FNR-105
forestry & natural resources
WOODLAND MANAGEMENT 
Grafting Black Walnut
Walter F. Beineke and Michael N. Todhunter,
Associate Professor of Forestry and Professional Forester, respectively
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.
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 late May.
Scionwood
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 or 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 3 months on the cooler shelves without plastic bags. Below freezing temperatures from 20° to 32°F may provide up to 6 months storage, but scionwood must be placed in non-vented plastic bags.
Grafting
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
PURDUE UNIVERSITY • COOPERATIVE EXTENSION SERVICE • WEST LAFAYETTE, INDIANA 47907