Myriophyllum spicatum aka Eurasian Water Milfoil

Myriophyllum spicatum has the common name Eurasian watermilfoil. It belongs to the Haloragaceae family. This water plant lives for years, with most shoots under water. Its native range spans Europe, Asia, and Africa. Kew includes tropical mountains and the Aleutian Islands in that range. It is native to Britain and Ireland. It grows in lakes, ponds, and ditches, including lime-rich waters. Humans have brought it to much of the United States and Canada, where it now grows.

RHS gives a garden height of about 0.3 feet [0.1 metres]. It lists a spread of 3.3–4.9 feet [1–1.5 metres]. NC State describes long stems under water, unlike these garden figures. Those stems reach 6–20 feet [1.8–6.1 metres]. They branch near the surface and feel smooth. Their colour ranges from reddish brown to pale pink-white. Soft, olive-green leaves grow in whorls, or rings, of four. Each leaf is pinnate, split into many fine, threadlike parts. RHS calls the foliage deciduous, meaning the plant sheds its leaves.

Small flowers rise above the water on slim summer spikes. Their colours include dull red, pink, and yellow. Each plant has both male and female flowers, kept separate. The flowers have four petals and grow along the spike. NC State notes blooms in mid-June and again in late July. Once blooms fade, the spike lies parallel to the water. The fruit is a capsule with four lobes. It splits into four nutlets. Plants make seeds, but most spread comes from stem pieces.

For pond use, RHS says to grow it in loam-filled baskets. Set these in water about 3.3 feet [1 metre] deep. Full sun suits this method, though NC State says it tolerates shade. RHS lists acid, neutral, and alkaline soils as suitable. Its UK hardiness rating is H4. NC State lists USDA zones 3–10 and notes tolerance of cold water. RHS suggests its use in wildlife ponds. It also calls for regular cuts to curb excess growth. Root sections can form new plants.

Few pests or diseases harm this species, but its spread causes serious problems. Stem pieces can grow after they break off or get cut. Dense growth crowds native plants and changes the places where animals live. It can block boat routes, water equipment, and other water uses. North Carolina lists it as a noxious weed. Many states restrict its sale. PFAF lists roots as food, eaten raw or cooked. It reports no known hazards but gives a low rating for food use. It also records past uses to soothe tissues and reduce fever.