The Practice of GraftingTodd Kennedy
The practice of grafting is an ancient one. Theophrastus, the Greek plantsman credited as the founder of botany who died ca. 288 BC, described the joining of a root and branch, as distinguished from the rooting of cuttings, and catalogued the feasible combinations of fruit trees. It was he who demonstrated that not just any fruits could be joined with others, though some could be made to grow when grafted on unlike sorts of fruits as rootstocks. This is a difficult concept for our members even today, and this article concludes with a list of compatible fruit combinations.
A form of surgery, grafting is that part of horticulture that brings us closest to the realization that plants and animals are not so very unlike after all. The grafter works with living tissue, and works quickly, as plant parts perish in the same manner, and with nearly the same speed, as their animal counterparts.
A reasonable degree of precision is required. Although it is not necessary to match nerve with nerve, the grafter must match specialized tissue with its like. Foreign body rejection is likewise found among plants, and the rootstock's first response is to expel alien tissue and close the wound. Infection with fungi, bacteria and viruses are hazards also. Clean tools, hygienic surroundings and healthy scionwood are necessary for success.
Grafting is the joining of the lower portions of a plant (the "rootstock") with a substantial piece of plant (the "scion"—pronounced "sigh-on") to grow as a new branch or a whole new tree on the rootstock. If the scion is insubstantial, typically merely a single bud to be inserted into the rootstock stem, the practice is called budding - never "bud grafting," which is nonsensical. Grafting may be done with dormant, deciduous fruit trees or with evergreen fruit trees, though a dormant scion with a dormant rootstock is what we teach in our Spring Event. Budding is invariably done with growing, fully leafed plants, typically in late spring or summer.
There are several methods used in grafting woody plants. Particularly noted are cleft, whip, whip and tongue, side, and crown grafting. Each has its particular purpose and is optimally utilized under particular circumstances. This article deals only with cleft grafting, which is most intuitively obvious for beginners and makes best use of the properties of the plant. We will emphasize stone fruits, which seem to be especially difficult for HOS members. The scion is inserted into the rootstock, and the rootstock clamps the scion in place until it grows. Cleft grafting is most successful with rootstocks established in the ground for only 1-2 years. Whip grafting is best applied to bareroot rootstocks, before planting out, typically in large numbers in production-line fashion. The scion and rootstock must be matched in diameter and in angle of cut, are bound together and the whole is then planted out in the field. This requires considerable skill and speed, and cannot be performed so late in the spring that the scion would be forced into immediate growth, before callusing can occur.
Whip-and-tongue grafting is an elaboration of whip grafting. The scion and rootstock are further split in the zone of union to form tongues, which are fitted together. This effectively doubles the zone of contact and therefore makes success more likely than in whip grafting; it is more time consuming and therefore used only for scionwood of dubious viability or fitness, usually of very small material. Side grafting—the insertion of a scion directly into the trunk at an oblique angle—is feasible only on rootstocks of mature size, with thick xylem and phloem layers, when the top of the tree is needed to nurse the young graft, or to produce a profitable crop during the season of grafting. This is only used with apples.
Crown grafting is used for the top working or conversion of an entire mature tree to another variety. The major branches are cut at considerable diameter, and multiple scions are inserted around the periphery of the cut. It is a superstition of many growers to leave one branch ungrafted to nurse the rest for the first year. Today, labor skilled in this technique is rare and expensive; crown grafting was once common in conversion of orchards, but makes most sense with stone fruits, where replant disease makes growers unwilling to grub out and replace an orchard of established trees.
Cleft grafting begins with the collection of scions. Pencilthick, pencil-length, straight sticks make the best scions. The source tree for the scions must be fully dormant at collection time. Avoid collecting, or even contacting by hand, any infected parts of the tree (gummosis, cankers, or fireblight). Do not wait until the Spring Event to collect scions; scion collecting is done at the plant's convenience, not yours. Soon after the winter solstice, many fruit trees have begun breaking dormancy in California to be well expanded by New Year's Day, and some plums ('Inca'), all ume (a Japanese apricot), and Chinese quince are in bloom by mid-January.
Expansion of the buds, as well as exposure to room-temperature heat, shortens or ends the viability of scions. Bag and label all scions, and refrigerate them immediately. Use zip-lock plastic bags with a few drop of water; never use paper, which can form botrytis mold that attacks the scions. Many or most of the scions offered at our scion exchange have been left out at ambient temperatures, and this is the most likely cause of members' failure at grafting. Store scions at a temperature as close to 32°F as possible. Leaving them outside at temperatures over 50°F for a day or two will destroy their viability. When scions are not refrigerated, enzymes and microbes act quickly to ferment the sugars in the scions and destroy nitrogen compounds in their cambium layer.
