Floating Mat of Roundleaf toothcup in Tuscaloosa, AL. (Photo by Tuesday M. Ervin)
Roundleaf toothcup is native to south and southeast Asia from India to Japan. In its native range, this Rotala
species is reported to occur primarily in mountainous areas, including altitudes of more than 2600m (8500ft). However, roundleaf toothcup has been recorded from canals in southern Florida and a single pond on the University of Alabama campus in Tuscaloosa. Roundleaf toothcup is planted in Florida water gardens as a transition plant because it grows well from shorelines out to open water. This flexibility gives roundleaf toothcup a similar advantage to alligator weed in fluctuating wetland margin habitats. Roundleaf toothcup was first observed in Florida in 1996, and by 2002 it was known to occur in three south Florida counties, in addition to the one Tuscaloosa, AL population.
This species was added as a Category One invasive species by the Florida Exotic Pest Plant Council in 2007. Presently, it is not listed as a potential threat by any other US state. However, it is a recognized weedy invasive species in Australia.
Roundleaf toothcup is readily distinguishable from native MidSouth species of the Lythraceae (loosestrife) family (other species of Rotala
, along with species of Ammannia
.). It has soft, somewhat succulent, stems that are dark pink to purplish, and branch abundantly, giving the plant a prostrate, creeping growth form. Aerial leaves (approx. 2.5cm [1in] long) are round to broadly ovoid and are either sessile or with short petioles. Submersed leaves are more linear to elongate-elliptical in outline, and appear distinctly four-ranked along submersed stems. Its creeping growth form permits roundleaf toothcup to form dense mats across the water’s surface or along shorelines, while producing very dense root systems.
Inflorescence of Roundleaf toothcup.
Roundleaf toothcup produces abundant rose colored flowers in dense racemose spikes at the tips of aerial stems. Both soil-rooted and floating plants are capable of producing flowers, and in some cases, both habits have been observed in flower simultaneously, during spring into early summer. Fruit are small capsules (~ 1.5mm long [0.5in]) that split along four sides to release seeds (~0.5mm length [0.2in]).
Roundleaf toothcup is capable of vigorous spread by stem fragments that root readily at nodes. In this capacity, it also is similar to alligator weed, and regrowth rates have been documented in laboratory studies to be comparable to those of alligator weed. A study in Florida demonstrated that roundleaf toothcup is capable of producing viable seeds that germinate and yield vigorous seedlings under moist, but unsaturated soil conditions.
Little else is known about the ability of this species to reproduce sexually in its exotic US range.
Roundleaf toothcup inhabits wetland areas, including low-lying fields, moist pond margins, and areas adjacent to dams and reservoirs. In the US, it is well known from urban canals in southern Florida. A population also was recorded in an urban pond in Alabama; that population is believed to have been extirpated. This species is a serious threat for expansion because of its desirability as an aquarium species and its high growth rates (which cause it to outgrow aquaria in short periods of time). In its native range, the related Rotala indica
(Indian toothcup, also non-native in the US) occurs as a weed in rice cultivation in LA and CA, but is not known to occur outside rice paddies.
The known US distribution for roundleaf toothcup at the time of publication is in two watersheds of south Florida.
The one population known from the MidSouth (Alabama) is thought to have been extirpated.
No biological control options are currently available for roundleaf toothcup.
Herbicide Rates for Control of Roundleaf toothcup
||1.28 fl oz/gal
||1.9 lbs ae/acre
||4 lbs ai/acre
||Short-term contact, fast acting
||1-2 lbs ae/acre
||Systemic, slow results
||0.5-1.5 lbs ai/acre
||Systemic, slow results
||0.03-0.09 lbs ai/acre
(2-5.6 fl oz/acre)
||Systemic, very slow results
(1.25 fl oz/gal)
|1.5-6 lbs ae/acre
The systemic herbicide 2,4-D has been specifically tested as effective on roundleaf toothcup. While other herbicides have not yet been tested, the contact herbicide diquat and the systemic herbicides glyphosate, imazapyr, penoxsulam, and triclopyr are likely to provide effective control. A nonionic surfactant is suggested for optimal control with all these products (use a surfactant approved for aquatic use). Use only herbicides labeled for aquatic use. Carefully read the label before using any herbicide.
While no control techniques have been evaluated for roundleaf toothcup, it is likely that hand removal including all stem fragments would be effective. Repeated mechanical removal may be required to remove materials reinfesting the site from stem fragments or seed germination.
While no physical control techniques have been evaluated, it is likely that benthic barriers and mulches would be effective in controlling roundleaf toothcup.
Ervin, G. N. and R. A. White. 2007. Assessing vegetative growth potential of exotic Rotala rotundifolia
(Roxb.) Koehne (roundleaf toothcup), in comparison with Alternanthera philoxeroides
(Mart.) Griseb. (alligator weed), a known successful invader. GRI #5015, Mississippi State University, MS. July 2007. 6 pp.
Jacono, C. C. and V. V. Vandiver, Jr. 2007. Rotala rotundifolia
, Purple Loosestrife of the South? Aquatics
Cook, C.D.K. 1979. A revision of the genus Rotala
Reese, N.L., Haynes, R.R. 2002. Noteworthy collections: Alabama. Castanea
United States Geological Survey, Florida Integrated Science Center. 2006. Rotala rotundifolia
. Online resource at cars.er.usgs.gov/Nonindigenous_Species/Rotala_rotundifolia/rotala_rotundifolia.html
accessed [10 April 2006].
US Forest Service, Pacific Island Ecosystems at Risk (PIER). Online resource at www.hear.org/pier/species/rotala_rotundifolia.htm
accessed [25 June 2007].
Dr. Gary N. Ervin
Dr. John D. Madsen, Geosystems Research Institute, Mississippi State University
Mississippi State University
Department of Biological Sciences
Mississippi State, MS 39762-9627