Grafting

Grafting is a method of plant propagation widely used in horticulture, where the tissues of one plant are encouraged to fuse with those of another. It is most commonly used for the propagation of trees and shrubs grown commercially. (Grafting is limited to dicots and gymnosperms. Monocots lack the vascular cambium required.)

In most cases, one plant is selected for its roots, and this is called the stock or rootstock. The other plant is selected for its stems, leaves, flowers, or fruits and is called the scion.

In stem grafting, a common grafting method, a shoot of a selected, desired plant cultivar is grafted onto the stock of another type. In another common form called budding, a dormant side bud is grafted on the stem of another stock plant, and when it has fused successfully, it is encouraged to grow by cutting out the stem above the new bud.

For successful grafting to take place, the vascular cambium tissues of the stock and scion plants must be placed in contact with each other. Both tissues must be kept alive till the graft has taken, usually a period of a few weeks. Successful grafting only requires that a vascular connection takes place between the two tissues. A physical weak point often still occurs at the graft, because the structural tissue of the two distinct plants, such as wood may not fuse.

Reasons for grafting

To induce dwarfing or cold tolerance or other characteristics to the scion. Most apple trees in modern orchards are grafted dwarf or semi-dwarf trees planted at high density. They provide more fruit per unit of land, higher quality fruit, and reduce the danger of accidents by harvest crews working on ladders.
Because the scion is difficult to propagate vegetatively by other means, such as by cuttings. In this case, cuttings of an easily rooted plant are used to provide a rootstock. In some cases, the scion may be easily propagated, but grafting may still be used because it is commercially the most cost-effective way of raising a particular type of plant.
To speed maturity of hybrids in fruit tree breeding programs. Hybrid seedlings may take ten or more years to flower and fruit on their own roots. Grafting can reduce the time to flowering and shorten the breeding program.
Because the scion has weak roots or the roots of the stock plants have roots tolerant of difficult conditions. e.g. many showy Western Australian plants are sensitive to dieback on heavy soils, common in urban gardens, and are grafted onto hardier eastern Australian relatives. Grevilleas and eucalypts are examples.
In order to provide a strong, tall trunk for certain ornamental shrubs and trees. In these cases, a graft is made at a desired height on a stock plant with a strong stem. This is used to raise 'standard' roses, which are rose bushes on a high stem, and it is also used for some ornamental trees, such as certain weeping cherries.
To provide pollenizers. For example, in tightly planted or badly planned apple orchards of a single variety, limbs of crab apple may be grafted at regularly spaced intervals onto trees down rows, say every fourth tree. This takes care of pollen needs at blossom time, yet does not confuse pickers who might otherwise mix varieties while harvesting, as the mature crab apples are so distinct from other apple varieties.
To repair damage to the trunk of a tree which would prohibit nutrient flow, such as the stripping of the bark by rodents which completely girdles the trunk. In this case a bridge graft may be used to connect the tissues receiving flow from the roots to the tissues above the damage which have been severed from the flow. Where a watersprout, sucker or sapling of the same species is growing nearby, any of these can be grafted to the area above the damage by a method called inarch grafting. These alternatives to scions must be of the correct length to span the gap of the wound.
To change the cultivar in a fruit orchard to a more profitable cultivar, called topworking. It may be faster to graft a new cultivar onto existing limbs of established trees than to replant an entire orchard.
For raising curiosities:
A practice sometimes carried out by gardeners is to graft related potatoes and tomatoes so that both are produced on the same plant, one above ground and one underground.
Cacti of widely different forms are sometimes grafted on to each other.
Multiple cultivars of fruits such as apples are sometimes grafted on a single tree. This so-called "family tree" provides more fruit variety for small spaces such as a suburban backyard, and also takes care of the need for pollenizers. The drawback is that the gardener must be sufficiently trained to prune them correctly, or one strong variety will usually "take over".
Occasionally, a so-called "graft hybrid" or "chimaera" can occur where the tissues of the stock continue to grow within the scion. Such a plant can produce flowers and foliage typical of both plants as well as shoots intermediate between the two. The best-known example is probably +''Laburnocytisus'' 'Adamii', a graft hybrid between laburnum and broom, which originated in a nursery near Paris, France in 1825. This small tree bears yellow flowers typical of Laburnum anagyroides, purple flowers typical of Chamaecytisus purpureus and curious coppery-pink flowers which show characteristics of both "parents".
Ornamental and functional, arborsculpture uses grafting techniques to join separate trees or parts of the same tree to itself. Furniture, hearts, entry archways are examples. Axel Erlandson was a prolific arborsculptor growing over 75 mature shaped and grafted trees.

