The Oxford Dictionary defines scion (also cion) as "a shoot of a plant, especially one cut for grafting or planting." Since our northern nuts do not root well from cuttings, we will be referring to scions for grafting (or budding), and specifically dormant grafting or budding. Dormant scion wood can be stored for a long time and is convenient for mailing to growers in other areas of the country.

Structure

Scion wood is a piece of twig, usually of the last year's growth, though sometimes older. The leaves have fallen by the time dormant scion wood is collected so all you see on the outside is bark, buds and leaf scars, from which the leaves separated or dehisced. If the wood is more than one-year old, there may be side branches or shoots, and there may be a ring of bud scale scars which marks the beginning of one flush of growth. This bud scale ring area is important to the hickory grafter because it contains invisible buds which can sprout a new shoot.

The buds of hickory and pecan look similar along the twig, though they vary in size. Usually the end or terminal buds are larger, and on hickory the terminal may be huge. Thus, the terminal end buds may be easier to handle in budding and may make a slightly bigger shoot when they first sprout. All of the buds of hickory and pecan make a shoot containing new leaves and new buds. Some of them also produce male and/or female flowers, though not all can do so. In hickory and pecan, all the flowers sprout from shoots of the current year's growth. The point is that hickory and pecan sprouts may have flowers, but they will all have leaves and shoots. Hence, any hickory or pecan bud can grow a new tree.

That is not the case with walnuts. Careful examination of many walnut shoots reveals two kinds of buds (Figure 1). It may take a magnifying glass to tell the difference. If you have a hard time, find a heartnut to examine, for they have the most distinctively different buds (Figure 2). Some buds will have few larger scales, and some will have a myriad of tiny scales which look like crosshatching. The larger-scaled buds are vegetative buds which will grow a shoot with new leaves and buds; some of them will grow female flowers. The tiny-scaled buds will grow male flowers or catkins, but no shoot! It is important to be able to distinguish the difference because if your successfully grafted scion contains only catkin buds, it will flower and die, and not grow into a tree. On walnut, the female flowers form on shoots of current year's growth, but the male flowers or catkins arise directly from one year old wood.

Figure 1.  Black walnut buds. "V" indicates the vegetative buds with large scales, and "C" indicates the catkin buds with tiny, more numerous scales. (Drawings by Virginia L. Rinkel adapted from the author.)

Figure 2.  Heartnut buds. "V" indicates the vegetative buds, and "C" indicates the catkin buds. The catkin buds on heartnut are most distinctly different from the vegetative buds. This is a good species to learn the difference between bud types (Drawings by Virginia L. Rinkel adapted form the author.)

If you look at the cut end of your twig or scion, you can see what's inside. To see better, cut is on a slant (Fig. 3 and Fig. 4). The outermost part of the bark is usually brown, and inside the wood is usually white. At the junction between the bark and the wood is a green line which is the cambium. If the cambium is brown your wood is probably dead. An exception to this would be a purple variety, i.e., 'Deming's Purple' black walnut. The cambium layer is the actively growing part of the twig, which gives rise to more bark and more wood. It is also the part which proliferates to form callus which knits together your scion to its rootstock. Hence the goal of grafting and budding is to align the cambium of the scion with the cambium of the root stock.

Figure 3.  Diagonal section of a Carya scion, showing the bark ("B"), cambium layer ("C"), wood ("W") and pith ("P"). Note that the pith in pecans and hickory is solid. (Drawings by Virginia L. Rinkel adapted from the author.)

Figure 4.  Diagonal section of a Juglans scion, showing the bark ("B"), cambium layer ("C"), wood ("W") and pith ("P"). Note that the pith in walnuts is chambered. (Drawings by Virginia L. Rinkel adapted from the author.)

