- Name also: Small Yellow Pond-lily, Least Yellow Water-lily
- Family: Water-lily Family – Nymphaeaceae
- Growing form: Perennial herb. Aquatic plant, with floating leaves and thick creeping rhizome.
- Height: 0.5–1 m (20–40 in.).
- Flower: Regular (actinomorphic), 1.5–3 cm (0.6–1.2 in.) wide. Corolla 8–13-leaved, yellow. Outermost 5(–6) outer surface green, petals at least 4–5 times longer than sepals. Large amount of separate stamens. Gynoecium formed of many fused leaves. Stigma disc star-shaped, 4–6 mm (0.16–0.25 in.) wide.
- Leaves: Stipules not present. Stalk of floating leaves quite flat. Blade elliptic, 8–12 cm (3.2–5 in.) long, fine-haired underneath. Veins laterally branched. Submerged leaves quite round, wrinkled.
- Fruit: Full of seeds, many-parted, bottle-shaped, 4–6 cm (1.6–2.4 in.) long capsule.
- Habitat: Lakes, ponds, slow-flowing parts of rivers.
- Flowering time: June–August.
Like white water-lilies (Nymphaea spp.), yellow water-lilies (pond lilies) are quite a primitive plant group, as is demonstrated by e.g. the tepals’ spiral form and their large number. Floating leaves on the other hand represent a long-term specialization. Yellow water-lilies are poisonous, perennial and strong-rooted water plants. Flowerless yellow water-lilies can be told apart from white water-lilies by their leaf veins, which on yellow water-lilies end straight at the leaf margin without forming a net-like pattern, as they do on white water-lilies.
Compared to yellow water-lily (N. lutea) least water-lily has smaller flowers and the greener sepals. The clearest identification marker is the star-shaped form of the stigma disc (round on yellow water-lily) and the smaller number of rays (yellow water-lily has 15–20, least water-lily has 8–11). There is also a difference in the hairiness of the underside of the leaves (yellow water-lily is glabrous, least water-lily is fine-haired) and the form of the basal indentation (least water-lily’s basal notch is wider, yellow water-lily’s lobes touch each other at the base). Classification can also be rather challenging as there is also an independent hybrid of these two species, (N. lutea x pumila, also Nuphar x spenneriana). It usually has 11–15 rays and the form is between round and star-shaped.