Grafting
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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.
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 or flowers, and is called the scion. The reasons for grafting are:
- 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.
- because the scion has weak roots or the roots of the stock plants have roots tolerant of difficult conditions.
- in order to provide a strong tall trunk for certain ornamental shrubs and trees. In these cases, a grafting is carried out 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.
- to provide pollenizers. For example, in tightly planted or badly planned apple orchards of a single variety, limbs of crab apple may be grafted periodically 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.
- 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 aboveground and one underground. Cacti of widely different forms are also sometimes grafted on to each other. Multiple cultivars of fruits such as apples are sometimes grafted on a single tree. This 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 caretaker must be sufficiently trained to prune them correctly, or one strong variety will usually "take over".
In stem grafting, 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 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.
Grafting can only be done between reasonably closely related plants. Most often the limits of success are with other species in the same genus, though in some cases plants in different but closely related genera can graft successfully (e.g. Larch will graft on Douglas-fir), and in other cases, not, even between some species in the same genus (e.g., Norway maple will not graft on Sugar maple).
See also: Fruit tree propagation for more on grafting and budding.
For grafting of animal tissue, see medical grafting.
Honeybee female larvae are "grafted" from their cells into artificial queen cell cups by honeybee breeders to rear queen bees.