Persicaria Mill.
Stipules entire when young, pale brown Persicaria
Flowers in spikes or spike-like racemes; if dense then more than 1 cm long
Stems erect or ascending. Spikes often paniculate.
Angles of branches and midveins of leaves ± lined with reversed bristles. Leaves ovate to lanceolate, bases hastate or sagittate, petiolate. Stems weak and straggling
Flowers arranged close together in ovate spikes on a branched peduncle. Leaves ovate, 3–7 cm long. Weak herb with stems up to 1.5 m long. Widespread. Wet places. Fl. summer. Spotted Knotweed Persicaria strigosa
Flower clusters distant from each other and the spike thus interrupted. Leaves narrow-ovate, 3–8 cm long. Weak herb with stems up to 1.5 m long. Widespread. Margins of swamps, ponds, etc. Fl. summer Persicaria praetermissa