This is a method of plant propagation in which a twig of one plant, called the scion, is made to grow on the roots of another plant, called the stock or the under stock. The scion is taken from the plant which is to be reproduced, and because this is an asexual means of reproduction, the resulting plant will be identical with the plant from which the scion was taken if no shoots are allowed to grow on the understock, or below the union of scion and understock.
In the first place, a scion and stock must be “compatible,” that is, they must be of a type that will grow together, make a firm union and continue to grow afterwards. This is found out by experience, and the art of grafting is centuries old. One should not expect a scion of Magnolia soulangiana to grow on apple or elm understock. In fact, it may not grow on all kinds of magnolia seedlings, but experience has shown that it may be expected to grow best on understock of M. kohus or M. tripetala. Sometimes several species in the same genus are equally good as understock, sometimes it has been found that under certain conditions one is better than another. English growers find that Prunus serrula is easily compatible with P. avium understock, while some American growers have better success with P. saigentii understock.
In the grafting operation the theory is to place the living cambium tissue of scion and under-stock in contact with each other. This is simply done by making the proper kind of cut into understock and gently slipping the whittle scion into it. This operation must be done both scion and understock is about ready for active growth. Actually, it is a greenhouse operation where the understock, in a pot, has already been forced into active growth and the scion is yet dormant. Usually Feb. or March is grafting time indoors.
As soon as the scion and stock are slipped together, they are bound tightly in place to prevent movement between them. Flat rubber bands are specially made for this and arc ideal since they can be bound just tight enough to hold the a together, but loose enough so that the rubber will give a little as the new graft increases in circumference. This “tie” remains on for a few months until stock and scion have closely knit, when it is simply cut and left on the union to eventually fall off.
Grafting is also done out of doors at a time just before vigorous growth commences on the understock. This is usually confined to trees that are being “made over” as will be explained below. Grafting small plants out of doors does not result in as much success as doing it inside under controlled temperature and moisture conditions.
Factors other than timing must be right. Air temperature must be in the 70’s or conducive to continued plant growth. Moisture must be present— the graft union must not be allowed to dry out in any way. Disease spores must not get into the union. These are the reasons why grafting is usually carried out in the greenhouse in “grafting cases,” places enclosed by glass or polyethylene with high humidity. This is also the reason why the graft union is covered with wax or polyethylene film as soon as it is made to keep the tender new-forming cells from exposure and possible drying out.
The most difficult time for the new graft is when it is noted that the scion has started into active growth. It must be kept in active growth, yet too high a temperature and too much moisture in the grafting case may cause it to grow too rapidly and fail to make a proper union. Here experience certainly aids the individual in properly regulating moisture and temperature.
The most important decision to make in grafting is to select the right kind of graft cut to make. Actually, if properly done, any one of the methods should result in a successful graft, but experience has shown that certain types of grafting cuts seem to result in better end results on certain species than others. This is not the place to go into this detailed discussion, but one should be familiar with the different types.
A double matching cut is made in both stock and scion. If this is to be used, the understock and the scion should be about the same dia. (lead-pencil size or slightly larger) and the top of the stock should be completely cut of an inch or so above the place of the proposed union. The whip graft is also ideal to use in root grafting, that is, in grafting a scion on to a piece of root (of the same dia.). If done properly, and it takes experience to make just the right cuts, it can be highly successful because there is so much of the cambium that can be fitted together in this double cut.
Merely making a slanting cut into the stock and inserting a wedge-shaped scion into it. This is often the method chosen when the stock is larger in dia. than the scion. Sometimes the top of the stock is severed just above the graft as soon as it is made. Other times it is left to grow for a week or two and then cut off. It is this method which is frequently used in bonsai culture to supply a new branch at exactly the right place it is needed.
