Ludwigia

1. Leaves opposite; plants prostrate or floating in shallow water, usually rooting at the nodes.

L. palustris

1. Leaves alternate; plants erect.

2. Flowers on distinct pedicels ca. 2–5 mm long; petals conspicuous, about equaling the sepals; roots usually tuberous-thickened.

L. alternifolia

2. Flowers sessile or nearly so; petals apparently none; roots not becoming thickened (but plant stoloniferous and the stem often with well developed spongy tissue at base).

3. Bractlets at or (usually) above base of ovary 2.5–6 (–8) mm long; capsules, leaves, and stem glabrous.

L. polycarpa

3. Bractlets at base of ovary 0–2.6 mm long; capsules, leaves, and upper part of stem at least sparsely strigose.

L. sphaerocarpa