The optimum rootstock is a young tree, in the ground for one year, with a diameter of 3/8 inch or so. Diameters over 1/2 inch are iffy, and rootstocks over one year old are predisposed to send up sprouts from below ground, in preference to uniting with the scion. Grafting onto established trees at a considerable distance above ground level is highly unsuccessful among stone fruits, always so if the entirety of the tree is not top-worked simultaneously.
Graft when freezing nighttime temperature will no longer occur. Cut the rootstock off within an inch or so of the ground using bypass-type shears (never anvil-type, which crush the wood beyond recovery). Loppers always crush the bark of the rootstock also. The bark and tissue immediately beneath it must be intact.
Scion preparation for the graft is a form of whittling, and may be done with a kitchen knife, so long as it has a well-honed cutting edge. Make a fresh cut at the bottom of the scion; then make two slanting cuts on the bias to meet at the center of the scion, the exposed wood of the cuts to be an inch or so in length. Slice away from your hands and body if you are a beginner; a band-aid around your thumb may give more confidence or serve as a reminder to be careful. At the sides will be a thin sliver of scion bark, diminishing to a sharp point. The resulting scion should be a pointed stick; cut the top, leaving three buds on the prepared scion.
For a cleft graft, lay the edge of the knife blade across the diameter of the rootstock and, with a hammer (a leather one makes discreet taps), pound it in, splitting the stock in two, to a length of an inch or so. Try to achieve straight, not ragged, sides to the split; this is difficult when using a steel hammer. A second knife may help to extract the first, at this stage. Keep the first knife inserted, at the center or one side of the split, and insert your scion at the other edge of the split. Except in the case of bench grafting, it is emphatically unnecessary to have the scion and rootstock of the same diameter; in fact, the writer always inserts two scions in every cleft graft, at opposite sides of the split. Sometimes they touch at the center, sometimes not—it hardly matters; but so done, the scions are of course always smaller than the rootstock in diameter.
The next step is the most important. With the knife still inserted and tweaked to allow movement of the scion in the split, jiggle the scion to the outer edge, then back ever so slightly, until the exposed green line in the cut surfaces of the scion matches with a comparable line in the periphery of the rootstock. This line is the cambium layer; it is the only living part of the tree trunk, and it is essential that the respective cambia are matched. Then insert a second scion at the opposite end of the split, if your rootstock can accommodate it, keeping the knife in long enough to allow the insertion. This second scion is more difficult to move and match with the cambium of the rootstock. Experience is the grafter's only teacher for matching of cambia.
Wrap the terminal portion of the rootstock upwards from where the split is evident with a non-adhesive nurseryman's tape. Never use electrician, Teflon, or masking tape. Pull the tape tightly, or the rootstock will squeeze the scions out. Never wrap across the top of the rootstock surface.
Do not wrap the scions because this will pull them out of cambium contact for sure. Wrapping the entirety of any part in Parafilm is unnecessary and practically guarantees that cambium contact will be destroyed. Use a small paint brush to paint the cut surfaces and taped portion of the rootstock with pruning paint—avoid the aerosol type, which has weed killer action. Using delicate strokes, paint only the cut tips and exposed cut surfaces of the scions. Never paint over the buds, which will prevent their growth. Avoid polymer paints, such as Doc Farwell's—they permanently interrupt the "knit" of the wound into continuous wood. Such grafts always fail to form a sound union.
The graft is done. Label it with name of the fruit variety and return periodically to monitor the rootstock and to pull away any sprouts from below the graft, during the next several months.
Rootstock Compatibility Chart
|European Pear (OHx)||European Pear, Northern Spy Apple only, Hawthorn (various species), Medlar (imperfectly compatible), Nashi (Asian pear)|
|Quince||Pear (always dwarfing, many varieties incompatible), Quince, Loquat (dwarfing)|
|Peach||Peach, Nectarine, European Plum (short-lived, many varieties incompatible), Almond, Ume, Apricot, Plumcot, Asian Plum|
|Almond||Almond Almond, Peach, Nectarine, Asian Plum, Apricot, European Plum (many varieties incompatible), Ume, Plumcot|
|Cherry (Mazzard & Mahaleb)||Sweet Cherry, Sour Cherry|
|Apricot||Apricot, Plumcot, Ume, European Plum (most varieties incompatible), Peach (short-lived), Nectarine (short-lived), Asian Plum|
|Cherry Plum (Myrobolan & Marianna)||Asian Plum, European Plum (some varieties incompatible), Plumcot, Ume|
|Asian Pear (Pyrus calleryana, P. ussuriensis)||Nashi (Asian pear), Asian pear hybrids, European pear (fruit quality affected, some partial or delayed incompatibility)|