Ludwigia decurrens Walter
Erect Primrose Willow
Family - Onagraceae
Stem - Erect to ascending, to 2 m, sharply 4-angled and 4-winged (from the decurrent leaf bases), glabrous.
Stem with decurrent tissue.
Leaf - Leaves alternate, sessile, to 20 cm, lanceolate, entire, usually glabrous. Venation conspicuous, pinnate, with lateral veins fusing to marginal vein at distal ends. Stipules present, to 0.5 mm long.
Flowers - Axillary, on short stalks, the bractlets obovate, usually glabrous, usually shed after flowering. Sepals 4, to 12 mm, ovate or less commonly lanceolate, angled to a sharply pointed tip. Petals 4, to 20 mm long broadly obovate, broadly rounded to truncate or with a shallow notch at the tip, yellow. Stamens 8, the filaments yellow. Pistil 1, with 4-locular inferior ovary.
Fruit - Fruits to 25 mm long, narrowly obconic, 4-angled and 4- winged, straight or rarely sharply curved, thin-walled, glabrous, dehiscing longitudinally. Seeds 0.5-0.6 mm long, free from the fruit tissue at maturity, the surface yellowish brown, nearly smooth, with a faint longitudinal patttern of fine lines.
Flowering - June - October.
Habitat - Streambanks, sloughs, pond margins, marshes, fens, swamps.
Origin - Native to the U.S.
Other info. - Historically this species was mainly found in the Mississippi Lowlands region of the state; however it has recently become quite common farther northward. It is ubiquitous along muddy banks of the lower Meramec River, for example. It is an easy Ludwigia to identify to species by virtue of the winged stems. These are not always visually conspicuous, but are easily detected by feel. The fruits are also characteristic in appearance, being square in cross section but much longer than the cubical fruits of seedbox (L. alternifolia).
Photographs taken at Shaw Nature Reserve, Franklin County, MO, 7-17-2012, at Tyson County Park, St. Louis County, MO, 8-6-2012, and at Pacific Palisades Conservation Area, MO, 8-12-2012 (SRTurner).