) rather than from juvenile suckers, to avoid delayed flowering of the grafted plant.
b. Progress has been made in some respects
Although there are many aspects of grafting which have changed little over the centuries, there are some areas of divergence between past and present understanding of the biology of grafting. Several examples, described below, are compatibility, timing, and moisture management.
- Compatibility refers to the closeness of the taxonomic relationship between stock and scion necessary for a graft union to form, assuming all other requirements for successful graft union formation are met.
- It is interesting to compare Virgil's understanding, in 29 BC, of the taxonomic limitations of grafting to that of the author of the Book of Romans in about the 1st Century A.D, and with our modern understanding.
- Virgil had an incorrect, but probably quite common misunderstanding of interspecific graft compatibility which suggests that he may not have had first-hand experience, despite having written extensively on the subject.
- In the Book of Romans, on the other hand, there is a clear understanding that successful grafting is more likely if stock and scion are of the same genotype, i.e. "self" - as closely related as possible.
- LH Bailey, in his Standard Cyclopedia of Horticulture (1928), noted Virgil's error (above) and speculated that the view that any species was compatible with any other may have been a popular misconception, while the true genetic limits of compatibility may have been held as "a professional or class secret in the ancient times."
- In Robert Sharrock's book, History of Propagation and Improvement of Vegetables (1672) there is a section in his chapter on grafting which is subtitled, "what plants take on different kinds," which addresses the limits of stock-scion compatibility. Most of the compatible combinations that Sharrock identifies are accurate (e.g. pear/quince, apricot/plum, rose/"briar"(wild rose)). On the other hand, part of this chapter is rather fanciful and profoundly inaccurate. Most notably, he calls the reader's attention to extraordinary reports by others (which he does not claim to have witnessed himself) of the most bizarre plant/animal and even animal/animal grafts. His statement that "there are almost infinite stories of strange conjunctions which urge earnestly for credit" suggests that he finds these plant/animal grafts a credible possibility. Such claims seem reminiscent of the fantastic, some would say hideous, paintings of the Flemish painter Hieronymus Bosch.
- Although modern horticulturists have a better understanding of the limits of graft compatibility than earlier writers on the subject appeared to have had, misconceptions still abound. Ask a few of your non-gardening friends about their understanding of compatibility, and you may find that times have hardly changed since Virgil, at least in our non-agrarian modern (U.S.) society.
- On the other hand, while living in Kenya, a much more agriculturally-oriented society, where I taught horticulture in 1989, I asked a young boy to explain grafting to me. The boy said, in Kiswahili,
"Unakata ndima na machungwa halafu unaziweka pamoja kwa mti moja ya machungwa ama ndima"
Which translates, "You cut a lemon and an orange, then you put them together on one tree of orange or lemon."
Was he right? Check the section on compatibility below.
(2). Scion selection and timing
- Pliny the Elder (23-79 AD), a Roman natural historian, lawyer, and admiral, wrote his 37 volume Natural History at roughly the same time as the biblical Book of Romans (above). He was strongly opinionated on the question of how and when a scion should be taken for grafting. He wrote that the scion should not be cut while the wind is blowing, and insisted that
"A graft should not be used that is too full of sap, no by Hercules! No more than one that is dry and parched...it is a point most religiously observed to insert the graft during the moon's increase."
(3) Moisture management
- In earlier times, the stock/scion junction was often packed with a ball of wet mud and dung, or wet moss, to prevent desiccation of the cut surfaces. This is still practiced in Madagascar by resource-poor Tanala farmers.
- The modern "invention" and horticultural application of plastic and rubber represent one the more important advances in propagation and other aspects of modern horticulture. Rubber and plastics, which are impervious to moisture, are commonly used to tightly bind scion to stock. For example, budding rubbers (rubber bands) are used to tie off bud grafts. Plastic strips, although less desirable, are sometimes used for the same purpose. Plastic bags are used as "humidity tents."
- For additional information on modern practices see the section on Avoidance of desiccation in Requirements for Successful Grafting and Budding.
2. Additional information about the history of horticulture
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