April 18, 1986: 32 years ago

​Keep an eye out for these bright yellow flowers blooming now along creeks across southwestern Pennsylvania. This specimen of lesser celandine (Ficaria verna, formerly known as Ranunculus ficaria) was collected in Edgeworth, Pennsylvania along Little Sewickley Creek by Myrta Macdonald and Jane Konrad in April 18, 1986. Native to Europe, lesser celandine was likely first introduced to the United States into gardens for ornamental purposes.  It is a spring ephemeral, meaning it blooms early in the spring and goes dormant by summer. Lesser celandine is among the earliest species to bloom in the spring, with bright yellow flowers.  In the past several decades, this species has become more and more common in southwestern Pennsylvania.  It grows in open woods, especially in moist soils along streams.  It forms dense mats that carpet the ground, capable of choking out native plants. 

Specimen below: Lesser celandine specimen from its native range in England, collected on April 13, 1826.

​Invasive lesser celandine is easily confused with the native wetland plant, marsh marigold (Caltha palustris).  Marsh marigold is found in somewhat similar habitats and has a similar appearance as lesser celandine.  However, there are several major differences.  Most notable, marsh marigold does not form dense, continuous mats along the ground, but instead plants are distinctly separate (although can be large and robust).  Also, lesser celandine has tuberous roots, while marsh marigold does not.  Compared to lesser celandine, marsh marigold is found in wetter habitats, has fewer flower petals (5-9 in marsh marigold vs. >8 in lesser celandine), and has larger leaves that are more rounded.

Specimen below: Be careful not to confuse lesser celandine (Ficaria verna) with the Pennsylvania native wetland species, marsh marigold (Caltha palustris).  This marsh marigold specimen was collected on April 28, 1887 in Westmoreland county, PA by P.E. Pierron.  The herbarium of St. Vincent College (Latrobe, Pennsylvania) became a part of the Carnegie Museum herbarium in 1991. 

We humans have moved plant species around the world at unprecedented scales. Human-mediated species introductions are a signature of the Anthropocene. Some of these non-native plant introductions were intentional, in the case of ornamental or food plants.  And some of these plant introductions were accidental, often in the case of “weeds” associated with human disturbance, cities, and/or agriculture. When introduced plants can survive and sustain a population without human intervention, we call these species “naturalized.”  When naturalized species are capable of reproducing at high rates and spreading across the landscape, far from the point of introduction, we call these species “invasive.”  This process from introduction to naturalization to invasion can occur quickly, but in many cases, there is a lag such that species introduction to full blown invasion can take decades to realize.
 
Herbarium specimens play a critical role in understanding species invasions, serving as a valuable archive of introduced plants.  When were species first introduced? Were they first introduced accidentally or were they intentionally planted in gardens? Where were species first introduced? Were they introduced multiple times at multiple locations? How have these introduced species spread in the decades to centuries since introduction? Have their morphology or genetic makeup changed since introduction?  Which introduced species become naturalized, which become invasive, and which species fail to establish altogether? These are just a few of the many questions that can be answered using herbarium specimens.