Black walnut is an unusually difficult species to propagate vegetatively, and until recently, interest in grafting black walnut has been mostly academic with the exception of the grafting of a few nut varieties. For example, since 1968, many grafting and cultural practices have been attempted through Purdue University's black walnut tree improvement program. From the annual accumulation of successes and failures over those years, researchers now can provide a fairly reliable, rapid, but unfortunately, expensive method for grafting and after-care of black walnut which can be used by walnut growers as well as hobbyists. This publication details this method, which uses a modified side graft of dormant scionwood and actively growing rootstock.
Rootstocks should be selected from the largest and most vigorous one- or two-year-old black walnut seedlings available. The seedlings should be planted in pots or half-gallon or gallon milk cartons in the fall or early spring. If fall potted, they should remain outdoors with a peat or sawdust mulch covering the containers by several inches. The potting mix should be approximately 1/2 part loamy topsoil, 1/4 part sand, 1/4 part organic matter, and a small amount of perlite. If rootstocks are spring potted, they should have the upper 1/3 of the stem cut off immediately after planting. For fall potted rootstocks, the tops should be cut off when they are taken inside to break dormancy.
Normally, the rootstocks are ready to graft one to two weeks after spring potting or moving fall potted trees inside. The proper time to graft can be determined by close observation of buds on the rootstock. The buds should show some green or red color, and a few small leaves should be expanding. As long as proper temperature and lighting can be maintained in a greenhouse, grafting can begin as early as February and continue until mid-June.
Collection of the scionwood (grafting wood) should be completed between January and early March. Temperature at time of collection has no effect on eventual grafting success. However, grafting wood should not be collected when wet unless the surface is allowed to dry thoroughly before storage.
Scionwood may be stored dry in ventilated plastic bags in a cooler or refrigerator at or just above freezing. Care must be taken so that condensation does not form on the scion inside of the bags. If it does, remove the scion, and allow the surface to dry. Excessive moisture encourages mold and fungus growth which damages the buds. In fact, scionwood can be stored for up to three months on the cooler shelves without plastic bags. Below freezing temperatures from 20 F to 32 F may provide up to six months storage, but scionwood must be placed in nonvented, sealed plastic bags.
An extremely sharp grafting knife is a must. Walnut wood is very hard compared to other commonly grafted woody plants and requires a great deal of control, strength, and practice for the proper cuts.
The rootstock should be cut first. A slanting cut, approximately 1 1/2 inches long, should be made into the stock as close to the soil level as can be comfortably worked (Figure 1). The depth of the cut can be adjusted to accommodate a wide variety of scionwood diameters. The cut should penetrate to the pith in the center for larger diameter scionwood. A cut closer to the outer edge of the stock will suffice for smaller diameter scionwood. The cut should be as straight as possible without curvature. The cut can be held closed to prevent drying with an ordinary spring-type clothespin while the scionwood is cut.
The scion is cut into a tapered "V" (Figure 2a and b). A single bud should be left so that it is on the side away from the rootstock. The "V" should be straight unless the rootstock cut is slightly curved. If the rootstock cut is curved, the "V" should be cut to fit the approximate curvature of the rootstock cut. All cuts should be as smooth as possible, and "whittling" should be held to a minimum. One long, clean cut gives better contact than several short, choppy cuts.
It is most desirable for both edges of the cuts to match the cut on the rootstock so that maximum cambial area is in contact (Figure 3). If the scion is much smaller than the rootstock at the point of grafting, then be certain that one edge of the scion lines up with one edge of the stock.
Once the scion is inserted into the stock, a red rubber budding strip should be wound (not too tightly) around the union area to prevent the graft from pulling apart (Figure 4). Depending upon rootstock size, a 5 x 3/16 inch or an 8 x 3/8 inch budding strip should be used.
Parafilm*, a plasticized paraffin with paper backing may be used to seal the grafting area so that the scion does not dry out before the union is formed (Figure 5). Usually a piece of Parafilm 5 x 2 inches stretched out to 8 to 10 inches is sufficient to wrap a graft. The entire grafted area should be wrapped, including two layers over the bud. Care should be taken not to stretch the film while in contact with the sensitive bud. Wrapping should be gentle with pressure applied only to the sides, top, and bottom around the bud to seal the film. Several layers of film should cover the spread between the top of the scion and the rootstock to prevent tearing of the film as the graft calluses and begins growth.