Inside the wood is a chamber of soft material called the pith. The pith of walnuts is chambered, while the pith of pecan and hickory is solid. If you cut a twig at different distances from the base of this year's shoot, you will usually notice that the base has mostly wood with small pith, and the tip is mostly pith with thin wood. The likelihood of a graft surviving is best if the wood is thick and the pith is small. Hence, grafts of wood from the base of a shoot are more likely to survive than grafts from the tip. Recalling that the most vigorous buds are on the tip, and least vigorous at the base, it follows that excellent scion wood comes form the center of the shoot which has relatively thick wood, relatively small pith, and fairly vigorous buds.

Now turn your attention back to the outside of the twig. At the base and middle of the twig, the bark is usually smooth and cylindrical, but near the tip it may look wrinkled or sunken. If you cross section the wrinkled or sunken area, you will find that it usually has very thin wood and large pith, and this will not make good scion. If that is all you have to use, it may grow, but is more likely to die than wood further down the twig.

Next, examine the distance between buds. This is called the internodal distance. If the shoot grew vigorously, the buds will be far apart which gives a nice cylinder on which to make your graft cut or lots of space from which to cut bud plates. If the shoot did not grow vigorously, the buds may be right next to each other, making the grafting or budding operation technically more difficult.

Considering all this, the best scion wood is cut from the middle portion of vigorously growing shoots with good size buds, thick wood, small pith, and a long enough internodal distance to make an easy graft cut. Also on walnuts, the buds must be vegetative rather than all catkin buds.

Frequently, you wish to cut scion wood from a tree, but there are no vigorous shoots. Go ahead and collect the poor shoots and try grafting them, but you can prepare the tree to grow better scion wood for next year by doing two things. First, cut back several branches to where the wood is _ to 1 inch in diameter and receiving plenty of light. Second, fertilize the tree with several pounds of high nitrogen fertilizer. The next growing season should produce several vigorous shoots, which you can then use if your grafts from poor wood fail.

Dormancy and Storage

During the growing season the leaves are green and the cambium is growing, but when fall arrives, the growth stops, and the leaves fall off, leaving dormant twigs. Even though a twig is dormant, it still has a little biological activity, and if it is cut off the plant it will begin to dry out and the cambium (which is still green even in dormancy) begins to turn brown and die. From this point of view, the best place to store scion wood is to leave it on the tree. There are two risks in doing so. First, the twigs may be frozen and killed by unusually harsh winters (especially for Persian walnut). Secondly, once the spring begins to warm up the buds become less dormant and begin to swell, and that may make our graft fail by beginning to grow a new shoot before the graft union calluses. Then the scion will wither because it needs food and water before the root stock can supply it. So scion wood should be cut after it becomes dormant, before it gets hurt by the weather, and before it loses dormancy in the spring. Hardy species can be cut any time before spring, and tender species (or new grafts or transplants which are made tender by stress) should be cut before the harsher parts of winter.

The traditional advice is to collect scion wood when the temperature has been above freezing for 24-48 hours. This is to minimize the likelihood of the scions being dried out or desiccated, but both Bob Martin and I have had success collecting scion wood of walnut and hickory in sub-freezing temperatures.

Once cut, the scions must be cooled or they will lose dormancy, the buds will swell, and the graft will fail. The early nut growing literature describes keeping scion wood in boxes buried outdoors or in cellars, but now we generally use refrigerators. Your kitchen refrigerator may have to do, but most have three problems. First, they are rarely cold enough, since the scions should be cooled to just above freezing. I have found that frost forming in the bag doesn't seem to hurt, but nobody recommends freezing scion wood. Check your temperature with a thermometer. Second, frost free refrigerators cycle to a warmer temperature briefly to defrost, and that shortens the life of scion wood. Third, some kitchen refrigerators may contain fruits which give off ethylene, and that makes scions begin to sprout. Ideally, scion wood should be stored in a non-frost-free refrigerator, adjusted to just above freezing, which does not contain fruits.

