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Hydrilla (Hydrilla verticillata)

Hydrilla Common Names: Florida elodea, Water thyme, Waterthyme

Description: This species has been listed as a noxious weed under federal law and is also prohibited under Michigan law.

Habit: Submerged, rooted, perennial; forms monocultures.

Leaves: Whorls of 3-10 thin (2-4 mm wide, 6-20 mm long), rough leaves at the node, small spines give the leaf margin a toothed appearance, midribs red and often spiny.

Stems: Usually rooted in water up to 6 m (20 ft) deep; little branching in deep water but dense at water?s surface; forms horizontal stems in water (stolons) and underground (rhizomes).

Flowers: Female flowers are small, white, 6 parted, and occur from leaf axils; male flowers are green in color and resemble an inverted bell.

Fruit and seeds: Seed pod, cylindrical, 1-5 smooth, tiny, brown in color.

Habitat: Found in lakes, ponds, reservoirs and ditches.

Reproduction: Vegetatively by tubers and vegetative winter buds (turions), fragments root at nodes, seed less important.

Similar species: Canadian waterweed (Elodea canadensis) lacks Hydrilla's toothed leaf margins, red veins, and spiny leaf midrib.

Monitoring and rapid response: Monitor aquatic habitats. Mechanical removal is problematic as fragments may produce new plants. Drawdown from fall through spring may reduce population levels. Herbicides with contact poisons have been used to control hydrilla but are non-selective and may be highly toxic to fish; systemic herbicides reduce the overall growth rate of this species, without eliminating it completely. BioControls are being tested. Permits are usually required for herbicide use in water bodies and wetlands. For information see MDEQs Aquatic Nuisance Control website at:,9429,7-135-3313_3681_3710---,00.html. As this species is not yet recorded in Michigan, it is important to document new occurrences. Please obtain flowering or fruiting specimens and submit to: Anton Reznicek, Curator (Vascular Plants), University of Michigan Herbarium, 3600 Varsity Drive, Ann Arbor, MI 48108-2287.

Credits: The Michigan Natural Features Inventory (MNFI) has partnered with MISIN to provide the information in this fact sheet. Species images and/or information were used with permission from "A Field Identification Guide to Invasive Plants in Michigan's Natural Communities" and "A Field Guide to Invasive Plants of Aquatic and Wetland Habitats for Michigan.


Priority Species

Common Name:


Scientific Name:

Hydrilla verticillata







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