Plant Propagation (Continued)
Propagating with Grafting
Grafting is the act of joining two plants to create one plant. The scion is the top part of the grafted plant and is a piece of a shoot with wood, bark, and dormant buds which will produce the stem and branches. The rootstock or understock is the bottom part of the grafted plant and will become the root and possibly part of the trunk. The cambium is the layer of cells between the wood and the bark which produces new bark and wood cells, and the cambium of the scion must be touching the cambium of the understock at the point where the two plants are grafted together. If a third layer is grafted between the scion and understock, then it is referred to as the interstem and will become part of the trunk.
Grafting is often used on certain cultivars that are not true-to-seed meaning the plant’s seed does not necessarily produce a plant with all the exact same characteristics as the plant from which the seed came. It is also used other plants that are problematic when propagating with cutting methods.
It should be noted that not all plants can be grafted. Typically, only plants that are closely related botanically can be successfully grafted. This is because the scion and understock must be compatible for the graft to form a strong union. There is no way to know two plants are compatible for grafting other than through trial-and-error. But generally, the closer the plants are taxonomically, the greater the chance they will be compatible and will form a strong union. Even if a union is formed, if it is weak then it may lead to poor growth, the scion breaking off from the understock, or the plant dying.
The ideal time to graft plants together is late winter after the last chance of severely cold weather or early spring before or just as new growth arises and before the summer heat arrives.
Types of Grafting Techniques
There are two basic types of grafting techniques. One type is where you are grafting a scion onto an understock of nearly equal diameters. The other type involves grafting one or more small scions onto an understock that is significantly larger in diameter. Regardless of the type of grafting technique used, four criteria must be met for grafting to succeed:
- The plants being grafted together must be compatible.
- Each plant grafted must be at the correct physiological stage of development.
- The cambium of the scion and understock must touch at the point where grafting occurs.
- The union between the scion and understock must remain clean.
Whip grafting is used to graft scions and understocks of nearly equal sizes, preferably ¼ to ½ inch in diameter. This type of grafting is relatively easy, and the union tends to heal rapidly.
When whip grafting, the understock can be growing in a field, potted, or a dormant bare-root understock. Grafting the latter two types of understock (potted or dormant bare-root) indoors is called pot grafting or bench grafting. The point on the understock where grafting is to occur should be straight grained and smooth, away from any point where lateral trigs or branches might have developed.
The scion should consist of 1-year old wood, ideally of approximately the same diameter as the understock to which it will be grafted so that the cambium of the scion and stock can meet all or most of the way around the grafted area. If the scion is smaller in diameter than the understock, then the cambium will only meet on one side of the graft. Never graft a scion with a larger diameter to an understock with a smaller diameter.
The first step is to prepare the stock and scion with matching cuts. Using a high quality very sharp knife, both the top of the understock and bottom of the scion should be cut all the way through at a diagonal 1 to 1½ inches long depending on thickness. The cut should be smooth and without waves or whittling.
You could graft the stock and scion together at this point, but it is better to use a whip-and-tongue system which is stronger. To do this an additional cut must be made to both the stock and scion. To form the “tongue” on the stock, about one-third of the way down from the tip of the previous cut, make another downward vertical cut about ½ inch deep as parallel as possible to the wood grain. The scion should be cut the same as the stock (including the tongue), only at the bottom of the scion.
Next the stock and scion should be fitted together as closely as possible. The cambia of both pieces should be well aligned so the cambium of the stock touches the cambium of the scion, hopefully all the way around. If the stock and scion are different sizes, then align the cambia along on side of the graft such that the lower tip of the scion does not hang over the stock.
Once the scion and understock have been aligned, carefully wrap the graft using rubber budding strips, grafting tape, electrical tape, or another type of plastic tape to provide strength to the graft and prevent drying. Once the tape has been applied, it should be waxed or painted uniformly with latex grafting paint to prevent drying. Throughout the wrapping and waxing steps, be sure that the cambia of the scion and stock remain aligned.
Cleft grafting is typically used to top-work a tree (i.e. to change it from one variety to another). This grafting technique can be used on juvenile and mature trees. Cleft grafting can be used on the trunks of young trees but should only be used on branches 2½ inches or less in diameter on older trees. Ideally, the area where grafting occurs would be fully exposed to sunlight in areas of active growth. It is preferable to graft on upright branches instead of horizontal branches.
