Roundleaf toothcup, Dwarf rotala
Rotala rotundifolia, or roundleaf toothcup, is a popular ornamental in the international aquarium industry that has more recently been promoted for water gardens. It is often referred to as “dwarf rotala” in the trade. It was first collected in 1996 from a flood control canal passing through a residential area in Broward County, Florida. Other naturalized populations have been found in Palm Beach County and Lee County, Florida. The only other known North American introduction point is Tuscaloosa, Alabama, where it was well-established around a campus pond until the pond was drained and left to dry. Rotala rotundifolia is native to India and Southeast Asia. It is in the same family, Lythraceae, as another aquatic weed, purple loosestrife (Lythrum salicaria).
Rotala rotundifolia is a creeping aquatic perennial with soft, dark pink stems that branch often to form low, creeping clumps. Leaves are rounded and attach closely to the stem without a leaf stalk. They are arranged oppositely on the stem. Flowers occur in spikes at the tip of stems. Plants flower prolifically in spring and early summer. Rotala rotundifolia can spread by floating stem fragments, which root adventitiously at lower nodes. The plant also produces viable seeds. Fruits are dry capsules that split open to disperse seeds. Other species of Rotala that occur in North America are the native Rotala ramosior and the introduced Rotala indica.
Rotala grows about 4 to 5 inches per week. Once the plant reaches the top of the water, it grows across the surface and quickly shades out other aquatic vegetation. It requires high sunlight and CO2 concentrations for optimum growth. Roundleaf toothcup belongs to the family Lythraceae that encompasses about 21 genera and 500 species. The family is mainly tropical but has a number of temperate members. Purple loosestrife (Lythrum salicaria) which is a troublesome invasive weed in the United States also belongs to the same family. Thus roundleaf toothcup has the potential to invade sites in the southeastern United States. Rotala grows submersed and emergent similar to hygrophila. In Florida, plants display both terrestrial and aquatic growth forms and produce many small seeds within capsules. Although little is known about the biology of roundleaf toothcup, the ability of the plant to reproduce from vegetative fragments, and its ability to produce seeds, further raises concerns about its potential invasiveness.
This plant will commonly infest drainage canals and prevent them from emptying rapidly during periods of heavy water load, thus causing flooding. Rotala rotundifolia will also impact navigation when waterways become obstructed.
Rooted plants, or plant fragments, should be removed and disposed of properly to prevent spread.
Few cultural/physical measures have been found successful for this plant. Since Rotala rotundifolia will root and establish above the waterline, dewatering events (drawdowns) often will not result in complete control.
Mechanical control is currently the most common and frequently used control method. Since this plant spreads by fragmentation, harvested plants should be removed from the site and not simply left to decompose on the bank. Mechanical removal will not commonly provide long-term control.
No insect species have been introduced as biological controls. Grass carp have also been observed to show a feeding preference for this plant. Therefore, grass carp will be of limited value to manage this species.
Studies were conducted at the University of Florida-IFAS, Center for Aquatic and Invasive Plants (CAIP), Gainesville, FL. Young roundleaf toothcup plants were planted in 30 cm diameter plastic pots filled with 2/3 potting media covered with 1-2 inches of sand. The plants initially were very slow in growth. After 4-5 months, these plants were transferred to 100 L tubs for treatment with different aquatic herbicides. Herbicides treatments included submersed application of triclopyr at 250, 500, 1000, 2500 ppb; endothall at 250, 500, 1000, 2500 ppb; flumioxazin at 50, 100, 200, 400 ppb; diquat at 50, 100, 200, 400 ppb; and 2,4-D at 250, 500, 1000, 2000 ppb. Plants were harvested 10 weeks after treatment (WAT) and above ground biomass was determined as dry wt/plant. Plants were dried for 1 week, weighed, and statistically analyzed for treatment differences.
The effects of herbicides on Rotala plants are presented in Table 1. Plants were harvested at 10 WAT to evaluate the effects of these herbicides on R. rotundifolia. ANOVA and regression analysis were utilized to determine rate effects of all the herbicides and make treatment comparisons. Conclusions from these studies are as follows:
The active ingredients that have been successful in treating Rotala rotundifolia include: