Grafting

Approach grafting or inarching is used to join together plants that are otherwise difficult to join. The plants are grown close together, and then joined so that each plant has roots below and growth above the point of union.[6] Both scion and stock retain their respective parents that may or may not be removed after joining. Also used in pleaching. The graft can be successfully accomplished any time of year.[7]

T budding

Bud grafting (also called chip budding or shield budding) uses a bud instead of a twig.[8] Grafting roses is the most common example of bud grafting. In this method a bud is removed from the parent plant, and the base of the bud is inserted beneath the bark of the stem of the stock plant from which the rest of the shoot has been cut. Any extra bud that starts growing from the stem of the stock plant is removed. Examples: roses and fruit trees like peaches.

Budwood is a stick with several buds on it that can be cut out and used for bud grafting. It is a common method of propagation for citrus trees.[9][10][11]

Successful cleft graft after 2 years' growth

Same graft after 4 years' growth

In cleft grafting a small cut is made in the stock and then the pointed end of the scion is inserted in the stock. This is best done in the early spring and is useful for joining a thin scion about 1 cm (3⁄8 in) diameter to a thicker branch or stock. It is best if the former has 3–5 buds and the latter is 2–7 cm (3⁄4–2+3⁄4 in) in diameter. The branch or stock should be split carefully down the middle to form a cleft about 3 cm (1+1⁄8 in) deep. If it is a branch that is not vertical then the cleft should be cut horizontally. The end of the scion should be cut cleanly to a long shallow wedge, preferably with a single cut for each wedge surface, and not whittled. A third cut may be made across the end of the wedge to make it straight across.

Slide the wedge into the cleft so that it is at the edge of the stock and the centre of the wedge faces are against the cambium layer between the bark and the wood. It is preferable if a second scion is inserted in a similar way into the other side of the cleft. This helps to seal off the cleft. Tape around the top of the stock to hold the scion in place and cover with grafting wax or sealing compound. This stops the cambium layers from drying out and also prevents the ingress of water into the cleft.

Graft needing attention the following season

Successful whip graft

In whip grafting the scion and the stock are cut slanting and then joined. The grafted point is then bound with tape and covered with a soft sealant to prevent dehydration and infection by germs. The common variation is a whip and tongue graft, which is considered the most difficult to master but has the highest rate of success as it offers the most cambium contact between the scion and the stock. It is the most common graft used in preparing commercial fruit trees. It is generally used with stock less than 1.25 cm (1⁄2 in) diameter, with the ideal diameter closer to 1 cm (3⁄8 in) and the scion should be of roughly the same diameter as the stock.

The stock is cut through on one side only at a shallow angle with a sharp knife. (If the stock is a branch and not the main trunk of the rootstock then the cut surface should face outward from the centre of the tree.) The scion is similarly sliced through at an equal angle starting just below a bud, so that the bud is at the top of the cut and on the other side than the cut face.

In the whip and tongue variation, a notch is cut downwards into the sliced face of the stock and a similar cut upwards into the face of the scion cut. These act as the tongues and it requires some skill to make the cuts so that the scion and the stock marry up neatly. The elongated "Z" shape adds strength, removing the need for a companion rod in the first season (see illustration).

The joint is then taped around and treated with tree-sealing compound or grafting wax. A whip graft without a tongue is less stable and may need added support.

Successful stub graft, healed

Stub grafting is a technique that requires less stock than cleft grafting, and retains the shape of a tree. Also scions are generally of 6–8 buds in this process.

An incision is made into the branch 1 cm (3⁄8 in) long, then the scion is wedged and forced into the branch. The scion should be at an angle of at most 35° to the parent tree so that the crotch remains strong. The graft is covered with grafting compound.

After the graft has taken, the branch is removed and treated a few centimeters above the graft, to be fully removed when the graft is strong.

The four-flap graft (also called banana graft) is commonly used for pecans, and first became popular with this species in Oklahoma in 1975. It is heralded for maximum cambium overlap, but is a complex graft. It requires similarly sized diameters for the rootstock and scion. The bark of the rootstock is sliced and peeled back in four flaps, and the hardwood is removed, looking somewhat like a peeled banana. It is a difficult graft to learn.

