Knotweed is a particularly difficult plant to control, even with herbicides. Usually treated patches need to be monitored for 5+ years to ensure no regrowth. It spreads predominately by fragmentation, mostly root but in summer it can be spread by cut stems as well. Depending on the specific species the plant can grow to as tall as 12’ making foliar application difficult especially when the patch is larger or mixed with other vegetation. My preferred technique for large stems is cut and paint(machete works best) and would raft the cut stems to keep them from re-rooting. This also prevents harming pollinators that are visiting the flowers during treatment window. It needs to be noted that with cut and paint there is a possibility to go over allowable