In addition to being cooled, the scion wood must be protected from drying. Polyethylene zip-lock bags work well since the polyethylene is permeable to oxygen and carbon dioxide but not water vapor. Many grafters put moist paper towel in the bag to increase humidity, but too much moisture causes mold to grow and that can ruin the scion. Most people address the problem of mold by limiting humidity, while others dip the scions in molten paraffin to seal them. This works well, but makes grafting messy. Plain paraffin is brittle enough to flake away when you make your graft unless you are working in the hot sun. The paraffin dip does not seem to hurt the scion so long as it is brief. Some people who don't dip the whole scion at least dip the ends to avoid drying. Another recommended end sealer is orange shellac.

Scion wood can be stored in long pieces and cut to 2 or 3 buds at grafting time or cut to size before storage. As the wood is cut it must be accurately labeled. Masking tape or freezer tape labeled with a porous tip marker or ball point pen is used by most people. If you apply labels before you dip the scion wood into wax to seal the ends, you can also dip the label in the molten wax, which protects it and keeps it from falling off. If you handle many varieties consider labeling each piece. That is especially important if going to a scion exchange, since bundles tend to get scattered and somebody will carry off the label, leaving a pile of unidentified sticks. I am sure many inaccurately labeled cultivars originate this way.

Properly stored, walnut scion wood will keep till June or after, and pecan and hickory wood will keep till at least July or August. These dates are well past the usual grafting season.

Pecan

Pecan scion wood is usually cut from the current season's growth, but the tip tends to be too small and pithy. Collect from the middle of the shoot for the best wood for all grafts.

If you can find vigorous shoots they work well. Hickory differs from pecan and walnut in that the terminals usually are not pithy, and the terminal makes good scion wood if the shoot if vigorous. Hickory terminals make great bench grafts but are frequently too small for 4-flap grafts. On forest trees or neglected trees, there are rarely vigorous shoots, in which case 2, 3, or 4-year-old wood is best. If you use older wood, include a ring of bud scale scars (Fig. 5) or a small side shoot which should include latent buds. This older wood is best for 4-flap grafts.

Figure 5. Detail of a hickory twig showing the bud scale scar ring ("BS"). This is the residual of the scales on the bud from which the shoot grew and is a site from which adventitious buds frequently grow. (Drawings by Virginia L. Rinkel adapted from the author.)

Walnut

Two problems are commonly encountered with walnut shoots. First, the tips are usually too pithy, and secondly, the buds may be too close together. If you do get vigorous growth, the middle portion of shoots are frequently too big for bench grafting, but make nice chip buds. Large, middle-shoot scions are great for 4-flap grafting or cleft grafting and are the most likely to succeed.

Persian walnut is very susceptible to winter freeze damage; so many nut growers collect scion wood in the beginning of winter before hard freezing occurs. Eastern black walnut is relatively easy to graft and winter hardy, so it can be collected any time in mid to late winter. Butternut and heartnut are harder to graft and are winter hardy, and some nut growers believe that success is greater when they collect their scion in late winter, just before losing dormancy. Even if the ideal collection time is not possible, with good handling success is still possible.

Literature Cited

Anonymous. Care of scionwood. The Nutshell. pp. 5-6. January 1961.
 
Gustafson, W.A. Jr. Collecting and storing propagation scion. University of Nebraska Extension Horticulture Newsletter. Issue 02-12-91.
 
McEachern, G.R. 1997. Collecting pecan graftwood. Pecan South, p. 60.
 
Machovina, P.E. 1959. The selection and storage of nut tree scion wood. The Nutshell. January.
 
McKay. J.W. 1962. The cutting and care of nut tree scion wood. The Nutshell. April, p. 40-42.
 
Madden, G.D. 1977. Selection, packing, and storage of pecan and hickory propagation wood. Annual Report. Northern Nut Growers Association 68: pp.67-69.
 
Monte, L & W.D Goff 1977. Storage and handling of pecan scionwood. The Pecan Grower, Dec. /Feb., p.12.
 
Weber, G.G. 1970. Commercial distribution of nut tree scion wood. Annual Report, Northern Nut Growers Association 61:47-50.

* David Johnson is a former President of NNGA and can be reached at Rt. 1, 3500 Cooville Station Road, Coolville, OH 45723-9421, e-mail: dgjohns@frognet.net