Propagation by Grafting
Grafting is an old and well-established technique by which parts of two separate plants are joined and then grow into one. The lower part, called stock or rootstock, typically provides a hardier or more disease-resistant basis to the plant, or may limit the mature size of the specimen, while the scion, grafted onto the stock, consists of the desired fruit or flower-bearing portion.
Stock and scion must be compatible: unrelated plants cannot be randomly joined together. The closer related species are, the more likely one is successful. Thus, wine grapes can be grafted onto North American grape rootstock, multiple apple varieties onto one apple tree base, desir-able tomatoes onto disease-resistant stock, even tomatoes onto their po-tato relatives.
Collect unfrozen, healthy scions, each 8 to 12" long, during the winter, a few weeks before grafting actually takes place. Store in the fridge, in damp peat moss and enclosed in plastic; soak a few hours before grafting. Then make fresh cuts to trim scions to their desired length; leave at least 4 buds. Don't touch the cut ends, and when grafting, ensure that the original orientation of stock and scion remains the same, that is, that up stays up, and down stays down.
A very sharp blade is used, so that plant tissues are cut, not crushed. For healing to occur, the cambium of the scion, a thin layer of plant tissue found just below the bark or skin of the plant, must be linked to the cam-bium of the rootstock; it is from this layer that the xylem and phloem, which carry water and nutrients, derive. Thus, the outer tissues of both plants must be lined up before being held into place.
Ordinary tape is as effective as anything else, in terms of holding scion and stock together, until the plant heals. However, enclosing the graft site in plastic may be helpful; it helps prevent the site from drying out. Remove tape once the scion shows signs of growing and the callus at the graft site seems to strain against the tape.
If moving a grafted specimen, make sure that the bud union stays at least 2" above soil level, so that the scion cannot itself grow roots directly into the soil. Should this happen, desired rootstock properties may be lost altogether.
Three techniques are described, and are illustrated in sketch form.
The whip graft
This graft is commonly used with fruit trees, and is used to unite parts of about the same diameter, and is performed early in spring, just before bud break. Cut diagonally through both stock and scion, such that cut surfaces are about 2" long. Then line them up; if parts are of different sizes, line them up along one side, and don't center. Cambium must connect with cambium! Wrap up well, and allow to heal. Simple.
The cleft or wedge graft
Used to connect one or two smaller scions to a larger rootstock, this tech-nique is commonly used with fruit trees. It should again be done early in spring, before bud break. Use scions no larger than 1/4", each with several buds, and cut off an angled, 2" long piece on either side, so that the bottom part of the scion tapers to a point. Cut the rootstock, 1 to 4" diameter, off horizontally, then make a vertical split through its diameter, some 3" deep.
Next, gently ease the scions into each end of the split in the rootstock (use a small wooden wedge or screwdriver to hold it open), aligning cambium with cambium. The wider part of the scion should face outward. Remove wedge. Scions should be tightly held by the rootstock. Cover all cut surfaces with grafting wax, or a commercial tree wound sealer.
If only one scion is eventually desired, allow both to grow for a year, then remove the one that is least vigorous.
Side-veneer grafting
Used mainly with evergreens, Japanese maple , suitable also for birch and rhododendron, this technique is performed in late winter, indoors, on specimens a few years old. The rootstock may have to be brought into the home or greenhouse; do so at least one month before grafting, decrease watering several weeks before surgery. Scions 4 to 6" long are ideal, though smaller can be used; scions must have at least some mature, brown wood.
Remove leaves from the lower 6" of the rootstock, then a shallow wedge of wood some 2" long from the side of the stock, not far from its base. Trim leaves from the lower half of the scion, then cleanly taper it to fit this wedge, and tape into place.
Bottom heat and high humidity can be helpful, provide good drainage, water little but frequently. Keep air temperatures fairly low. After a few months, harden the plant off and move outside. Remove one-third off the top of the rootstock, another third in several weeks, and any remaining stock above the graft site later that summer, so that only the scion and any rootstock below it remain.
Bud grafting
Bud grafting consists of grafting a piece of scion bearing a single bud, on-to the rootstock. Two techniques are mentioned here, and may often be used interchangeably: T-budding, and chip-budding. Only the latter is illustrated in the Pictures section.
In T-budding, a 'T' is cut into the rootstock, and the bark is gently peeled back, so that a small section of scion, with bud, may be slid in behind these flaps of bark, and secured until the two unite. Even when the graft has taken well, however, the rootstock should not be cut back until the next winter.
A variant known as chip-budding may also be used: rather than cutting a 'T' into the rootstock, a small chip of wood is removed from the rootstock, and a matching chip of scion, with bud, is grafted into the gap. With both techniques, be careful when handling the bud chip: do not handle by the moist cambium on the inside of the new chip, but handle by the bud or outer bark only.
