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.