Reynoutria japonica Houtt.
Asiatic knotweed
Rhizomatous herb forming large clumps; stems usually 1-2-(2.5) m tall, c. 1.5 cm diam. at the woody base, ± zigzagging, glabrous, ± glaucous to reddish. Petiole 1-5.5 cm long, ± dark crimson, channelled above, with a single pit at base on dorsal side. Lamina 8-23 × 5-17 cm (infl. lvs often smaller and more elongated), broad-ovate, somewhat glaucescent below, with < 14 pairs of lateral veins, often minutely puberulent on veins near base above, otherwise glabrous, ± undulate; base usually truncate, sometimes shallowly subcordate especially on vegetative shoots; apex cuspidate. Ochreae to c. 3 cm long, deciduous, glabrous. Panicles with branches clustered, to 6 cm long, > petioles, erect or suberect, densely puberulent. Bracts small, glabrous or puberulent, cuspidate or cuspidate-acuminate. Pedicels > bracts, glabrous, jointed in lower 1/2. Perianth 1.5-2.5 mm long at anthesis (excluding decurrent base), white; segments obovate to oblong-obovate, keeled. Filaments to 2 mm long. Stigmas fimbriate. Valves of fr. 7-11 mm long (including broad wing); apex notched. Nut 2.5-3 mm long, obtusely angled, glossy brown.
N.; S.: Marlborough, Canterbury, locally common in S.E. Nelson, locally abundant in Westland.
Japan 1935
Roadsides, waste places, river banks, usually near dwellings, gardens.
FL Dec-Mar.
R. japonica is generally commoner than R. sachalinensis. The attractive R. japonica cv. 'Compacta' (often cultivated as Polygonum reynoutria) is widely grown and has a tendency to spread within gardens. It is a ♀ plant with crimson frs, petioles and lf veins, and is < 1 m high. The sp. was first recorded as Polygonum cuspidatum.