To prepare the stock for a cleft graft, you will first need to carefully saw off the trunk of a small tree or a branch of a large tree at a section where it is smooth, knot-free, and straight grained. It should be sawed off at a right angle to the grain while taking care not to split, tear, or otherwise damage the bark. Be sure to leave one or two nurse limbs to supply the tree with energy until the graft is growing well. Next use a grafting tool or large, sharp knife tapped with a mallet to split the stock through its center about 2 inches deep.
To prepare the scion for a cleft graft, first select a long one-year old twig or branch that is about 3/8ths to 5/8ths inch in diameter where you plan to cut it. Ideally, the cut scion should have 3 buds so the scion can be inserted with the bottom bud just above the split in the stock. The scion should be collected during the dormant months of January or February and stored in a sealed plastic bag containing moist sphagnum peat moss, sawdust, or paper towels to prevent the wood from drying out. The sealed bag containing the scion should be refrigerated at 34° to 38° F until the understock has started growing.
When you are ready to graft the scion onto the stock, make a long, smooth 1 to 1½ inch cut starting just below the lowest bud and cutting toward the base of the scion. Flip the scion over and make a second smooth cut the same length on the opposite side. The side containing the lowest bud should be slightly thicker than the other side. The scion wedge does not have to be sharp at the tip. In fact, it is preferable for the scion wedge to be blunt at the tip.
Open the crack in the stock just wide enough for the scion to be inserted using some sort of wedge or grafting chisel. Orient the scion so that the thicker side (and lowest bud) is closest to the outside of the stock and the scion’s cambium makes good contact with the stock’s cambium. The best contact point is typically about ¼ inch below the top of the stock. It is recommended that you place two scions in each stock slit, one at each end, to increase the odds of getting a graft to grow. After the scions have been properly positioned, the wedge holding the stock’s split open can be removed.
After inserting the scions into the stock, all cut surfaces of the cleft graft should be waxed. Cracks can occur after the wax dries, so check for cracks after a few days and then every few weeks and reapply wax as needed to keep all surfaces covered.
During the first growing season, do not prune any branches that begin to grow from the scions. If the grafts grow quickly, then you may want to pinch off the tip to promote branching. Cleft grafts should grow vigorously and only require light pruning.
After the first year, you may be required to trim some branches. If both scions take hold and grow, shorten the weaker one to allow the other to grow more and become dominant. However, you should not remove the second graft until much later as it will help the graft to heal and cover the wound much faster.
Bark grafting is another method of grafting. It is simple, requires no special tools, and can be used on branches from one to several inches in diameter.
In bar grafting, the stock is prepared the same as in cleft grafting by sawing it at perpendicular to the grain. This can only be done in mid to late spring when growth begins, and the bark easily separates from the wood. Once the understock has been cut, either a single slit 3/4ths of an inch long or double slits 3/4ths of an inch long and the width of the scion should be made in the bark.
The scion should be made from the last seasons growth and should be collected during the dormant months of January or February. The wood should be 1/8th to 1/4th inch in diameter. Once collected, they should be stored in a sealed bag of moist sphagnum peat moss, sawdust, or paper towels to avoid the wood drying out. The sealed bag should be stored in the refrigerator or somewhere else at a temperature between at 34° to 38° F.
When you are ready to graft, remove the tip of the scion and recut its base. The scion should be 4-5 inches long and contain 2 to 3 buds. About 1 to 1 ½ inches above the new base of the scion, cut most of the way through perpendicular to the grain of the wood before turning downward to the base of the scion. This forms a shoulder and a long, smooth cut. This long downward cut should be about 1/3rd of the length of the scion. On the opposite side of the long downward cut, make a short cut forming a small wedge at the bottom tip of the scion.
Once the scion and stock have been prepared, center the long, smooth, downward cut of the scion in the understock’s slit or between both slits if two were cut. Push the long, smooth cut of the scion down and behind the bark of the stock until the shoulder of the scion rests on the shoulder of the stock. If the scion is large, a nail or two may be driven through the long, smooth cut and into the side of the stock to help hold it in place. But if the scion is not too large, you can use electrical tape or masking tape to wrap the graft pulling the surfaces of the scion and stock tightly together. Ideally, all cut surfaces would be covered and remain covered by grafting wax until the graft completely heals.
