The Connecticut Nursery and Landscape Association (CNLA) recently called for a voluntary ban of 25 cultivars of Japanese barberry (Berberis thunbergii). The parent species – in photo to the right – has overtaken much of Connecticut forestland and large stands of the thorny green woody shrub appear to act as optimal host areas for Lyme-disease carrying ticks. As reported by CNLA, the 3-year phase-out includes the 25 cultivars that are most prolific at seed production. Another 18 cultivars – those at the bottom 10% for seed production – are not included in this voluntary ban. Click here to read the full CNLA report and the list of banned and permitted barberry cultivars.
It takes years of research to determine just how invasive a plant is in a specific area. We are fortunate to have an active research facility at the University of Connecticut. There, Dr. Mark Brand and colleagues continue research into the invasiveness of Japanese barberry. To date, research shows 13 of the banned cultivars producing more seeds – from 9900 to 1150 seeds per plant – than the wild/parent Japanese barberry which produces about 1100 seeds. Seed production is key to invasiveness. Birds eat seeds, partially digest them, then deposit still viable seed wherever their droppings drop.
While I applaud CNLA’s urging of its member nurseries to ban the heaviest seed producing barberry shrubs and its call for increased public education of the issue, I remain concerned that non-CNLA affiliates might ignore the ban and continue selling potentially invasive Japanese barberry to an unsuspecting or uneducated gardening public. These are popular shrubs, particularly the red- and gold-tinged cultivars, and what the public wants the public usually gets, one way or another.
Therefore, the best way to complement CNLA’s voluntary ban is to inform the gardening public of the potential invasiveness of most, and possibly all but a handful of Japanese barberry shrub types. Research currently shows the cultivars Aurea Nana, Aurea, Bagatelle, Golden Devine, and B. x mentorensis with the lowest seed production – ranging from zero to 11 per shrub. Of these Aurea Nana, Aurea, and Golden Devine sport yellow leaves, while Bagatelle is red and x mentorensis is green.
If you must have this thorny shrub in your landscape, at least make sure to plant one of the types least likely to produce a plethora of seeds for birds to eat and spread through their droppings.
The parent cultivar has done a bang-up job of spreading itself throughout much of Connecticut’s woodlands. It’s not an attractive plant and it’s thorniness makes what should be a pleasant stroll through the woods a very unpleasant experience. Then there’s the tick factor, and the last thing we need is ever-expanding tick habitats.
Read related stories in Garden Center Magazine and the Hartford Courant. To learn more about invasive plants in Connecticut visit Connecticut Invasive Plant Working Group’s website or, better yet, attend the invasive plant symposium on October 14, 2010 at UConn in Storrs.