Awl grafting takes the least resources and the least time. It is best done by an experienced grafter, as it is possible to accidentally drive the tool too far into the stock, reducing the scion's chance of survival. Awl grafting can be done by using a screwdriver to make a slit in the bark, not penetrating the cambium layer completely. Then inset the wedged scion into the incision.

Veneer grafting, or inlay grafting, is a method used for stock larger than 3 cm (1+1⁄8 in) in diameter. The scion is recommended to be about as thick as a pencil. Clefts are made of the same size as the scion on the side of the branch, not on top. The scion end is shaped as a wedge, inserted, and wrapped with tape to the scaffolding branches to give it more strength.

Rind grafting involves grafting a small scion onto the end of a thick stock. The thick stock is sawn off, and a ~4 cm long bark-deep cut is made parallel to the stock, from the sawn-off end down, and the bark is separated from the wood on one or both sides. The scion is shaped as a wedge, exposing cambium on both sides, and is pushed in under the back of the stock, with a flat side against the wood.

White spruce can be grafted with consistent success by using 8–10 cm (3–4 in) scions of current growth on thrifty 4- to 5-year-old rootstock (Nienstaedt and Teich 1972).[16] Before greenhouse grafting, rootstocks should be potted in late spring, allowed to make seasonal growth, then subjected to a period of chilling outdoors, or for about 8 weeks in a cool room at 2 °C (Nienstaedt 1966).[17]

A method of grafting white spruce of seed-bearing age during the time of seed harvest in the fall was developed by Nienstaedt et al. (1958).[18] Scions of white spruce of 2 ages of wood from 30- to 60-year-old trees were collected in the fall and grafted by 3 methods on potted stock to which different day-length treatments had been applied prior to grafting. The grafted stock were given long-day and natural-day treatments. Survival was 70% to 100% and showed effects of rootstock and post-grafting treatments in only a few cases. Photoperiod and temperature treatments after grafting, however, had considerable effect on scion activity and total growth. The best post-grafting treatment was 4 weeks of long-day treatment followed by 2 weeks of short-day treatment, then 8 weeks of chilling, and finally long-day treatment.

Since grafts of white spruce put on relatively little growth in the 2 years after grafting, techniques for accelerating the early growth were studied by Greenwood (1988)[19] and others. The cultural regimes used to promote one additional growth cycle in one year involve manipulation of day length and the use of cold storage to satisfy chilling requirements. Greenwood took dormant potted grafts into the greenhouse in early January then gradually raised the temperature during the course of a week until the minimum temperature rose to 15 °C. Photoperiod was increased to 18 hours using incandescent lighting. In this technique, grafts are grown until elongation has been completed, normally by mid-March. Soluble 10-52-10 fertilizer is applied at both ends of the growth cycle and 20-20-20 during the cycle, with irrigation as needed. When growth elongation is complete, day length is reduced to 8 hours using a blackout curtain. Budset follows, and the grafts are held in the greenhouse until mid-May. Grafts are then moved into a cooler at 4 °C for 1000 hours, after which they are moved to a shade frame where they grow normally, with applications of fertilizer and irrigation as in the first cycle. Grafts are moved into cold frames or unheated greenhouse in September until January. Flower induction treatments are begun on grafts that have reached a minimum length of 1.0 m. Repotting from an initial pot size of 4.5 litre to 16 litre containers with a 2:1:1 soil mix of peat moss, loam, and aggregate.

