Japanese Knotweed is also called Japanese bamboo and Mexican bamboo. The bamboo comes about because the stem becomes woody and has enlarged nodes similar to those of a bamboo fishing pole. Once introduced it spreads rapidly by rhizomes and is extremely competitive. It also is extremely strong in that it can push through macadem. There is a related plant, sakhalin knotweed (Polygonum sachalinense), which has much larger leaves but is much less of a problem.
Photo by Dr. John Meade, weed scientist emeritus
Rutgers NJAES Cooperative Extension