- Name also: Water Pineapple
- Family: Frogbit Family – Hydrocharitaceae
- Growing form: Perennial herb. Water plant. Submerged plant, weakly rooted to the bed, rising to the surface to flower. With runners. Forms wide stands.
- Height: 15–50 cm (6–20 in.). Stem very short.
- Flower: Plant dioecious (staminate and pistillate flowers on different plants, only female plants in Finland). Corolla regular (actinomorphic), white; petals three, 15–25 mm (0.6–1 in.) long. Sepals 3. Waste-stamens 15–30. Pistil of 6 fused carpels, styles 6, stigmas divided in 2. Flowers solitary, flower-stalk shorter than leaf, flat, with barbed teeth along edge. Blooms quite rarely.
- Leaves: In a rosette, stalkless, 15–40 cm (6–16 in.) long submerged leaves. Blade narrowly lanceolate–linear, rigid, fragile, serrated, teeth with up-curving spine. flower stalks’ tip with 2 small upper leaves.
- Fruit: Capsule. Fruit probably doesn’t develop in Finland.
- Habitat: Small, nutritious, neutral lakes, Sheltered bays in large lakes.
- Flowering time: July–August.
Water soldier is mainly invisible as it grows under the water and attaches itself to the bed. During its flowering time the stem breaks off and rises to the surface, and the tips of the leaves and the inflorescence rise above the surface of the water. The smell of decomposing flesh that the flower emits attracts flies and perhaps some butterflies to pollinate it. The plant flowers quite rarely, but it is not much of a cause for joy for this dioecious plant: it only produces a male or a female flower, but there are no pollen-producing male plants in Finland. It spreads well vegetatively, however: the flowering rosette develops short runners whose tips develop small, overwintering lateral rosettes at the end of the summer. The following spring the rosettes become separate plants when the runners wither. The plant’s vegetative spread is quite limited, however. Floating rosettes sometimes find new habitats.
Water soldier grows rarely in different parts of Finland as far north as Kittilä. This demanding water plant arrived in Finland’s lakes around 11,000 years ago during the warm period at the end of the Ice Age. At that time, like many other plants, it would have grown larger in the north than it does now. Most of the flora from that warmer time have since disappeared, but water soldier has clung on in shallow lakes. In western Finland the species is completely absent and around coastal regions it only grows around the Bay of Bothnia estuary and in a couple of places along the coast in the province of Satakunta. Water soldier is also necessary for Finland’s rarest dragonflies, who won’t lay their eggs anywhere else. The spiny plant appears to provide protection for the grubs from fish and other potential predators.