In one of the first accelerated growth experiments, white spruce grafts made in January and February that would normally elongate shortly after grafting, set bud, and remain in that condition until the following spring, were refrigerated for 500, 1000, or 1500 hours beginning in mid-July, and a non-refrigerated control was held in the nursery.[19] After completion of the cold treatment, the grafts were moved into the greenhouse with an 18-hour photoperiod until late October. Height increment was significantly (P 0.01) influenced by cold treatment. Best results were given by the 1000-hour treatment.[19]

The refrigeration (cold treatment) phase was subsequently shown to be effective when applied 2 months earlier with proper handling and use of blackout curtains, which allows the second growth cycle to be completed in time to satisfy dormancy requirements before January (Greenwood et al. 1988).[19]

As humans began to domesticate plants and animals, horticultural techniques that could reliably propagate the desired qualities of long-lived woody plants needed to be developed. Although grafting is not specifically mentioned in the Hebrew Bible, it is claimed that ancient Biblical text hints at the practice of grafting. For example, Leviticus 19:19, which dates to around 1400 BCE, states "[the Hebrew people] shalt not sow their field with mingled seed..." (King James Bible). Some scholars believe the phrase mingled seeds includes grafting, although this interpretation remains contentious among scholars.

Grafting is also mentioned in the New Testament. In Romans 11, starting at verse 17, there is a discussion about the grafting of wild olive trees concerning the relationship between Jews and Gentiles.

By 500 BCE grafting was well established and practiced in the region as the Mishna describes grafting as a commonplace technique used to grow grapevines.[25]

According to recent research: "grafting technology had been practiced in China before 2000 BC".[26] Additional evidence for grafting in China is found in Jia Sixie's 6th century CE agricultural treatise Qimin Yaoshu (Essential Skills for the Common People).[27] It discusses grafting pear twigs onto crab apple, jujube and pomegranate stock (domesticated apples had not yet arrived in China), as well as grafting persimmons. The Qimin yaoshu refers to older texts that referred to grafting, but those works are missing. Nonetheless, given the sophistication of the methods discussed, and the long history of arboriculture in the region, grafting must have already been practiced for centuries by this time.

In Greece, a medical record written in 424 BCE contains the first direct reference to grafting. The title of the work is On the Nature of the Child and is thought to be written by a follower of Hippocrates. The language of the author suggests that grafting appeared centuries before this period.

In Rome, Marcus Porcius Cato wrote the oldest surviving Latin text in 160 BCE. The book is called De Agri Cultura (On Farming Agriculture) and outlines several grafting methods. Other authors in the region would write about grafting in the following years, however, the publications often featured fallacious scion-stock combinations.

During the European Dark Ages, Arabic regions were experiencing an Islamic Golden Age of scientific, technological, and cultural advancement. Creating lavishly flourished gardens would be a common form of competition among Islamic leaders at the time. Because the region would receive an influx of foreign ornamentals to decorate these gardens, grafting was used much during this period.[25]

Possible deliberate grafts on a sessile oak in Ayrshire, Scotland

After the fall of the Roman Empire, grafting survived in the Christian monasteries of Europe until it regained popular appeal during the Renaissance. The invention of the printing press inspired a number of authors to publish books on gardening that included information on grafting. One example, A New Orchard and Garden: Or, the Best Way for Planting, Graffing, and to Make Any Ground Good for a Rich Orchard, Particularly in the North, was written by William Lawson in 1618. While the book contains practical grafting techniques, some even still used today, it suffers from exaggerated claims of scion-stock compatibility typical of this period.

While grafting continued to grow in Europe during the eighteenth century, it was considered unnecessary in the United States as the produce from fruit trees was largely used either to make cider or feed hogs.[25]

Beginning in 1864, and without warning, grapevines across France began to sharply decline. Thanks to the efforts of scientists such as C. V. Riley and J. E. Planchon, the culprit was identified to be phylloxera, an insect that infests the roots of vines and causes fungal infections. Initially, farmers unsuccessfully attempted to contain the pest by removing and burning affected vines. When it was discovered that phylloxera was an invasive species introduced from North America, some suggested importing rootstock from the region as the North American vines were resistant to the pest. Others, opposed to the idea, argued that American rootstocks would imbue the French grapes with an undesirable taste; they instead preferred to inject the soil with expensive pesticides. Ultimately, grafting French vines onto American rootstocks became prevalent throughout the region, creating new grafting techniques and machines. American rootstocks had trouble adapting to the high soil pH value of some regions in France so the final solution to the pandemic was to hybridize the American and French variants.[25]