- Name also: Common Mare’s Tail
- Family: Plantain Family – Plantaginaceae
(formerly Mare’s-tail Family – Hippuridaceae)
- Growing form: Perennial water and shore plant. Rootstock long, horizontal. Often forms dense stands.
- Height: 10–60 cm (4–25 in.). Stem unbranched, hollow, green, aerial stems erect–ascending, submerged stems limp.
- Flower: Petals and sepals vestigial, flower small, approx. 1 mm (0.4 in.) wide. Stamens 1. Pistils 1, stigma thread-like, unbranched. Flowers axillary.
- Leaves: Whorled, stalkless. Blade linearly needle-like, tapered, with entire margins, glabrous, submerged leaves thin and limp. Whorls usually with 8–12 leaves (submerged 12–16), leaves longer than internodes.
- Fruit: Elliptic, barrel-shaped, 2–3 mm (0.8–1.4 in.) long achene.
- Habitat: Ponds, lake and sea inlets, rivers, streams, ditches, clay pits, mud pits. Usually in shallow or quite shallow water, sometimes completely submerged.
- Flowering time: June–August.
Mare’s-tail is easy to recognise by its spruce-like appearance – small stands spreading out through ponds and puddles look like miniature spruce forests. It usually grows half above and half under the water, but it can also grow on the water line on dry land or completely submerged. Mare’s-tail can stand very different kinds of conditions and even mild pollution seems to do the species good – although submerged stands suffer from eutrophication. Mare’s-tail has spread throughout Finland, although it is not very abundant anywhere. The species’ overall habitat is remarkably wide as it can not only be found all over the northern hemisphere but also in southern temperate zones. It is a rare example of a true cosmopolitan, a citizen of the world that has spread out under its own steam to every corner of the planet.
Mare’s-tail’s inflorescence is so inconspicuous that most people would miss its flowering time entirely. Vestigial axillary flowers with a single stamen and pistil grow quite abundantly, the wind takes care of pollination and the waves efficiently carry off the achenes with the seeds.
Fourleaf Mare’s-tail & Hybrid Mare’s-tail
Hippuris tetraphylla & Hippuris x lanceolata
Mare’s-tail has a very simple construction and not many feasible identification markers. According to some researchers mare’s-tail is the only, highly diverse plant in its entire genus. Other people are however of the opinion that endangered and protected fourleaf mare’s-tail (four-leaved mare’s tail) is sufficiently clearly different to deserve species-status: its whorls usually only have 4–5 leaves which are shorter than its nodes, blunt, wide and obovate. The most difficult species to clearly identify are those that grow on the coast as the number and form of their leaves is in between the two previously-mentioned species. Sometimes they are grouped into their own species (H. lanceolata). They could be a hybrid (H. x tetraphylla) between mare’s-tail and fourleaf mare’s-tail, but usually they are seen as a variation of the former. At the end of the day it is a question of preference if there is one very varied species growing in Finland or if there are two or three.