I have been working on a grafting article for several years now. It seemed like I could never finish it. Well, now I have. If you have ever wondered how to graft, this post should give you enough information to actually perform the operation. You can click on any of the pictures to see the full size photo. Questions and comments appreciated.
Grafting is a procedure for clonal propagation. The resulting plants will have identical characteristics to the parent plant, in most cases a named cultivar. The understock characteristics (what it is grafted to) do not change the genetic integrity of the scion (the plant you graft to the understock). In some cases the understock will speed up or slow down the scion growth, but that is the only effect. Remember that the roots of your grafted plant retain the genetic integrity of the understock (sometimes referred to rootstock).
Grafting for bonsai requires special procedures not required for ordinary landscape plants. These problems are fully discussed in the article:
Root Grafts for Bonsai
This article will focus on the actual procedure in detail. Since decent grafted material is so hard to obtain, grafting can be a valuable skill. The procedure described is specifically for pine grafts, but it can be adapted for almost any species used for bonsai. The same technique (with appropriate modification) can be used for branch grafting on larger trees, either to add foliage to a naked branch, or to completely graft over to a new species such as grafting 'Shimpaku' juniper to 'San Jose' or collected mountain and desert junipers.
Grafts are generally made using the same species for understock because grafts between species (different species for understock and scions) are not always compatible. Pines are a bit unusual in that there is a broad range of species compatibility. Japanese white pine, Pinus parviflora, are mostly, grafted to a different species. In the West, it is mostly commonly grafted to P. strobus (Eastern White Pine), the common understock for all white (five needled) pines. However, in Japan, P. thunbergii (Japanese Black Pine) is the most common understock for P. parviflora. I use P. thunbergii understock for all my black and white pine cultivars.
Preparation of Understock
I have spent a lot of time studying how to prepare understock. Beginning with bareroot seedlings, I originally believed the premise that saving as much root as possible (and still be able to get it in the pot) was the best thing to do. To further relieve the stress, I also pruned back the top. These small bareroot young pines didn't usually have more than one adult node, so this meant cutting off the main terminal bud. This turned out to be a bad idea. I lost many of these pines, and the ones that survived usually didn't grow strongly enough to make good understock the first year.
Then I started applying the ideas that I talk about in the article Root Pruning Bareroot Seedlings. Even though the article is geared toward deciduous trees, the principles also apply to pines if you take some additional precautions for the unbalanced system (more foliage than root). Briefly, what I do for deciduous dormant bareroot trees is to leave as much top as possible and rootprune rather severely to remove the tap root. This allows me to be able to get it in a pot that is only 3 inches deep. Since buds already contain everything they need to open into a leaf, the roots have only to supply water at the start. So, if you can shelter these plants from heat stress, you get a fast recovery because you leave them with the maximum photosynthesis potential. I have done comparison tests on identical plants, so I know this works.
In pines and other evergreens, the principles are the same, but they are not leafless when bareroot, so there is always a transpiration loss, even when dormant. This doesn't create a problem until it gets warm and they start to grow, but it does make them more susceptible to freeze drying than deciduous plants. By leaving one or more strong terminal buds, you provide a means of quick growth and recovery from the root pruning if you can match the transpirational losses to the impaired root system. This can usually be done by keeping them shaded somewhat, out of the wind, and partially covered. I really like a product called Reemay. It is a spun crop cover that is white and gives you a slight shade factor, almost completely reduces wind drying, but allows air circulation, and allows rain or irrigation to flow through it. The only problem with it is that you can get dry spots at times during the winter. I have changed to system of overhead water frost protection that works even better, but the Reemay is a good simple, inexpensive solution.