Once the graft has taken and the scion has started growing, you should cut off any side shoots that might shade or slow the growth of the new graft.
Budding is a grafting method that uses a single bud rather than a longer inch section of stem containing multiple buds. It is used to propagate a wide range of woody plants. It is often used to propagate varieties that are not true-to-seed. Its commonly used for producing stone fruit trees (e.g. cherry, plum, and peach trees), roses, and ornamental trees and shrubs. It can also be used on trees not easily cleft grafted or whip grafted.
T-budding is faster, typically has higher success rates, and forms a stronger union than other grafting techniques. It is well suited for shoots from ¼ to 1 inch in diameter, and can be done in March and April (called spring budding), late May and early June (called June budding), and late July to early September (called fall budding). Fall budding is easiest for beginners at grafting, and can be used for fruit trees like peaches, pears, and citrus as well as ornamental trees like dogwoods and flowering cherry trees.
Propagation with June budding is done in the summer as soon the current season’s buds mature and can be cut which is usually between early June and mid-July. After budding has occurred for two to three weeks, the rootstocks should be cut off about ½ inch above the bud and all suckers should be removed which should cause the bud to grow.
When propagation occurs in late summer it is called dormant budding. This is best done in late August or early September. In dormant budding, however, the buds are not forced until the following March when the rootstock is cut off about ½ inch above the bud.
To get started with budding, you must first collect terminal shoots from the current season’s growth called budsticks. When selecting budsticks, be sure they are from parts of the plant that are growing vigorously and free of diseases and insects. Buds from the center of the budstick are generally better than those from the tip or base.
Once budsticks have been collected from tree or shrub of the variety to be increased, cut off the leaves leaving ¼ to ½ inch of leafstalk. The partial leaf stalk will make the buds easy to handle. Ideally, budsticks should be used when you harvest them, but they can be stored by wrapping them in a moist paper towel, placing them in a sealed plastic bag, and refrigerating for up to 3 days or so.
Once budsticks have been collected, the next step is to select your understock. The ideal understock is a young vigorously growing stem between ¼ and 1 inch in diameter located 2 to 12 inches above the surface of the soil. However, you can topwork a larger plant by attaching the bud higher up.
To prepare the understock, it is important to have a sharp knife as with other types of grafting. They make budding knives specifically for making the necessary cuts and peeling back the bark so that a bud can then be inserted.
First wipe the understock clean of all soil and leaves in the smooth, branch free areas where budding is to occur. Make a 1½ inch vertical cut in the stock parallel to the wood grain by pulling the knife upward. You should cut through the bark but do not cut into the wood.
Next, cut across the stem perpendicular to the wood grain at the top of the vertical cut to form a “T”. The crosscut at the top of the “T” should be made with the blade tilting slightly downward. This will make it easier for to insert the bud later. Again, the cut should only be through the bark, not into the wood.
The next step is to remove a bud from the middle portion of the budstick. To do this, start the knife blade 1/2 to 3/4 inch below the selected bud to be removed and make a smooth upward cut that extends to about 1/2 to 3/4 inch above the selected bud, taking with it a sliver of underlying wood. Make sure the sliver of wood is cut straight to ensure good contact a successful union once the bud is grafted. Then cut horizontally just above the bud to free it from the budstick. Use the petiole or leafstalk that was left when the budstick was first harvested.
The bud should be immediately inserted into the “T” cut on the stock before drying of either can occur. To insert, gently raise the two flaps of the “T” cut. Holding the bud shield by the petiole, slide the bud into the stock’s vertical slit and behind the two flaps until the top of the bud shield is even with the crosscut or slightly below it. The bud shield may need some trimming to get it to fit properly and make good contact.
Once the bud is in place, it should be wrapped 3 to 4 times with a rubber budding strip above and below the bud to hold it tightly into place and ensuring good contact. Start above the bud to prevent it from slipping out of the slit making sure to completely cover the horizontal cut at the top (preferably with a single wrap of the rubber budding strip) but not covering the bud itself. Finish the wrapping with a self-binding loop.
