Polygonum persicaria L.
willow weed
Glabrous or glabrate, taprooted annual; stems erect to spreading, 0.1-1 m tall, often reddish, glabrous or with scattered antrorse hairs. Lvs on main stem similar to those on branches; petiole 2-12-(20) mm long. Lamina (1.2)-4-15 × 1.2-4 cm, lanceolate to linear-lanceolate, usually with a central dark blotch above, with scattered appressed hairs or more obviously hairy on midrib below, inconspicuously glandular-punctate, ciliate, entire; base cuneate; apex acute to acuminate. Ochreae to c. 1.5 cm long, fringed, often hairy, initially truncate, green or pinkish. Fls in erect, dense, spike-like, eglandular infls; spike usually not interrupted below, cylindric, sometimes very narrow and open, mostly terminal, to 4-(5) × 0.7-1.5 cm; peduncles eglandular; pedicels short, ± hidden amongst ochreae-like bracts; bracts always long-ciliate. Perianth 2-3 mm long, somewhat accrescent, eglandular, usually pale to deep pink, occasionally white; segments ± broad-elliptic, imbricate; veins straight or gently curved. Nuts 2-2.5 mm long, biconvex or trigonous with rounded margins, sometimes slightly biconcave, brownish black, shining.
N.; S.; St.; C.: throughout.
Temperate N. Hemisphere 1878
Near settlements, especially gardens, arable land, waste places, sometimes a serious weed in cultivated ground on rather wet soils, also riverbeds, roadside banks, cliff talus.
FL Jan-Dec.
Possibly poisonous (Connor 1977).
Willow weed is extremely variable in N.Z., especially in stature. The dark lf blotch is not always present, and some plants have a slender, cylindric, rather open raceme 3-5 cm long instead of the usual dense, rather blunt, cylindric one which rarely exceeds 3 cm.
Healy, A. J., Identification of Weeds and Clovers (1976), recorded an unidentified Polygonum sp. from cultivated land in Waikato and Auckland districts. The specimens on which this record was based (CHR 243400, 243402, 243403), and some more recent specimens from waste ground around Auckland, are probably hybrids. They are most similar to P. persicaria, which is probably one parent. The strongly glandular perianth and more slender spike reminiscent of P. hydropiper suggest this sp. as the other parent, although the glands in P. hydropiper are never stalked as in the hybrid. Allan (1940) recorded P. hydropiper × P. persicaria from several localities in N.Z., but without supporting specimens. P. salicifolium, with its normally slender raceme and ciliate ochreae, should also be considered as a possible parent of these hybrids, although it has an eglandular infl. These putative hybrids are fairly common locally. They are robust plants; stems to 2.5 m × 1.5 cm, glabrous or with scattered antrorse hairs; lvs mostly broad-lanceolate, with a faint dark blotch or unblotched; ochreae strigose, with apex fringed; infl. very slender, 3-6 × c. 0.5 cm; peduncles and perianth pink or red, glabrous or with very short glandular hairs.
