Hydrilla verticillata aka Florida Elodea

Hydrilla verticillata is a rooted water plant called hydrilla or waterthyme. It belongs to the Hydrocharitaceae family and is a perennial. Its native range spans Poland and Asia, Australia, and the western Pacific. It also includes parts of Africa, from Uganda to northern Zambia. The plant now grows across much of the world. Aquarium sales helped spread it into waterways in the United States.

Hydrilla grows underwater, with roots anchored in the bottom. Its thin green stems branch and form large masses. Pond accounts describe stems reaching about 6.6 feet [2 metres]. Other sources report lengths up to 25 feet [7.6 metres]. These are stem lengths, rather than heights above the ground. The small green leaves have pointed tips and toothed edges. Each leaf measures about half an inch long. Four to eight leaves form a whorl around each node. Some leaves have sharp teeth beneath the central vein.

Small white flowers appear from late summer into fall. They grow singly on long stalks and have six petals. Some plants bear both male and female flowers. Others bear just one sex on each plant. Hydrilla spreads through seeds and pieces of stem. Even small broken pieces can root and start new growth. It also makes tubers and other buds that produce new stems. Tubers help it survive winter in colder places.

This plant needs water and cannot survive on dry land. It grows in both sun and shade. It tolerates water with high or low nutrient levels. Habitats include lakes, rivers, marshes, springs, ditches, and tidal waters. It can grow in shallow water or far below the surface. PFAF notes a preference for alkaline water, but broad pH tolerance. NC State lists USDA hardiness zones 5 through 11. PFAF describes its use as a pond oxygenator.

Its fast spread makes it a poor choice for open waterways. Dense mats displace native plants and hinder boats and water flow. They also reduce light and oxygen below the surface. Hydrilla appears on the federal noxious weed list. NC State reports no major pest or disease problems. However, it can host bacteria linked to brain disease in waterfowl. PFAF lists no known food uses or direct hazards. That does not establish food safety. PFAF records traditional wound treatments and a use in sugar refining.