Sagittaria platyphylla (Engelm.) J.G.Sm.

Erect, perennial herb to 150 cm, with corms and stolons. Emergent leaves sheathing, petiolate; lamina 40–210 long, 20–100 mm wide, narrow or broad-elliptic, ovate or linear-ovate, glabrous, base cuneate and gradually tapering or occasionally sub-cordate, apex acuminate or acute, margin entire; petiole 240–657 mm long, 5.0–10 mm wide, trigonous, septate, conspicuously winged, glabrous. Submersed phyllodial leaves ribbon-like, 100–410 mm long, 6.0–23 mm wide, apex acuminate to obtuse, margin entire. Inflorescence a whorled raceme, pyramidal, bracteate, 50–150 mm long, 30–120 mm wide, 3–9 whorls of three pedicellate flowers; scape 150–650 mm long, 3–6 mm wide, erect, trigonous, emergent. Staminate flowers about 20 mm diameter, sterile carpels absent, pedicels spreading to ascending 0.5–0.8 mm wide, terete; sepals 4–9 mm long, 3–6 mm wide, ovate, concavo-convex, with conspicuous membranous margins; petals white, 8–10 mm long, 10–12.5 mm wide, broad-ovate with a narrow claw, delicate, apex rounded, irregularly crenate; stamens 17–41; filaments 1.5–2.5 mm long, dilated, densely pubescent; anthers yellow, 1.0–1.5 mm long, elliptic. Carpellate flowers, carpels many, staminodes absent, pedicels spreading, becoming recurved in fruit 1.0–2.0 mm wide, distinctly thicker than staminate pedicels, terete; sepals 5–8 mm long, 3–5 mm wide, ovate, concavo-convex, with membranous margins, spreading to reflexed, not enclosing fruiting head; petals white, delicate, deciduous 4.5–5.0 mm long, 5.5–6.0 mm wide, broad-ovate with a narrow claw, apex rounded. Fruiting head 5–15 mm diameter. Achenes 2.0–2.5 mm length, 0.5–1.5 mm wide, obovate with a lateral beak, without keel, winged, tuberculate, brown. Seeds about 1.0 mm long, 0.5 mm wide, uncinate, brown.

Sagittaria platyphylla has non-lobed elliptic or ovate leaves (which can be narrow or broad), distinguishing it from other large species of Sagittaria present in New Zealand: S. montevidensis and an unidentified species (in the Sagittifolia group), both of which have obviously lobed, arrow-shaped leaves. There is no coloured spot on the petals of S. platyphylla, in contrast to S. montevidensis, and the petiole of Sagittaria platyphylla is trigonous rather than D-shaped (semi-terete). Sagittaria platyphylla emergent leaves are like those of Alisma spp., especially A. lanceolatum. The petiole of S. platyphylla is trigonous in contrast to the D-shaped (semi-terete) petioles of Alisma spp. When flowering, Alisma spp. have broader, taller inflorescences with smaller, but many more, pink or lilac flowers rather than white.

Sagittaria platyphylla is a designated unwanted organism in New Zealand under the Biosecurity Act, is banned from sale and distribution, and is targeted for eradication wherever it occurs.

Wetlands, in streams and along edges of waterways, ponds, drains.

Notes: spreading by seed in waterways, and by rhizomes and corm fragmentation.

AK 185308, P.D. Champion, Tamaki, Glenfield, Becroft Park, 1989.

Flowering: Dec.–Apr. Fruiting: Mar.–May.

2n = 22 (Fedorov 1969)