Hydrilla
Scientific: Hydrilla verticillata
Note: Modern phylogenetic evidence supports that Florida is invaded with Hydrilla verticillata subsp. verticillata which is also referred to as the ‘invasive dioecious biotype’ [66].
Common: hydrilla, water thyme, waterthyme, Florida elodea, whorled hydrilla, Indian star-vine
Family: Hydrocharitaceae (Frog’s-bit family)
Introduction History: Introduced to Florida in 1950s-1960s from aquaculture trade [2, 7].
Origin: Tropical Asia [10, 45, 57, 60]
General Description: A submerged aquatic perennial that forms dense underwater mats in lakes, ponds, rivers, and canals. Slender stems can grow several meters long and bear small, needle-like leaves arranged in whorls around the stem. Surface mats often impede water flow, navigation, and recreational activities.
Habitat: Grows rooted to substrate in freshwater environments [7, 30]. Freshwater ponds, lakes, rivers, streams, canals, springs, reservoirs, retention areas, bayous [15, 30, 70]. Exhibits physiological plasticity that permits invasion in a variety of aquatic conditions [36, 56].
Invaded Range: Globally cosmopolitan distribution. Invaded Eastern United States. Present throughout Florida [74].
Leaves: Narrowly lanceolate leaves (0.4-0.8 inches long) with serrulate or serrate margin and terminal spine at leaf apex. Leaves on lower portions of stem are oppositely arranged, transitioning to verticillate arrangement- whorls of 4-8 leaves in middle and upper portions of stem [7, 30, 75].
Flowers: Inconspicuous spathe-type inflorescence with two connate bracts produced from apex of stem near or at the water surface [7, 10, 30]. One flower per spathe typically produced June-August [30, 70].
Seeds: Sexual reproduction is reported as rare or uncommon in Florida [7].
Other (Asexual reproduction): More commonly disperses and reproduces asexually by fragmentation of stems or clonal spread via subterranean horizontal stems (rhizomes) or by turions [25, 49, 50, 51, 54]. Turions are asexual propagules which provide energy storage and complicate effective management of hydrilla [54]. They are 0.12-0.59 inches and vary in color (green, white, cream brown). They form either on erect stems within the water column, on floating fragmented stems (axillary turions), or on the end of the horizontal stems within the soil substrate (subterranean turions) [75].
Regulation/Categorization: Federal Noxious Weed (USDA), FISC Category I (FISC, 2025), Florida Noxious Weed, Florida Prohibited Aquatic Plant, UF/IFAS Assessment “Prohibited”
Forms dense vegetative mats which block light within water column, outcompetes native submersed vegetation [19, 22, 36, 69, 77].
Negatively impacts freshwater benthic biodiversity and community dynamics [29, 58, 77]. Although some research suggests the ecological effects of hydrilla to native ecosystems are context-dependent rather than universally detrimental [34].
Avoid introducing hydrilla into water bodies. Use best practices to prevent introduction by cleaning boat trailers, propellors, diver gear and live bait wells. Transporting plant fragments on boats, trailers, and in livewells is the main source of introduction into new lakes and rivers. Do not use hydrilla in aquariums or ornamental ponds. Opt for native submersed aquatic plants such as, sago Pondweed (Potamogeton pectinatus), bladderwort (Utricularia floridana), coontail (Ceratophyllum demersum), fanwort (Cabomba caroliniana) or southern naiad (Najas guadalupensis).
Small infestations of Hydrilla may be removed either manually or using hand tools, such as a rake. In some cases, lake drawdowns may help manage hydrilla by letting the exposed plants die and decompose.
Mechanical harvestors can be used to remove hydrilla from the water and transport it to shore for disposal. One drawback in the use of mechanical harvesters is that cuttings of hydrilla, which are not removed from the water, help to spread this weed.
Currently, four insects and one fish have been released to control hydrilla, but only two of these insects are established, and only one is commonly associated with hydrilla in the southeastern U.S. Click here to learn more.
Several registered aquatic herbicides provide temporary control of hydrilla. See: Efficacy of Herbicide Active Ingredients Against Aquatic Weeds to learn more.
