Japanese knotweed is one of the most difficult weeds to eradicate due to its extensive woody root system. In England it is notorious for caving in basement walls where it was planted as a foundation plant. At least 15 years ago I discovered a 'nice' patch of Japanese bamboo along the banks of the Skunk River near the old city pool (Figure 1). It looked like the patch had gotten started when they had put in the new pedestrian bridge over the river, and over time it spread about 100 yards downstream. I contacted the Ames Parks Department to notify them of the problems associated with this plant. They thanked me for calling, said they'd always wondered what the plant was, and that they would initiate a control program.
It is now at least four years since they started their control program, and I am happy to report that it looks like they have finally eradicated the problem (Figure 2). Although I haven't talked to them about the specific program they used, based on the injury symptoms and disturbance in the area I think they used a combination of glyphosate and tillage to eradicate the weed. The important thing is that they were persistent in their control efforts, it was obvious that city workers returned multiple times each year to retreat the area and prevent the Japanese knotweed from replenishing its root system. Like any good perennial weed, a single application of a herbicide or any control tactic is not going to be successful at eliminating an established population of Japanese knotweed.
It is great to see the city take action to control this weed. Many/most populations of Japanese knotweed are sterile, and thus do not produce viable seed. The main means of spread is transportation of the rhizomes, either by human activities or water movement. The location of this patch on the bank of the Skunk River would make it a source of new infestations downstream during the frequent flooding events that erode the river bank and dislodge rhizomes. The only bad thing about this is that I lose a spot to take my 217 class to during field trips, but unfortunately there are plenty of weeds elsewhere.