I appreciate the comments that several members have contributed. The plants are growing in high clay soils in a dredge disposal site; the clay overlays sand (parent ground material). So it stays wetter longer than the soils around it, despite being elevated about 6 ft from the native soil level. I do think the plant or plants arrived via dumping by a community resident. As they now cover less than 100 sq ft, this is a good time to contain and control the population. If the site were drier, I'd probably leave them alone, but it's not. I'll be talking to the manager soon, to come up with a suitable control process. Thanks, Kathleen