The bud should be checked within 7-10 days. It should be obvious by then whether a union was formed. If the bud is dry and shriveled up, then it failed possibly leaving you with enough time for another attempt at budding. If the bud and shield look fresh, then the union has taken place. If elastic wrappings were used, then they can be removed anytime. However, if nonelastic wrappings were used, then they should be removed.
When buds are forced depends on whether June budding or dormant budding was used to propagate. June buds should be forced within two to three weeks of successful grafting. But dormant buds grafted in late summer should not be forced until March of the following year.
To force budding, cut the stock just above the grafted bud, typically ½ to 2 inches above the top of the “T” cut. This will break the existing apical dominance on that stem of the plant and “force” the new bud to begin to grow, becoming the apical bud. If other non-grafted buds from the rootstock begin to develop, remove them when they begin to appear. The new branch resulting from the grafted bud should not be pruned the during the first year of growth. If there is any danger of the new branch being broken, then some method of bracing or staking should be employed to provide the new growth with additional support.
Patch budding is used on thick-barked trees like walnuts, pecans, and other relatives where T-budding cannot be used. It is slower and more difficult than T-budding. Patch budding is typically done in late summer or early fall, but it can be done in spring when the bark starts to slip.
Ideally, the budstick and understock will be about the same diameter, but you can graft buds onto stocks as thick as 4 inches in diameter. Furthermore, it is crucial that the bud and its attached bark are the same size as the patch cut prepared on the understock. Double bladed knives and other special tools have been created to ensure perfectly parallel horizontal cuts of a specific width, typically an inch apart.
Budstick selection should be done in late summer two to three weeks before patch budding is to occur. Once you have identified the budsticks you plan to use, cut the leaf blades from the stick leaving the petioles. But do not remove the budsticks from the plant. The petioles will drop off over the next two to three weeks before budding, and the leaf scars where the petioles were attached will have healed by then.
Two or three weeks later when you are ready to begin budding, cut the budsticks keeping them moist and out of direct sunlight or heat while you work to prepare the stock. Prepare the stock by removing a 1-inch square patch. The stock patch should be removed from a clean, straight grained portion of the stock. Once the stock patch area has been identified, make horizontal parallel cuts one inch apart in the stock using a double-bladed knife. Make similar vertical parallel cuts one inch apart. Then remove the patch from the stock by pushing it to the side.
Once the patch is removed from the stock, it is time to remove a bud from the budstick by making horizontal parallel cuts an equal distance above and below the bud with a double-bladed knife. Then make similar vertical parallel cuts an equal distance to the left and right of the bud. The goal is to create a roughly 1-inch square patch with the bud in the center. Remove the patch and bud from the budstick by pushing it to the side as you did to remove the stock patch. Do not otherwise try to pull the patch and bud from the budstick as this could result in pulling off the patch bark with the outer portion of the bud leaving the needed center of the bud behind on the budstick.
Once the bud patch has been removed from the budstick, it should immediately be inserted into the stock where the bark was removed. Removing the patch from the stock, removing the bud patch from the budstick, and inserting the bud patch into the stock need to all happen quickly so that neither the bud patch nor the stock patch has time to dry out. It is best if the bud patch fits snuggly into the stock on all four sides, but most important that the top and bottom are snug.
Immediately after the bud patch is inserted into the stock, the patch should be wrapped. If the bud patch is recessed because the tree bark is thicker than the bark from the budstick, then shave down the stock tree bark around the bud patch so that the stock bark is flush with the bud patch bark. This will keep the bud patch tightly in place after wrapping. Cover all four cuts with the wrapping, but do not cover the bud. The edges of the patch should be waxed. Then the bud can be wrapped in place with nursery adhesive tape, masking tape, or heavy cotton string.
After about 10 days, check the buds. If it appears union has occurred, you should release tension from the wrap. Cut the wrap on the opposite side of the stock from the patch, but do not pull the wrapping off if it looks as though it is stuck to the stock bark or bud patch.
Like other budding techniques, do not force budding until union is complete. When dormant budding is done in the fall, wait until growth begins the following spring before cutting the stock back to force budding. When budding is done in the spring, the stock can be cut back to force budding about 10 days after inserting the bud patch when a union is formed.