Persicaria prostrata
Stems prostrate or spreading. Perennial with appressed hairs. Perianth greenish or brown. Leaves lanceolate; stipular sheath sparsely hairy on the outside, ciliate on the margin. Flowers in spike-like axillary or terminal racemes 1–2 cm long. Widespread. In or near water. Fl. summer. Creeping Knotweed or Trailing Knotweed