Horticultural strategies to counter invasive Japanese barberry (Berberis thunbergii DC.)

Japanese barberry (Berberis thunbergii DC.) is an important nursery crop that has established invasive populations across the United States. A research program was undertaken to analyze the reproductive potential of this shrub\u27s ornamental cultivars and pursue development of sterile forms. The propagule production and early seedling growth of four primary commercial genotypes, B. t. var. atropurpurea (purple foliage), \u27Aurea\u27 (yellow foliage), \u27Crimson Pygmy\u27 (purple foliage) and \u27Rose Glow\u27 (purple foliage), was evaluated over three years using landscape specimens. It was found that \u27Aurea\u27 and \u27Crimson Pygmy\u27 produced significantly fewer seeds (75 and 90, respectively) than var. atropurpurea and \u27Rose Glow\u27 (2968 and 762, respectively). Young seedlings derived from these cultivars also expressed reduced vigor. A survey of additional Japanese barberry cultivars revealed that all forms produced some seedlings with green-leaf phenotype that resembled feral barberry. \u27Rose Glow\u27 specimens located adjacent to additional purple-leaved genotypes yielded significantly fewer green seedlings (45%) than specimens isolated from purple B. thunbergii (88%). Seedling offspring derived from the three primary purple-leaved B. thunbergii cultivars were also evaluated under shade levels ranging from 00% to 95% to approximate performance in the forest understory. As shade levels increased, seedlings with purple genotype displayed a less purple (more green) phenotype. Seedling growth was vigorous within the range of 00% to 85% shade with optimal performance at 30% shade.^ Efforts to induce tetraploidy in Japanese barberry as a preliminary step in a sterile breeding program were undertaken using pre-germinated seeds and sterile nodal explants derived from B. t. var. atropurpurea. Seeds were exposed to the mitotic inhibitors colchicine (.02%, .05%, .1% and .2%) and oryzalin (.002%, .005%, .01% and .02%) for 6, 12 and 24 hours and ploidy was determined by flow cytometry. Both mutagenic agents successfully induced tetraploidy. The most efficient seed treatments were .005% oryzalin for 6 hours and .05% colchicine for 24 hours. Oryzalin exposure produced high mortality of nodal explants with few lines and no tetraploids produced. The most productive explant treatment was 0.05% colchicine for 1 day which yielded 4 polyploid lines (two 2N+4N and two 4N) out of a total of 7 lines.