Paraffin or grafting wax can be used in place of Parafilm. Paraffin wax can be melted and brushed over the entire area of grafting and the scion. The proper temperature for application is when the paraffin is barely in the melted state. As soon as it is applied, it should congeal into a white solid with little runoff. Paraffin heated above that point may damage the buds.
The identity of the graft can be marked on the carton or pot if the identity is to be maintained. Tags may be fastened to the graft, but they can girdle and eventually kill the rapidly growing graft if not removed when out-planted in the field.
The most crucial condition required to promote grafting success in black walnut is the proper temperature during callusing and early growth. For maximum success, temperatures should be between 65 F and 90 F, with 82 F being optimal. Temperature stability is the major reason a greenhouse is an expensive necessity for consistent grafting success. Supplemental lighting may be required during short-days early in the season. A 16-hour day length seems to promote better callusing and growth. Applications of fungicides, hormones, water, and fertilizer to the cut scion, stock, or developing graft have not improved grafting success.
Small buds, often red in color, will sprout along the exposed rootstock stem above and below the graft union area and must be removed. Debudding of the rootstock must be attended to on a three- or four-day cycle until the grafted bud initiates growth and becomes dominant.
One week to 10 days after grafting, the remainder of the rootstock above the grafted scion must be cut off to stimulate the grafted bud into growth.
Humidity should be kept as high as possible, but direct misting or sprinkling of the grafts is not recommended. Too much moisture leads to disease problems which could kill the expanding grafted bud. Soil media in the pots should be kept moist at all times, but again, too much water can be detrimental. After the grafts begin to take and the leaves begin to expand, Miracle-Gro or Rapid-Gro liquid fertilizer may be applied to the foliage and/or soil once per week. Follow label directions carefully.
Field (outdoor) grafting may be successful some years provided that the temperature remains mostly between 65 F and 90 F which is seldom the case in May in the Midwest.
Grafts held over winter and planted the following spring usually survive the first winter in the field better than grafts planted the same spring they were grafted. Grafts should be placed in winter storage after leaf fall and before severe freezes occur-usually late October to mid-November. The best storage method, but unfortunately the one requiring the most space, is to place the carton or pot intact in cold storage at just above freezing. Also, grafts may be removed from the cartons and stored fairly dry in a cooler or a refrigerator in paper or ventilated plastic bags. A quick water dip two or three times during the winter may be necessary.
Grafts can also be stored in or out of the carton by burying in sand or other material providing good drainage and low water retention. Grafts should be placed horizontally on a mound of sand about a foot high and then covered with at least 18 inches of sand on all sides. White plastic may be placed over the mound to keep excessive moisture out and reflect sunlight that could cause alternate freezing and thawing of the sand.
Grafts may be planted in the field during the spring of grafting after all danger of late frost is passed. If grafts in cartons are planted, the hole should be dug large and deep enough to accommodate the carton. The carton bottom must be cut off and the hole filled around the carton as it is slipped up and over the top of the graft and removed. Soil should be packed around the root ball so that no air spaces remain. Bare-root grafts may be planted with a planting bar in the same manner as bare-root seedlings. Weed control is an absolute must. Cultivation, herbicides, or to a certain extent, mulches, will control problem weeds. Specific herbicide recommendations can be obtained from your District Forester or Extension Forester at the state university.
If the soil dries during the first summer, watering may be necessary, and a mulch may be applied to help retain water, cool the soil, and suppress weed growth. Maps can be made of the planting site immediately after planting so that different varieties are properly recorded in their permanent locations.
Careful attention to detail and patience are required for successful walnut grafting. Several seasons of grafting may be needed to obtain satisfactory grafting success. But by using the procedures outlined in this publication, a consistent success rate of from 70 to 90% can be achieved.
*Parafilm is a product of American Can Company, Greenwich, CT 06830
Cooperative Extension Work in Agriculture and Home Economics, State of Indiana, Purdue University and U.S. Department of Agriculture Cooperating. H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the Acts of May 8 and June 30, 1914. It is the policy of the Cooperative Extension Service of Purdue University that all persons shall have equal opportunity and access to our programs and facilities.