September 8, 1991: 26 years ago today

​Collected on September 8, 1991, this specimen was found near Tarentum, PA by Walt Zanol.  If you had to pick the most aggressive invasive plant in the Pittsburgh area, knotweed would be among the top choices.  This particular specimen is Bohemian knotweed (Fallopia xbohemica), a hybrid between giant knotweed (Fallopia sachalinensis) and Japanese knotweed (Fallopia japonica).  Japanese knotweed was introduced from East Asia and Giant knotweed came from Sakhalin (Russia).  The hybrid likely originated when these two species “met” after they were introduced in Europe.  Both species and their hybrid can be found around Pittsburgh, often in enormous dense clusters along highways and waterways.  Take note on your drive to work or walk in the neighborhood – knotweeds are all around! 
 
Giant knotweed is distinguished by its large (usually much larger than your hand), heart-shaped leaves.  Japanese knotweed and the hybrid Bohemian knotweed are much more difficult to distinguish, with much variation in leaf shape.  In fact, the hybrid was only recognized in the early 1980s and was largely overlooked in the US until even more recently.  Some suggest it invades more aggressively than its parents. Most specimens in the Carnegie Museum of Natural History’s herbarium were originally identified as Japanese knotweed.  Last year, Allison Cusick, Research Associate in Botany at the museum, went through all 212 knotweed specimens and reidentified many as the hybrid.  In fact, only 3 of the specimens from Allegheny county were identified as Japanese knotweed!

For more info, see Zika & Jacobson (2003) Rhodora 105:143-152. https://www.jstor.org/stable/23313523?seq=1#page_scan_tab_contents

All three knotweeds collected a couple weeks ago at the same site near the Allegheny River and Barking Slopes Conservation Area, New Kensington/Plum, PA.  Left to right: Japanese knotweed (Fallopia japonica), Bohemian knotweed (Fallopia xbohemica), Giant knotweed (Fallopia sachalinensis).