Purpleinopp, thank you very much for answering my question. I thank you because even though your identification was incorrect, it was close. It led me along the right path and enabled me to find the correct answer. Very unfortunately, the plant is Japanese Knotweed, an invasive species that is evidently very hard to get rid of due to its thick web of roots - rhizomes. A speck of the root that breaks off is likely to start another plant. Now I have to deal with getting rid of this. I believe this came into my yard because if work that was done to install a new septic leach field. This required that an excavation company come in and dredge out 3/4 of my small yard, and because there was no place to put the soil, they took it offsite and then when the work is done they brought it back. It seems likely that some pieces of the root got mixed into my soil.