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).

The dioecious biotype (plants with either male or female flowers) were introduced to Florida in the 1950s, and the monoecious biotype (plants with both male and female flowers) were introduced in the late 1970s (Cooke et al. 2005). Monoecious biotypes may be better adapted than dioecious biotypes to cool temperate climates due to its ability to form overwintering tubers quickly when photoperiods are short (Langeland 1996). Hydrilla verticillata can reproduce both asexually through vegetative fragmentation and tubers, and sexually where female and male plants occur in the same location. Sexual reproduction among and between monoecious and dioecious strains is possible (Steward 1993), but its importance is unknown (Langeland and Smith 1984). This plant mainly spreads vegetatively through dispersal of plant fragments, axillary turions, and tubers (Langeland and Sutton 1980).  Vegetative fragmentation is likely the most common method of reproduction for this species.

Tubers remain viable out of water for several days (Basiouny et al. 1978) and in undisturbed sediment for over 4 years (Van and Steward 1990). Viability remains after ingestion and regurgitation by waterfowl, although passage of vegetative propagules through the digestive tract likely renders them non-viable (Joyce et al. 1980).

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 can be detrimental to native species and the ecosystem. This species grows aggressively and competitively as dense mats that can displace or shade out native submersed plants. In the southeast U.S., hydrilla effectively displaces beneficial native vegetation, such pondweeds (Potamogeton sp.), eelgrass (Vallisneria americana), coontail (Ceratophyllum demersum) and southern naiad (Najas guadalupensis) (Estes et al. 1990, Langeland 1996, Rizzo et al. 1996, Van Dijk 1985). Infestations may reduce seed production of native aquatic plants, which may reduce the number of native species in the community (De Winton and Clayton 1996). Experimental evidence suggests that hydrilla has high allelopathy potential and can inhibit the growth of lettuce seedlings (Lactuca sativa L.)  and duckweed (Lemna minor L.) (Elakovich and Wooten 1989). Furthermore, infestations of this species may shift phytoplankton compositions and alter chlorophyll content (Schmitz et al. 1993).

Infestations of hydrilla may alter water chemistry, decrease oxygen levels, increase pH, and increase water temperature (Woodward and Quinn 2011). Abnormal stratification of the water column (Rizzo et al. 1996, Schmitz et al. 1993), decreased oxygen levels (Pesacreta 1988), and fish kills (Rizzo et al. 1996) have been documented in hydrilla infestations.   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 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).

*Page last updated 7/05/2022. Always check federal, state/provincial, tribal and local regulations directly for the most up-to-date information.

Control

Control of this species requires a multi-year effort; typically years 1-2 are focused on removal of overall biomass and fragments while years 3-8 control the tuber bank (Weibert et al. 2024).

Biological
Biological control methods in conjunction with herbicide treatments have been implemented on lower level infestations. On these occasions, treatment methods typically utilized sterile triploid grass carp (Ctenopharyngodon idella) (USACE and ERDC 2019). However, the utilization of grass carp presents challenges as they consume and reduce other aquatic plants and can also migrate to different water bodies.

Experimental trials have indicated that several other species could act as biocontrol agents on hydrilla, including Hydrellia spp. flies, Bagous spp. weevils, and polyphagous moths, herbivorous gastropods, and herbivorous fish (Ribas et al. 2017, Bownes 2018, Harms et al. 2021, Weeks et al. 2022).

Physical
For new populations, it is recommended to limit public access to prevent further spread from boating and fragmentation.

Unless the hydrilla infestation is small, mechanical controls are not recommended because it can be expensive, labor intensive, and further the spread of hydrilla (USACE and ERDC 2019). Benthic mats (i.e. burlap barriers) and limnocorrals (e.g. impermeable enclosures) can be used to confine smaller infestations in shallow, low-velocity waters to prevent spread and isolate for direct herbicide application(USACE and ERDC 2019).

Chemical
For rapid response to new populations, application of contact herbicides is recommended. Fluridone and endothall are the most frequently used herbicides.  Chemical treatment is recommended from mid- to late summer to kill new tuber sprouts. (USACE and ERDC 2019).

Initial research suggests florpyrauxifen-benzyl may also be effective (Weibert et al. 2024).

Other