Understanding Invasive Plant Cultivars
When a plant species is found to be invasive, it is not always clear if the risk extends to all cultivars, particularly those promoted as low-fruiting or sterile. Unfortunately, some such cultivars have appeared to be sterile in their early years, but eventually started producing viable seed.
To guard against such situations, New York State law takes a “guilty until proven innocent” approach to cultivars. This means that no cultivars of a state prohibited species can be sold until researchers or industry advocates can supply sufficient evidence to prove that the cultivar is not capable of contributing to invasions.
The summer of 2018 marked the first gardening season since the rules went into effect in 2015 when one such prohibited, but wildly popular species could be sold again in New York: Berberis thunbergii (Japanese barberry). Four sterile cultivars were approved by the state; three from the University of Connecticut and the fourth, ‘Aurea’ pictured below, from an earlier class of “sterile” cultivars that withstood the test of time.
In the Midwest, Minnesota, Wisconsin and Indiana also regulate Japanese barberry. The Minnesota and Wisconsin rules both have many cultivar exemptions. Indiana’s rule, which went into effect in April of 2020, does not include any exemptions for sterile or low fruiting cultivars.
Even the new B. thunbergii cultivars remain guilty in some minds – see the article “Developing Sterile Invasives (Why Bother?)” here. Many in the invasive plant field recall having been burned by cultivars in the past. For example, all Callery pear (Pyrus calleryana) cultivars were widely advertised as self-sterile, which they are. The problem is that they are not cross-sterile and have become hugely invasive in some areas. Japanese barberry, sterile or not, is associated with higher black legged tick populations, but it is less clear if increases in tick population would occur in low-density planting situations with sterile plants that do not self-seed. Barberry is also known to alter soil chemistry by impacting pH (acidity), carbon cycling, and nitrogen cycling, though these impacts will be less dramatic in a low-density garden situation than in a heavily invaded site. In many minds, the verdict is still out. In the meantime, there are lots of great native alternatives to barberry. Check them out in the Landscape Alternatives section of the WIGL Japanese barberry profile!
Written by Clair Ryan, Midwest Invasive Plant Network Coordinator, August 2018, updated June 2020