Myriophyllum aquaticum aka Brazilian Water Milfoil

Myriophyllum aquaticum lives in fresh water and grows for years. Its common name is parrot's feather. It belongs to the plant family Haloragaceae. Kew places its home range in central and southern tropical America. It grows in streams and on nearby wet mud or gravel. It also thrives in some warm temperate and subtropical areas. Plants that escape gardens have spread far beyond this range.

Its stems branch under water and form mats on top. Their leafy tips rise above the water. RHS gives a garden height near 0.3 feet [0.1 metres]. Upright shoots may reach about 0.5 feet [0.15 metres]. Its listed spread is 3.3–4.9 feet [1–1.5 metres]. Other sources give stem lengths of 6 feet [1.8 metres]. Useful Tropical Plants gives a full height of 6.6 feet [2 metres]. These larger sizes differ from the short shoots above water. Tough stems range from green to reddish green. Small leaves with fine splits grow in rings called whorls. Leaves under water are bright green. Those above water are a deeper blue-green. RHS calls the leaves deciduous, which means they fall off.

Tiny yellow-green flowers add little to the garden's looks. They grow where leaves above water meet the stems. NC State reports flowers in spring. Missouri Botanical Garden gives July–August as the bloom time. Male and female flowers tend to grow on separate plants. Useful Tropical Plants says some plants have both sexes. Reports list only female plants in the United States and Britain. NC State reports no fruit in the United States. Bits of roots or stems can start new plants.

The plant needs wet ground or water, not dry soil. Sources list full sun, though it also takes part shade. RHS lists loam and acidic, neutral, or alkaline soil. NC State lists soil rich in organic matter and standing water. Its hardiness ratings are USDA zones 6–10 and RHS H4. Growers have used it in ponds, water gardens, and aquariums. Leaves under water shelter fish and add oxygen. They also take up excess nutrients. Missouri Botanical Garden says the plant draws birds too.

Fast growth can make this plant a serious weed. Dense mats crowd out native water plants and hinder water sports. They can also give mosquitoes places to breed. Missouri Botanical Garden reports no major pest or disease problems. RHS reports a sales ban and limits on release into the wild. It also calls for control of plants people already hold. Useful Tropical Plants lists cooked young shoot tips as food. That source lists no known hazards or uses as medicine. Its hazard note does not prove the plant is safe for pets.