This is the type of graft employed in grafting trees in the open. It is frequently employed in grafting apples, where a tree of an old outmoded variety is to be “changed over” to a new and better variety. In this case, all the main limbs are sawed off carefully and “clefts” or splits are made in them with a large grafting tool similar to a butcher’s knife. Two or 3 such openings can be made in a 4-6 in. branch. Then, 3 or more wedge-shaped scions are inserted carefully, so the cambium tissue of stock and scion meet exactly, the entire union is painted with wax and one then awaits developments. Only 1 of the 3 or 4 scions will be allowed to grow eventually, but it is a quick method of making over a tree of bearing size. In fact, the newly made-over tree may grow so well that it may begin producing the new variety of apple in 3 or 4 years. Because the stock is so large, it is not necessary to stick stock and scion together in such a graft, for the properties inherent in the wood of the large branches are enough to hold the scions tightly for all practical purposes.
This is done on the cut limbs of trees, merely by slitting the bark a few inches in a straight line from the cut surface, and then inserting the wedge-shaped scion just between the bark and wood of the stock. Three or 4 of these can be inserted, but the whole should be tied tightly to prevent the bark of the branch from curling away or splitting farther and thus exposing the scion to drying out.
Consider 5 small plants in pots, one of a very rare type, the other a worthless seedling. The seedling would act as the understock for the rare plant, the whole idea being that one does not risk loss of the rare plant in this method. A cut is made in the side of the understock or some of the bark is very carefully removed just to the cambium. A matching cut is made in the rare plant or a matching part of its bark is carefully removed. Then they are joined together, tied and painted, while the tops of both are allowed to grow. If it is obvious that the union is growing together, the part of the understock above the graft can be removed. When the union is solidly made, the scion material can be partially cut away from the scion plant, and later another deeper cut made so that the severance of the scion from the parent plant takes place over a period of weeks. This is a method used by the experts to graft two plants together that are otherwise difficult.
Sometimes a plant is growing with the wrong kind of understock and the tree needs a better root system. Young plants are established at the base, a narrow strip of bark removed on the tree to correspond with a similar width of bark removed on the new plants. Several such plants can be stabilized and grafted to one trunk.
Bridge grafting over a jagged cut breaking the bark completely around a tree. The scions are taken from the living branches of the same tree and covered with wax to finish the operation.
The bridge grafts can be nailed or tied in place and the whole exposed wound painted with wax. The idea here is to cut the top and bottom of the scions in such a way that they can be inserted underneath the bark above and below the wound, with the cambium tissues of stock and scion in contact with each other. Hence these shoots will, if they grow, act as bridges over the injured trunk for the upward and downward flow of nutrients and foods. Many a damaged tree has been saved in this way. However, such an operation is best done when the tree is dormant, certainly not when the scions are in leaf. If properly done, these scions will gradually increase in size and may completely heal over the injured trunk by growing solidly together.
Sometimes this method is used, especially to produce a dwarf plant or when one kind of plant material is not compatible with the under-stock. Grafting is done in the normal way using an “intermediate” scion, that is, a scion from a plant that will be compatible with the understock and the plant to be grafted as well. An example is the grafting of Bartlett Pear on quince roots for dwarfing. These a are not compatible but if a seedling pear is grafted on the quince roots, then the Bartlett Pear grafted on the seedling pear, this results in a good tree. This can be done in different ways. The Bartlett Pear can be grafted on pieces of the seedling pear and the grafted pieces put in moistened peat moss in a cool place for a few weeks until callused, when this piece (with scion and intermediate graft) is grafted onto a dormant quince root. Or, the seedling pear can be budded onto rooted cuttings of the quince one summer, and then the Bartlett Pear can be budded on the seedling pear the next.
So, grafting can be a complicated process but, if done properly, results in fairly good trees. Sometimes, years after the grafting, one notices a large hump at the graft union, showing clearly that the stock has grown much faster than the scion. Sometimes just the reverse is true. Frequently this is not serious but, whenever possible, it is always best to select an understock that grows at the same rate as the plant from which the scion was taken.