Fecundity of Japanese Barberry (Berberis thunbergii) Cultivars and Their Ability to Invade a Deciduous Woodland

Japanese barberry is an important landscape shrub that has a demonstrated potential to be invasive in ∼30 states across the central and northern United States. Forty-six cultivars were evaluated for their potential to produce fruits and seeds in a randomized, replicated field planting. Seeds from a subset of cultivars were evaluated for their ability to germinate and survive as seedlings in a deciduous woodland. Seed production for cultivars varied from no seeds to more than 12,000 seeds plant⁻¹ and the number of seeds per fruit ranged from 0.1 to 1.8. Five cultivars produced fewer than 100 seeds plant⁻¹, and two cultivars failed to produce fruit. When plants were allowed to mature for 4 to 5 yr beyond the first evaluation time, cultivars exhibited significant increases in fruits per plant, producing as much as 35,000 fruits plant⁻¹ (‘Sparkle’). ‘Golden Devine’ and ‘Red Chief’, fruitless cultivars at the first evaluation, produced 165 and 20 fruit plant⁻¹, respectively, at the follow-up evaluation, demonstrating that long-term evaluation of cultivars is necessary to accurately assess sterility. Between 12.5 and 31% cultivar seed sown in a deciduous woodland germinated, and seedlings survived at rates between 5.6 and 29.3%. Coupling cultivar seed-production data with germination and survival data in a deciduous woodland suggests that even cultivars producing as few as 100 seeds annually have the potential to contribute a few seedlings each year to a natural area.Nomenclature: Japanese barberry, Berberis thunbergii DCManagement Implications: Japanese barberry cultivars represent a substantial economic value to the nursery and landscape industries because they are popular, durable plants, widely used for landscaping. Unfortunately, Japanese barberry is also an invasive plant, and some states have imposed laws preventing the propagation and sale of the species and all cultivars. Cultivars of Japanese barberry look and behave differently from the species, with many being dwarf forms with purple or yellow foliage. The dramatic visual differences between cultivars of Japanese barberry and the species have led some to suggest that certain cultivars may be noninvasive or have a lower potential for rapid establishment in natural communities and could be exempted from species bans. The objective of this study was to evaluate the fecundity of a wide range of commercially available Japanese barberry cultivars to determine whether any could be considered noninvasive. We determined fruit production, seed production, and germination rates for 46 cultivars of Japanese barberry and its hybrids. Using seed from a representative subset of cultivars, we also determined the potential each cultivar had to germinate, survive, and grow in a deciduous woodland. Cultivars varied considerably in their seed production from more than 12,000 seeds to no seeds, and several cultivars produced fewer than 100 seeds plant⁻¹. When plants were allowed to mature for 4 to 5 yr beyond the first evaluation time, cultivars exhibited significant increases in fruits per plant. ‘Sparkle’ produced more than 35,000 fruits plant⁻¹ and ‘Golden Devine’ and ‘Red Chief’, which initially appeared to be fruitless, produced 165 and 20 fruit plant⁻¹, respectively. To accurately assess reproductive potential of cultivars, it is necessary to conduct evaluations for durations that are longer than those typically reported and to provide ample opportunities for cross-pollination. Cultivar seed germination rates were between 12.5 and 31% in a deciduous woodland, and seedlings survived at rates between 5.6 and 29.3%. By combining seed production data with establishment data in a deciduous woodland, even cultivars producing as few as 100 seeds yr⁻¹ in the landscape have the potential to contribute a few seedlings each year to a natural area. Most barberry cultivars clearly cannot be considered noninvasive, and even those that are the least fecund, can still be minimally invasive if they reach an age of 10 yr or more.