With this new system, my losses are much less, under 10%, and sometimes nearly all the seedlings will survive. It works better with younger plants than with older ones, probably because younger, smaller plants are easier to protect. As I said, many of these are root established and ready to graft eight or nine months later. In addition, the root pruning allows me to start working on the nebari even before the trees are grafted. I begin selecting surface roots; the tap root is cut off; I can remove or shorten overly strong side roots, and position the top roots radially. I also pot the seedlings high in the pot with the soil mounded up, so that I can low graft right on top of the surface roots without digging down into the pot. This greatly simplifies the grafting process and keeps the cut cleaner. Notice the root position of the understock picture above.
For more information on understock preparation see the blog post “Where It All Begins”.
Preparation of the Scions
The scions are usually 1/4 inch caliper or less and from growth less than a year old (tips) with a single node (bud or whorl of buds) at the end. For smaller cultivars, the scion is usually cut just above the last node (not counting the node at the tip). When the last internode section is very long, a stub can be left, or the scion shortened later. Typically, scions are one to three inches long. Always inspect the terminal bud when selecting scions. Sometimes what appears to be a really nice stick of new growth will have only a very tiny terminal bud at the tip. While these may “take”, the success rate is certainly higher when there is a nice fat bud at the tip and even better when the other buds around the terminal bud are also visible. They are an insurance policy should the terminal bud not survive. Remember that the node at the base of the terminal bud is going to provide the first branch on the grafted tree, so you don't want a really long stick of a scion; the shorter the better. The bottom needles are removed and the bottom of the scion is cut on both sides to make a wedge that fits into the slit in the understock(if only it were that easy!). There is a long side of the wedge that should be as long as the inside of the slit on the understock. The short side of the wedge is mostly to make it pointed.
The cutting part of grafting requires a great deal of practice and skill. As mentioned above, the knives have to be razor sharp. I use Japanese carving knives and water stones to sharpen them, finally finishing the edge with a razor strop. I restrop the knives after every twenty to forty grafts. Getting the knife to shave hair is the starting point. You want the knife so sharp, that the hairs of your arm will pop right off when you touch them. Some people use single edge razor blades and discard them after a few grafts, but personally, I prefer knives.
If you choose a knife, a high quality laminated steel single bevel knife is best. Knives that are beveled on both sides (most western knives) are not suitable; you will never get a surface that is flat enough for really good grafts. Most Japanese carving and grafting knives are flat (or hollow ground as can be seen in the photo) on one side and beveled on the other. Because of the single bevel, these knives are either right or left handed. Be careful of left hand v. right handed terminology. It is usually for carving, meaning that you are going to use the knife by carving away from you (picture an old guy whittling). Grafting is done by moving the knife toward you. So the terms are usually backward. I am left handed, so I use right handed knives (pictured). Right handed people will usually want left handed knives. If this totally confuses you, Picture yourself holding the knife, and moving it toward you to make the cut. The flat side should be on the bottom, and the bevel side should be on the top. You always want the flat side next to the mating surfaces of the scion and the understock.
I actually use two knives. One knife is ground down so that the blade is very thin and fragile. This is used for making the understock cut only. The thinner and sharper it is, the easier and better the cut will be. Thick knives (short bevel) will stretch the cut, and thus lose some of its holding power. The other knife uses a standard bevel (just as it comes from the factory) and is used for making the scions cuts. Some people manage to make the scion cuts while holding the scion in their hand. I have never been able to do this and get a good result. I prefer to use a block to support the scion (pictured in the photo above). The problem with using a block is that the scion cut touches another surface that can scratch it, and can allow dirt and debris to enter the union. Therefore, it is essential that the block be kept clean and smooth. I use a poly cutting board set sideways in a 2 x 4 that I can plane smooth after it begins to roughen from the cuts. You must also keep it meticulously clean. A wipe with alcohol or WD40 after every graft is a good idea.
