Grafting

I

Introduction

Grafting, joining the cut surfaces of plants under circumstances that will allow physiological union. The supporting plant is called the stock; the plant part that is grafted to it is called the scion. The scion may be a twig, stem, bud, or other part of a plant; grafting in which the scion used is a bud is called budding. The ability of the cut surface to heal is dependent on intimate contact between the cambium layers of the scion and the stock. The cambium layer, a ring of reproductive tissue around the stem, produces callus tissue; this tissue is composed of large, undifferentiated cells. Callus tissue, in a successful graft, differentiates to form food-conducting vessels, water-conducting vessels, and a cambium layer, which connect the corresponding tissues of scion and stock.

Grafting is frequently used to combine advantageous characteristics of scion and stock. For example, branches or buds of trees known to produce good fruit are grafted to stronger trees that produce fruit of indifferent quality. Grafting is also used in propagating seedless fruits, such as seedless oranges and grapes.

II

Grafting Methods

Grafts are usually unsuccessful unless plants of the same or closely related kind are used. A seedling or cutting, usually chosen for hardiness and resistance to pests and diseases, or for special characteristics such as dwarfing, is commonly used as the stock in grafting. When a seedling is used as a stock, it is first allowed to establish its root system; a graft is then inserted at the base of the stem. As soon as the union of scion and stock has become established, any remaining portions of the shoot of the stock are cut away; all substances absorbed by the root system are thereafter available to the scion for its development. When a cutting is used, the graft is made first, and then the cutting is rooted. Nursery stock for rubber plantations and for many fruit orchards is produced by the seedling type of grafting.

The area surrounding the juncture of scion and stock is protected by paraffin or by grafting wax, which consists of a mixture of beeswax, beef tallow, and resin. Grafting tape is usually used to cover the waxed wound, so as to exclude moisture and prevent attack by diseases or pests.

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III

Grafting Types

The most popular types of professional grafting are splice grafting, whip or tongue grafting, saddle grafting, cleft grafting, and side grafting. In splice grafting, a simple diagonal cut is made in scion and stock. The cut surfaces are joined, covered with grafting wax or paraffin, and bound with grafting tape. In whip grafting, a sawtooth diagonal is used to provide a larger surface. Saddle grafting is accomplished by cutting one stem, usually the stock, in the shape of a wedge, and cutting the other stem in the shape of a slot that will fit the wedge tightly. In cleft grafting, branches of the stock are sawed straight across, and the stub of the branch is split to depth approximating the diameter of the stock. A temporary wedge, sufficient to open the slit, is inserted at the center of the split. Two scions, each having several buds, are inserted at the edges of the split, and are adjusted so that the lowest bud is close to the top of the stock and facing outward. The wedge is removed without displacing the scions, and the cut is covered with grafting wax and, if necessary bound with grafting tape. Sometimes, more than two scions are used, with all the scions but the strongest being removed in the following season. Cleft grafting is the most widely used method for grafting scions that produce high-quality fruit on mature trees. Side grafting is useful for grafting new branches onto bare spaces of tree trunks. In this method, a longitudinal cut is made through the bark layer. The scion, with its base cut in a wedge shape, is inserted beneath the bark, tied, and then waxed.