Grafting Methods

The easiest and most common form of grafting is cleft grafting. This method simply splits the stock and inserts the scion. The stock is best when 2-7 cm in diameter and the cleft is optimum when around seven centimeters deep. At that point the scion (best when they have 3-5 buds.) can be cut in a wedge shape and inserted into the tree with the cambium. Use grafting compound to cover the bare stock. This must be done because otherwise the cambium layer will quickly dry out and the graft will be unsuccessful.

Cleft grafting requires a large section of the tree’s framework to be cut. Stub grafting however retains the shape of a tree and is useful when a stock of 2-7 centimeters is not available. Also scions are generally of 6-8 buds in this process. Take the branch and make a small incision of one centimeter above. Then as before, wedge the scion and force it into the branch making sure that the angle of the scion to the parent tree is no more than 35º so that the crotch may remain strong. Cover it all with grafting compound.

Awl grafting takes the least resources and the least time must is best done by an experienced grafter for fear that the grafter would accidentally drive his tool too far into the stock and then the scion would have a reduced chance of survival. Awl grafting can be done by using a screwdriver to make a slit in the bark but just so far as the cambium layer. Then inset the wedged scion into the incision.

This final method is for stocks that are far bigger than practical for cleft grafting. Veneer or inlay grafting is for stocks that are larger than three centimeters in diameter. A stock is taken and cut. Then clefts are made of the same size as the scion (which is recommended to be the thickness of a lead pencil). Then simply wedge the scion and insert it. Wrap the scion in PVC tape to the scaffolding branches to give it more strength. Remember to set the scions to the side of the scaffolding branches and not on top.

Another great way to graft is something called Renewing fusion in which new species of plants can be easily created. What you do is take a limb at least a centimeter wide, then you find the tree you would like to graft it to, find one the main branches on that tree, measure to make sure it is at least 3 inches long and 2 and a half inches long and divide by how long the branch is. Shave that many centimeters of the bark. Place the branch you would like to graft on the center of the shaved area then drill in the circle to the core of the branch then insert the twig. Tape around it with a thin strip of duct tape to prevent insects from getting in the excess part of the whole and hold it up with diagonal lashings.

Natural grafting

The tree roots of the same species will sometimes naturally graft where they contact each other. A group of trees can share water and mineral nutrients via root grafts, which may be advantageous to weaker trees. A problem with root grafts is that they allow transmission of certain pathogens, such as Dutch elm disease. Natural grafting also sometimes occurs where two stems on the same tree, shrub or vine contact each other. This is most common in plants such as strawberries and potatoes.

Scientific uses

Grafting has been important in flowering research. Leaves or shoots from plants induced to flower can be grafted onto uninduced plants and transmit a floral stimulus that induces them to flower.^[1]

The transmission of plant viruses has been studied using grafting. Virus indexing involves grafting a symptomless plant that is suspected of carrying a virus onto an indicator plant that is very susceptible to the virus so shows symptoms quickly.

Herbaceous grafting

Nonwoody plants are often grafted including tomato, cucumber, eggplant and watermelon.^[2] The main advantage of grafting is for disease-resistant rootstocks. In Japan an automated process using grafting robots exists.

Other meanings

See: graft (disambiguation page)
Honeybee female larvae are "grafted" from their cells into artificial queen cell cups by honeybee breeders to rear queen bees.

External link

Citations

[1] Lang, A., Chailakhyan, M.K. and Frolova, I.A. 1977. Promotion and inhibition of flower formation in a dayneutral plant in grafts with a short-day plant and a long-day plant. Proc Natl Acad Sci U S A. 74(6): 2412-2416. [1]
[2] http://www.ars.usda.gov/is/AR/archive/jul05/fruit0705.htm, Core, J., Grafting watermelon onto squash or gourd rootstock makes firmer, healthier fruit., 53, Agricultural Research, 2005.

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