Making the Cuts
There are many subtle tricks to making a good cut, and not everyone is going to approach it the same way. I will try explain what has helped me to make good, flat, straight cuts. First, for both the scion and understock cuts, this is not a whittling exercise. The cuts should be, as much as possible, a single, straight, smooth stroke. Once you begin making the cut, the knife should not stop moving. The purpose of the flat side of the knife is make a cut that does not tend either to curl in, or curl out of the cutting plane. You need flat cuts. This is perhaps the hardest skill to obtain. It will take hundreds, if not thousands, of cuts until you can do this perfectly and effortlessly.
Understock cuts are fairly simple and you will perfect these first. You start about an inch above the existing surface roots. You want to finish the cut just above the roots. The knife should by angled so that a cut of 3/4 to 1 inch will go about 1/3 of the way through the trunk. You will know if you go too far, because suddenly the knife resistance will be gone, and the top will be limp and the cut will not close. You need to go far enough into the trunk to create a tightly closed cut after you withdraw the knife. Too deep and the trunk will split or the top will lose its stability. Too thin and the outside of the cut won't have enough wood to make the graft tight. One third the distance of the understock is just about right. Of course, none of this will mean anything to you until you try it.
The scion cuts are more difficult. In many cases, you are working with a stick of wood little more than two or three inches long. You have to remove the lower needles so that you can position the knife properly. Make the long side of the wedge first. I have gone back and forth on this, and currently I feel it is best to do the long or mating side first. It makes the scion more stable when cutting the other side to make the wedge. Lay the scion flat on the block, and angle it so that the whorl and buds are to one side of the edge of the block. You want the section you are going to cut flat on the block for support. Position the knife and scion so that you are pulling the knife toward you, flat side of the knife down against the scion. Pull the knife through to the bottom of the scion to make a cut that is about 3/4 inches long, and make it go halfway through the scion, thus removing a small U shaped bit of wood from the bottom of the scion.
To make the scion pointed, turn the scion over and now position the cut you have just made flat against the block. This takes some practice. You must feel that it is flat against the block. Now make the shorter 1/4 inch cut by pulling the knife down through the scion in the same fashion, except that this time the knife is going to go all the way through the scion and into the block to make a flat sharp edge to the bottom of the scion. If the edge is not sharp, you will have to repeat the cut, starting at the top again. If you remove too much material, the long side will now be too short, start over.
That's the general instructions for the scion cut. Now for the detail. The sharp bottom edge of the scion must be at 90 degrees (right angles) to the scion. If it is not, then, you have to re cut one or both cuts.
Here is what happened: The two cuts are flat intersecting planes. If they don't intersect at right angles to the edge, that means that one side of the scion wedge cut is thicker than the other. This is evidenced by the lack of the right angle at the bottom edge. It is difficult to tell with the naked eye whether these two planes intersect properly except by looking at the resulting angle at the bottom of the cut (see pictures). Since the scion wedge is going into an understock cut with parallel sides, one side of the scion cut cannot be thicker than the other or there will be a gap between the cut surfaces on one side of the graft or the other. If this gap is on the mating side of the graft, chances are good that it will fail. The best grafts will make contact on all portions of the cut surface of the scion, short and long side. Whew!
You won't make perfect cuts on your first attempt until you have made a few hundred (or thousand) cuts, so it is ok to go back and repeat the cuts if the planes don't intersect properly. When repeating a cut, always start over by beginning the cut in fresh bark at the top of the previous cut. If you don't do this, and start in the middle, the cut will not be flat (in a plane). With most pines, it is fairly easy to get the cut fully through the scion with one stroke. However, with some pines and especially with hardwoods, such as maples, you may find you can't cut through the desired distance with one stroke. In this case it is perfectly acceptable to first make a shallow cut, or two, to remove some material before making the final cut as described as above. Without having to push through so much wood, the cut will be much easier to make.
