- Name also: Common Waterplantain, European Water Plantain, Great Water Plantain, Mad-dog Weed
- Family: Water-plantain Family – Alismataceae
- Growing form: Perennial herb, aquatic plant.
- Height: 30–100 cm (12–40 in.).
- Flower: Regular (actinomorphic), approx. 1 cm (0.4 in.) wide. Petals 3, separate, white to pale pink, yellowish spot at base. Sepals 3, separate, green, clearly shorter than petals. Stamens 6. Several carpels. Pedicels 1–3 cm (0.4–1.2 in.). Flowers in whorled groups forming a narrowing (cone-like) raceme.
- Leaves: Most leaves aerial, floating and submerged leaves fairly common in flowing water. Aerial and floating leaves long-stalked, 8–25 cm (3–10 in.) long. Blade ovate, with rounded or cordate base and entire margins. Submerged leaves may have a strap-shaped blade.
- Fruit: Achene.
- Habitat: In shallow water of lakes, rivers, and brackish water bays especially on grazed shores, ditches, ponds.
- Flowering time: June–September.
(Common) water-plantain starts to flower in June, and continues throughout the summer. However, individual flowers are very short-lived. The flower opens in the afternoon, and withers six hours later. The pollination is carried out by dipterans which are attracted to the flower by the nectars produced by the twelve nectaries. The ripe achenes may float for several days and, thus, spread over long distances.
Another possible Alisma -species in Finland is Alisma wahlenbergii which is an endemic submerged plant in Baltic sea area.