Factors Affecting Nut Tree Propagation

F. L. O'Rourke, Department of Horticulture, Michigan State College

Propagation of nut trees is primarily involved with the problems affecting the perpetuation of selected clones by vegetative means. It has been indicated by Morris (14), Reed (18), and others that trees produced from seed are of inferior value for nut production. Seed propagation, however, must be practiced to produce the necessary rootstocks upon which the selected varieties are budded or grafted.

Seed Propagation

Barton (1) indicated that while some few seedlings may be produced without prior seed stratification, after-ripening of the seed for 2 to 4 months at 35° to 50° F. markedly increased seedling production with hickory and walnut. Chase (4) found that black walnut seed sown in November yielded more and larger seedlings than when planted at a later date. Chase (5) also reported that nuts containing larger kernels produced larger seedlings, and that planting 1 to 2 inches beneath the surface yielded larger seedlings than deeper placement. There have apparently been little or no observations made on the performance of seedlings for rootstock purposes between different parental strains except for Chinese chestnut as reported by McKay (12).

Clonal Rootstock Propagation

The difficulty of propagating any selection of nut trees by vegetative means has discouraged selections for rootstock purposes. Only filberts offer such an opportunity for selection on somewhat the same basis as the East Malling clones of apple rootstocks which produce different sized scion varieties after grafting. Unfortunately, no non-suckering desirable clones of filberts have yet been reported and even the non-suckering Turkish tree hazel is grown from seed when such rootstocks are used (16).

Propagation by Cuttings

Gellatly (7) quoted the success of the East Malling Research Station in England in rooting cuttings of walnuts grown in the greenhouse and reported on his own experience in producing short roots on dormant cuttings of heartnut and Persian walnut. The writer (15) has occasionally produced roots on softwood cuttings of pecan and hickory set in a mist humidified greenhouse but the cuttings did not survive. Mist humidification has been a distinct aid in retaining foliage on softwood cuttings of filbert and Chinese chestnut until roots were formed but unless the axillary buds were developed sufficiently to make new growth immediately thereafter, little or no survival was secured. Apparently when the cuttings were succulent enough to form roots the buds were too immature to put out new shoots. If one waited until the buds were developed the tissue at the base of the cutting was too highly lignified for root formation. The use of synthetic plant hormones on cuttings of nut-tree species has been of questionable value.

Propagation by Layers

Mound layers are used quite successfully for the propagation of filbert varieties but have not proven of value with other nut-tree species. Chinese chestnut has been reported to layer easily but experiments with both mound and trench layers of selected varieties of this species at the Glenn Dale, Maryland Station of the U.S. Department of Agriculture gave negative results. The writer (15) has occasionally rooted pecan, hickory, and Chinese chestnut by aerial layering. A marcot box containing sphagnum moss kept moist by a glass wick immersed in water from a bottle at the lower end was employed. The time and labor involved were so great that the experiments were discontinued.

Propagation by Grafting

Bench grafting of walnuts and hickories has been adequately described by Bernath (3), Hardy (8), Lounsberry (10), Slate (24), and others. This method has been tested on a commercial basis and apparently should be considered as one of the most efficient ways to produce nut trees quickly and cheaply in large quantities. Greenhouse and storage facilities are required and keen expert attention must be given the newly-made grafts to assure success.

Reports on top-working and field grafting are both numerous and voluminous. Morris (13), MacDaniels (11), Wilkinson (29), and others have demonstrated the value of cutting back the stock a week or more before setting the scion in order to avoid injury from excess flow of sap. Reed (17), Stoke (27), Morris (14), Shessler (21), Sitton (23), and others have described methods of preparing and setting scions in the stock. All writers agree that greater success is secured when dormant scions are set relatively late in the season. Becker (2) stated that greater success was secured when scions were set from time leaves were full-grown until catkins fell. Protection of the scion by waxes, paper bags, and shading has been advocated by Morris (14), MacDaniels (11), Shelton (20), Shessler (21), and others.

Propagation by Budding

The shield or T bud has not been considered suitable for thick-barked trees such as hickory and walnut due to the difficulty of preventing "air-pockets" beneath the bark. Shaving the edges of the bark at the side of the shield may eliminate this difficulty. Joley (9), reported variable success in shield budding of walnut in California. Patch budding, either by the annular method or with the Jones patch-budding tool was described by Reed (17), and is reported by Chase (6), Zarger (30), and others to be the most practical method of propagation with walnuts. Pecans and hickories are commonly patch-budded in summer in commercial nurseries. The thin-barked Chinese chestnut is usually budded by the shield-or T-bud method as reported by Hardy (8) and McKay (12).

Scion and Budstick Handling

Sitton (22) reported that two-year wood of black walnut was superior to either older or younger wood. MacDaniels (11) advocated the base of the scion to be in the two-year wood and the tip in the one-year wood.

Shelton (19) reported that scions could be kept moist until used by storing in a closed container with a small amount of sodium sulphate, commonly known as "Glauber's salt". The usual method of scion storage is to pack in moist but not wet peat or sphagnum moss and place in a refrigerator at about 35° F. Waxes and resins have been used successfully to prevent undue loss from the plant tissues while in storage.

Waxes and Dressings

Propagators seldom agree in their choice of a wax and wound dressing. In a series of carefully controlled tests, Sitton (23), found that a rosin and beeswax mixture with a filler gave results with pecans superior to the so-called "cold waxes" or asphalt emulsions. Paraffin and polyvinyl resin are often used for scion covering and to protect newly set buds. Shelton (20) has indicated certain qualities of a satisfactory wax.