And lastly, side veneer grafts are designed to make smooth undetectable unions. This means that there is only a slight angle between the scion and the understock; both are upright. Often, these short scions will have a fairly fat whorl of buds. You must position the scion so that no bud is directly against the understock. If you do, the bud will often touch and thus push the scion away from the understock stem. If this happens, you will not be able to close the graft. So, while you are holding the scion, preparatory to making the cuts, examine it and decide which side is going to go against the understock. This side should be able to lie nearly flat against the understock, and should have clean straight sides if cut from this position. Similarly, examine and decide where to make the cut in the understock. You want to make the cut just above existing roots in an area with relatively smooth bark and no distortions so the sides of the cut will be straight. I will sometimes even make a cut go below an existing root on the backside of the understock.
Fitting the Scion to the Understock
Insert the scion into the undertock by gently opening the understock cut by bending the stem away from the slit, and then gently positioning the scion to the very bottom of the cut. This may take a light pushing effort. When you are getting the cuts right, the scion will fit right into the slit and the rounded tops of the two cuts will come together evenly. If they don't, the cut on the understock should be longer than the cut on the scion, or the scion will dry out and die. The scion should be held tightly to the understock by the force of the cut closing on the understock when you release it. After inserting the scion, examine the graft for flatness. The cut should be closed and the surfaces mating perfectly. If you can see any light through the mating surface, it has to be closed by the rubber or tape pressing the graft together, but it is far better to have a perfectly mated surface without even touching the graft. That should be your goal, alas, it is not easy.
I recommend that you practice on willow wood for several hours before experimenting on valuable scion wood. I have students create faux grafts of willow wood because it is soft and straight. Here's how you do it. Take a small block of 2x4 lumber and drill a 1/4 inch hole in it. With a reamer or knife, taper the hole slightly so that you can insert a six inch willow stick into it tightly. That is your understock. You can practice making understock cuts on this. Then practice making scion cuts on other pieces of willow slightly smaller than 1/4 inch. You can test your skill by inserting them in your 'understock'. You can even practice tying the grafts, which is an art onto itself.
Aftercare is a whole other procedure to learn. If you are winter grafting, you want the grafts to stay cool and humid with indirect sunlight only (50% shadecloth) for the entire spring and summer following the grafting. If your graft 'takes', the scion bud will probably break by midsummer. Often the scion will stay alive, but the bud won't break until the following spring. If the graft fails, the scion will be dead by the first summer. There are any number of ways of achieving the proper conditions. Some people recommend placing a plastic bag over the scion. I caution you about the bag method: You cannot let direct sunlight hit the plastic bag or you will cook your scion. Most people use some sort of glazed frame that is not completely enclosed, or a shaded area that is wind protected. Greenhouses can be used if there is good ventilation, humidity control, and the temperature doesn't get too high. Fertilize the understock normally. You want it as vigorous as possible given the low light levels. I even have seen some people create little sunscreens from foil for the scions so the rest of the understock can have more light.
Our spring weather (high Coastal Mountain valley of Northern California) is very fickle, so climate control has been exceptionally difficult for winter grafts (Dec through Feb). I had many losses. Finally, I heard about Oregon grafters grafting in August, also when budding is done. I have always had a pretty decent automatic mist arrangement for cuttings, and as cutting production is pretty much completed in August, I thought I would give summer grafting a try. It has been a godsend for me. I have found I have to graft later than August since it is still very hot here then. I am now grafting at the end of Sept and putting the new grafts under mist in a small greenhouse for about a week to ten days. The temperature is kept between 50F (night) and 95F (day) to get the grafts to knit as quickly as possible. Since both scions and understock are active, this is possible in a very short period. After a the mist is stopped, the greenhouse humidity is still kept at 60%. As the weather cools and the rains begin, I take them out of the greenhouse. Previously, I kept the grafts in a cold greenhouse for the winter from about 40 to 60F, but since that greenhouse was remote, I never got there often enough for proper care. So now I just put them in a shadecloth hoop house once the rain has started. I don't let them get very cold, trying to keep them above 25F by watering them in the middle of the night when it's below freezing and encasing them in ice. This prevents them from freeze drying too. In colder areas, a cold greenhouse with that is kept humid would be perfectly acceptable.
