Florida elodea, waterthyme

The distribution of biotypes is changing rapidly, however, southern populations were predominantly dioecious female (plants having only female flowers) that overwinter as perennials (the monoecious biotype has spread south through Georgia, South Carolina, Tennessee, and Alabama). Populations north of South Carolina were often monoecious (having both male and female flowers on the same plant) (Cook and Lüönd 1982; Madeira et al. 2000). Fertile seed production was reported in the monoecious type (Langeland and Smith 1984). Both biotypes depend on tubers for overwintering, although monoecious hydrilla exhibits a more annual habit than the dioecious type, with abundant tuber/turion production around September (Owens et al. 2012).

Morphologically similar species include exotic Brazilian waterweed (Egeria densa), native western waterweed (Elodea nuttallii), and native (except Alaska and Puerto Rico) Canadian waterweed (Elodea canadensis). E. densa, E. nuttallii, and E. canadensis have 3-6 leaves per whorl, with inconspicuous leaf serration and no dentation on midrib, but E. densa leaves are 2-3 cm long, and both E. nuttallii and E. canadensis usually has 3 leaves per whorl near stem base (Langeland et al. 2008, Wunderlin and Hansen 2011, Rybicki et al. 2013).

Recent research into molecular techniques for identifying hydrilla and its biotypes has proven successful (Verkleij 1983; Ryan et al. 1995; Madeira et al. 2004). An early method used isoenzyme patterns in hydrilla to distinguish origin and biotype (Verkleij 1983). A later method used a random amplified polymorphic DNA (RAPD) procedure to find DNA markers in hydrilla samples (Ryan et al. 1995; Les et al. 1997; Madeira et al. 1997, 2000). A relatively inexpensive alternative method used “universal primers” to sequence hydrilla DNA (Madeira et al 2004; Benoit and Les 2013; Rybicki et al. 2013).

Models indicate that the top watersheds at risk for further hydrilla invasion include St. Clair-Detroit River corridor, western Lake Erie, southern Lake Erie and southern Lake Ontario, (USACE and ERDC 2019).

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

Hydrilla infestations slow the movement of water, potentially causing stagnation, flooding and other hydrology-related impacts (Carlson et al. 2008).

The cyanobacteria Aetokthonos hydrillicola is linked to avian vacuolar myelinopathy (AVM) and frequently grows on hydrilla (Haram et al. 2020, Breinlinger et al. 2021).
Hydrilla alters predator-prey relationships and food web interactions as a food source and habitat for waterfowl and fish. Sportfish exhibited lower weight and size when hydrilla occupied the majority of the water column, which suggests that foraging efficiency was reduced as open water space and natural vegetation gradients were lost (Colle and Shireman 1980).

Hydrilla verticillata has a high socio-economic impact in the Great Lakes.
Hydrilla causes major impacts on infrastructure and is among the worst aquatic plants in the southeastern U.S., causing costly damage to irrigation and hydroelectric power projects, and recreation (Cooke et al. 2005). Hydrilla can reduce the flow in drainage canals, which can result in flooding and damage to canal banks and structures (Langeland 1996). This species can clog intake pumps used for irrigation. This species is a nuisance for navigation of recreational and commercial waters and interferes with swimming (Langeland 1996). The dense mats of hydrilla create stagnant waters that can be used as mosquito breeding habitat (Kerr Lake Guide 2013), making this species a risk for human health.
Large hydrilla mats prevent access to many of the prime locations used for waterfowl hunting and most warm water sport fishing. Low oxygen levels in these mats make them unsuitable for the growth and survival of sport fishes and most other aquatic animals. Heavy hydrilla infestations (those that cover more than 30%) eliminate fish habitat, cause stunting, and reduce the number of harvestable fish. Thus, hydrilla usually is detrimental to sport fishing over the long term (North Carolina Agricultural Extension Service 1992).

Hydrilla verticillata has a moderate beneficial impact in the Great Lakes.
Hydrilla may increase water clarity by reducing sediment resuspension and reducing phytoplankton populations (Langeland 1996). It might also improve water quality by stabilizing nutrient cycling in eutrophic water bodies and accumulation of heavy metals (Shrivastava and Srivastava 2021).
Hydrilla may benefit some species as a food source, but only when its coverage is below 30% (Cole and Shireman 1980, Estes et al. 1990, GISD 2006).