Eight Corylus L. (hazelnut) species were intercrossed in all possible combinations to reveal genetic relationships. Pollinations were made on either individually bagged branches or trees covered entirely with polyethylene using mixtures of pollen of five genotypes to minimize low cluster set due to single incompatible combinations. Percent cluster set, seed germination, and hybrid seedling survival were determined. Hybridity of seedlings was verified by inspection of morphological traits. Based on percent cluster set, seed germination, and hybrid seedling survival along with observed morphological similarities, Corylus species were placed in three groups: 1) the tree hazels C. colurna L. (turkish tree hazel) and C. chinensis Franchet (chinese tree hazel), 2) the bristle-husked shrub species C. cornuta Marshall (beaked hazel), C. californica (A.DC.) Rose (california hazel), and C. sieboldiana Blume (manchurian hazel), and 3) the leafy-husked shrub species C. avellana L. (european hazel), C. americana Marshall (american hazel), C. heterophylla Fischer (siberian hazel), and C. heterophylla Fischer var. sutchuensis Franchet (sichuan hazel). The two tree hazel species crossed with each other readily, as did the three bristle-husked shrub species. The frequency of blanks was low (<20%) for crosses of the tree hazels, and <50% for interspecific crosses within the group of bristle-husked species. The leafy-husked shrub species could be crossed with each other in all directions, although cluster set on C. heterophylla was low. For crosses of species belonging to different groups, set was generally low and the frequency of blanks high. Nevertheless, a few hybrid seedlings were obtained from several combinations. When used as the female parent, C. californica set nuts when crossed with all other species, indicating possible value as a bridge species. Crosses involving C. avellana were more successful when it was the pollen parent. In crosses with C. avellana pollen, cluster set on C. chinensis was better than on C. colurna and the frequency of blanks was much lower, indicating that it might be easier to transfer nonsuckering growth habit from C. chinensis than from C. colurna. Reciprocal differences in the success of crosses was observed. The following crosses were successful C. californica × C. avellana, C. chinensis × C. avellana, C americana × C. heterophylla, C. cornuta × C heterophylla, C. californica × C. colurna, and C. americana × C. sieboldiana, but the reciprocals were not.
`Barcelona' hazelnut (C. avellana L.) shoots were girdled and stool layered in a factorial design with three tissue removal (leaf, bud and meristem removal) and two hormone (with or without 750 ppm IBA) treatments. Percent rooting, rooting grade (0 to 5), shoot length, shoot diameter, and total number of buds were determined. Average percent rooting was >90% for all treatments. Girdling alone gave as high percent rooting as hormone application. The main effect of IBA was on root quality rather than percent rooting. About 75% of the hormone-treated rooted layers could be directly planted (grades 3 to 5), compared to 44% for the control, but shoot length, shoot diameter and total number of buds decreased with IBA application. Bud removal did not affect average percent rooting while meristem removal reduced it slightly. The percentage of layers having grades 3 to 5 was lower for the meristem and bud removal treatments than for leaf removal. Our results support the adoption of stool layerage with girdling and IBA application for the production of strong, well-rooted trees suitable for planting directly in the orchard.