Knotweed

This vigorous, but noninvasive perennial flowers from early summer into autumn with bright red, purple, or white blossoms. The narrow blossoms are up to 4 inches long, and are held on long stalks above pointed, slightly puckered leaves. It grows to 4 feet high and wide.
Noteworthy Characteristics: Persicaria are rhizomatous or stoloniferous plants that are spreading and sometimes invasive. They have fleshy stems and conspicuously veined leaves, some of which exhibit autumn color. Many are suitable in a mixed container or border, others naturalized in a woodland or meadow. All plant parts may cause skin irritation on contact and stomach upset if ingested.
Care: Grow in moist soil in full sun to partial shade.
Propagation: Divide plants in spring or fall. Sow seed in spring in a cold frame.
Problems: Aphids, Japanese beetles, slugs, snails
Read more: http://www.finegardening.com/bistort-persicaria-amplexicaulis#ixzz3XrliltWw