Reynoutria japonica   Japanese knotweed
           (Alt: Polygonum cuspidatum, Fallopia japonica )
invasive                      herbaceous           
Distribution: Greenbelt

USDA Plants Link: Reynoutria japonica   (POCU6)

Japanese knotweed was once widely planted as a tall, easy-to-grow plant with edible shoots, large leaves, and sprays of small white flowers.  Unfortunately, it also spreads aggressively and is almost impossible to eradicate, with roots that can cause damage to structures, so you should resist planting it even if you find it available in a nursery, or if a neighbor offers some to you.  A saving grace is that it does not appear to spread except vegetatively, and is not known outside of residential areas.

The use of Reynoutria japonica here follows the treatment by James L. Reveal in Intermountain Flora (vol 2a, 2012); however, Flora of North America (vol 5, 2005) and Flora of the Pacific Northwest use Fallopia japonica instead.  Most earlier treatments included the species in a broadly defined Polygonum, where the correct name is Polygonum cuspidatum (due to the combination Polygonum japonicum being already taken by an unrelated species).
The hybrid between Japanese knotweed and giant knotweed (Reynoutria sachalinensis), known as Bohemian knotweed (Reynoutria ×bohemica, with the “×” used to indicate a hybrid species) was collected in Boise in 1964 (Higgins 9-64, ID088466).  This is possibly the source of Reveal’s (2012) specifically mention of Boise as the sole location of R. ×bohemica within the bounds of the Intermountain Flora; however, no current populations have been located of either R. ×bohemica or R. sachalinensis anywhere near the Boise area

Japanese knotweed leaves are always less than 20 cm long, relatively flat-based, and nearly glabrous except for tiny bumps along the veins.  Giant knotweed, in contrast, has much larger (often > 20 cm) leaves with a more deeply cordate base and multicellular hairs along the veins.  Bohemian knotweed is somewhere in between, with sharp unicellular hairs scattered along the veins on the undersides of the leaves.

Reveal, J. L.  2012.  Reynoutria.  Intermountain Flora 2a: 218–220.

Trail guides, walks or articles that mention Reynoutria japonica