The Rootstock Problem

In the Pacific Northwest Painter (16) stated that some Persian walnut varieties on Juglans hindsi (the northern California black walnut) develop a fatal graft blight due to delayed incompatibility at about 20 years of age. This is the so-called black-line disease. McKay (12) found great differences in survival of buds of Chinese chestnut placed on five seedling strains and Hardy (8) suggested that more attention should be paid to the parental relationship of stock and scion in the chestnut. Weschcke (28) reported that black walnuts grafted on butternuts yielded poor crops and that bitternut was a satisfactory stock for shagbark varieties and shagbark hybrids. Smith (25) advocated shagbark stocks for shagbark varieties but found bitternut to be practically as good. Stoke (26), and Smith (25) found eastern black walnut to be the best stock for all walnut species, including heartnuts and butternuts.

Nursery Practices

Commercial nurseries have adopted various methods to discourage the normal tap-rooting habit of nut trees and stimulate lateral and fibrous root production. Planting seed over screen wire, undercutting the seedling each year in the nursery row, frequent transplanting, and root pruning are methods commonly used. Attention must be given to the production of an adequate root system to help the grafted tree withstand the shock of transplanting to its permanent location.

Summary

The chief obstacle to the large scale growing of selected nut varieties is the difficulty in propagation. Careful workers with a background of knowledge and experience and skilled in craftmanship are successful in a limited way. Quantity production is apparently dependent upon specialized facilities and efficient labor programs. The need for extensive rootstock research is keenly felt by growers of walnut, hickory and chestnut.

Literature Cited

1. Barton, Lela V.—Seedling Production in Carya ovata, Juglans cinerea, and Juglans nigra. Cont. Boyce Thompson Inst. 8:1-5. 1936

2. Becker, Gilbert—Notes from Southwestern Michigan. Rept. North. Nut Grow. Assoc. 28:135. 1937

3. Bernath, Stephen—Propagating Nut Trees under Glass. Rept. North. Nut Grow. Assoc. 37:90. 1946

4. Chase, Spencer B.—Black Walnut Nursery Studies. Rept. North. Nut Grow. Assoc. 37:40-41. 1946

5. Chase, Spencer B.—Eastern Black Walnut Germination and Seedbed Studies. Jour. For. 45:661-668. 1947

6. Chase, Spencer B.—Budding and Grafting Eastern Black Walnut. Proc. Amer. Soc. Hort. Sci. 38:175-180. 1947

7. Gellatly, J. U.—Notes on Nuts and New Combinations of Old Principles. Rept. North. Nut Grow. Assoc. 29:115-120. 1938

8. Hardy, Max B.—The Propagation of Chinese Chestnuts. Rept. North. Nut Grow. Assoc. 40:121-126. 1949

9. Joley, Lloyd E.—Personal Correspondence. July, 1951

10. Lounsberry, C. C.—Bench Grafting of Black Walnuts. Rept. North. Nut Grow. Assoc. 28:60. 1937

11 MacDaniels, L. H.—Some Experiences in Nut Tree Grafting at Ithaca, New York. Rept. North. Nut Grow. Assoc. 28:52. 1937

12. McKay, J. W.—Results of a Chinese Chestnut Rootstock Experiment. Rept. North. Nut Grow. Assoc. 38:83-84. 1947

13. Morris, R. T.—Top Working Hickories—Rept. North. Nut Grow. Assoc. 11:105. 1920

14. Morris, R. T.—Nut Growing. 1931. Macmillan, New York

15. O'Rourke, F. L.—Unpublished data. 1940-1945

16. Painter, John H.—Personal Correspondence. July-August, 1951

17. Reed, C. A.—Nut-Tree Propagation. U.S. Dept. of Agr. Farmers' Bul. 1501. 1926

18. Reed, C. A.—Seedling Chestnut Trees versus Grafted Varieties. Rept. North. Nut Grow. Assoc. 32:79. 1941

19. Shelton, E. M.—Glauber's Salt for Humidity Control in Scion Storage. Rept. North. Nut Grow. Assoc. 28:70-71 1937

20. Shelton, E. J.—A Laboratory Experience in Testing Wax Mixtures for Use in Plant Propagation. Rept. North. Nut Grow. Assoc. 28:72-75. 1937

21. Shessler, Sylvester—Grafting Walnuts in Ohio. Rept. North. Nut Grow. Assoc. 39:145. 1948

22. Sitton, B. G.—Vegetative Propagation of the Black Walnut. Mich. Agr. Expt. Sta. Tech. Bul. 119. 1931

23. Sitton, B. G.—Pecan Grafting Methods and Waxes. U. S. Dept. Agr. Circ. 545. 1940

24. Slate, George L.—Grafting Walnuts in the Greenhouse. Rept. North Nut Grow. Assoc. 39:146-147. 1948

25. Smith, Gilbert L.—Our Experience with Rootstocks. Rept. North Nut Grow. Assoc. 40:62-64. 1949

26. Stoke, H. F.—Nut Nursery Notes—Rept. North. Nut Grow. Assoc. 34:96. 1943

27. Stoke, H. G.—Grafting Methods Adapted to Nut Trees. Rept. North. Nut Grow. Assoc. 37:99-102. 1946

28. Weschcke, Carl—The Importance of Stock and Scion Relationship in Hickory and Walnut. Rept. North. Nut Grow. Assoc. 39:190-195. 1948

29. Wilkinson, J. F—Preparation of Stocks for Propagation. Rept. North. Nut Grow. Assoc. 28:65-66. 1937

30. Zarger, Thomas G.—Nut-testing, Propagation, and Planting Experience of 90 Black Walnut Selections. Rept. Nut Grow. Assoc. 36:23-30. 1945