Part of the Water-Milfoil family this aquatic perennial can be seen projecting from still and flowing water, in burns, lochs and ditches. It survives quite well even in the fastest flowing streams. It grows all over Scotland, but more commonly in the North West and Isles.
The slender stems and leaves are entirely submerged and only the flower stalks can be seen above the surface of the water so it is only noticeable during its flowering season.
The stems are slender, branched and hairless. Leaves are absent towards the base of the stem, but soon appear in whorls of 4 or sometimes 3. Each leaf is deeply lobed into about 6 – 18 feathery segments. Leaves are about 15 to 30mm in length.
At the top of the stem tiny flowers, about 2 to 4mm across, grow alternately (hence the species name) up an ascending spike, growing in the angles of small leafy green bracts (tiny leaf-like structures). Male flowers grow at the top of the spike, female flowers below. Each have 4 petals, though minute on the female flowers, which are yellow with red streaks. There are slightly larger petals on the male but they fall quickly. 8 yellow stamens are prominent on the male flowers and 4 short stigmas on the female.
In Spiked Water Milfoil (M. spicatum) flowers tend to grow in rings of 4. This is less common in Scotland. Another variety is Whorled Water-Milfoil (M. verticillatum), which has five leaves to a whorl and more prominent toothed bracts also in whorls or 4 or 5 at the tip of the stems.