.................Invasive plants can weaken biological diversity and transform the look of the land. Japanese knotweed, or Fallopia japonica, is one of the more nightmarish members of this group, botanists say, because it is hard to get rid of and it spreads quickly. ....Knotweed thrives in areas that have been disturbed, such as land used by people, who trample plant beds and create trails, causing native plants to wither, said Peter Del Tredici, a senior research scientist at the Arnold Arboretum. ''These plants aren't intrinsically evil," Del Tredici said. ''They are just taking advantage of the disturbance humans create." Park officials...