Basically, proper aftercare must ensure that a high level of humidity is maintained, and that temperatures remain moderate. You want neither high heat, nor hard freeze. The temperatures and conditions vary depending on whether you are doing winter or summer grafting, but the essential principle is not to stress the plants in any way.
The understock top (above the scion) is removed after the graft takes and is well established. There are varying opinions about when and how to remove it. I do it in stages. As the scion grows, I remove more and more of the understock. You have to keep the root system strong and this is difficult to do that if you whack off the top as soon as the scion begins to grow. I remove the last of the understock after about two years.
My understock is about two to three years old when I use it for grafts. This is smaller than most grafters use, but I like having vigorous material that is only slightly larger than the scions. I start with one year old field grown bareroot seedlings and root prune and pot them into 2 ¾ inch square pots (see the blog article above for more detail). This is done in late winter. By September these black pines are mostly root established and strong enough for understock. I use only the stronger plants that are stable in the pots. The weaker plants are culled out to be used the following year.
The grafts are made with the understock in these 2 ¾ inch pots. The tops of the understock are not pruned, and branches are not removed except where they interfere with the placement of the scion. The finished grafts are placed in flats using empty 2 ¾ inch pots to position them in every other space. A standard 17 inch flat will hold 36 2 ¾ inch pots (6 x 6), so this allows 18 finished grafts per flat. This arrangement gives the grafts more light and air and is especially important in the darker and very humid atmosphere on the mist bench and in the intermediate house.
By late winter, usually February, I can usually tell if the grafts will take. I remove the failed grafts and repot the good ones into 4 inch pots. It is important not to let them get rootbound, which they will if not repotted at this time. No pruning is done. The rootball is loosened, but disturbance is minimal. I used to try to separate the roots at this stage and set up the nebari, but the result was fairly heavy losses due to the disruption. Grafting is a traumatic process, and everything possible must be done to eliminate stresses. In the 2 ¾ inch pots the understock is planted high to expose the roots for grafting, but when shifting to the 4 inch pots the roots and the bottom of the graft are buried to make sure the roots survive.
The grafts stay in the 4inch pots for a year and still kept in the shade house (30% shade factor). They are fed regularly and the tops are pruned back during the summer once the roots have nearly colonized the pots. The understock is not totally removed, but rather reduced to encourage the scion to grow while restraining the understock. It is also important to make sure that the scion is not getting shaded out by the understock top.
During the second winter the 4 inch potted grafts are shifted to one gallon cans. Again, only minimal separation of the roots can be done to avoid stressing the plants. The following spring the grafts and the understock both really take off. The cans are nearly root colonized by summer. They are still kept under 30 to 40% shadecloth. It's a judgement call on how much of the understock to remove during the summer. I prefer to leave quite a bit to ensure thorough root colonization, but sometimes removing the leader will release a stubborn scion that doesn't want to break new buds. Often the black pine scions will be growing faster than the understock at this point, and the understock can be removed entirely. The dwarf white pines are usually not strong enough to grow vigorously without the help of the understock.
By autumn (two years since they were grafted), most of the black pine grafts are essentially finished and the understock is removed and the tape removed. The larger white pine grafts such as 'Ara Kawa' will be finished as well. It is at this stage that I begin selling them because I know I have a healthy stable plant with a very high chance of success, even in the hands of a beginner. Most of the white pines will have to spend another winter and summer with all, or a portion of their understock still attached to reach this same stage. They are then sold as 3 year grafts the following fall.
In most areas of the US, the black pines can be placed in full during the third winter. Winter is a good time to do this since it allows them to acclimate slowly to increasing day length and sun intensity in the spring. In our area, the white pines must remain under 30 to 40% shadecloth all summer, but in milder areas they may be able to take full sun, or full morning sun.
Thanks to Bob Potts, my apprentice, we a little video of the grafting process. Click on the link below to see the video: