Management and Control
A wide variety of methods have been developed for controlling invasive aquatic plants (Madsen 1997). These include manual, chemical, and biological control methods, as well as various habitat manipulation techniques. The best method by far, however, is to prevent the spread of curly pondweed to new watersheds. Fragments of aquatic invasives that include seeds or (especially) reproductive turions can be carried to uninfested waters on boats, motors, bilges, live wells, and bait buckets. Therefore public education is invaluable in limiting the spread of curly pondweed and other invasive aquatic species.
Small populations of curly pondweed in otherwise uninfested waterbodies should be attacked aggressively (Madsen 1997). Madsen (1997) recommends hand-pulling, suction dredging, or spot treatments with contact herbicides for these situations. Methods such as cutting that cause fragmentation of the plants should be avoided, and a determined effort should be made to remove all plant pieces. Treatment of the site should be followed by careful monitoring of the treated patch and nearby areas.
Control of large populations usually requires a long-term commitment (Madsen 1997). Under the best circumstances, this may entail only periodic monitoring and occasional removal of scattered plants.
Permits from the appropriate state are generally required before implementing mechanical and chemical control methods for aquatic plants.
If used properly, mechanical methods can successfully control and reduce dense populations of curly pondweed and other aquatic plants. Unfortunately these methods are often applied with little biological or ecological knowledge (Madsen 1997). Whatever method is used, native vegetation should be left undisturbed if possible.
Hand-picking: Although labor intensive, hand-pulling may be the best solution for curly pondweed and other invasive macrophyte populations of less than 0.04 hectare (0.1 acre) (Madsen 1997). Hand-picking is inexpensive for small populations (and larger populations, if volunteers are available) and has the great advantage of being highly selective, leaving competing native plants in place. For dense populations, hand-cutters or rakes are often used (Madsen 1997). If the goal is to remove small population(s) in an otherwise uninfested water body, great care should be taken to remove all roots and plant fragments, to keep them from re-establishing (Hoffman and Kearns 1997).
Barriers: Bottom-covering barriers work quite well for small populations. Typically plastic sheeting is used (Madsen 1997). The color is unimportant, and even clear sheeting will work (Madsen 1997). As opposed to organic materials such as tarps, etc., plastic sheeting tends to collect gas beneath it, which can then lift it out of position (Gunnison and Barko 1992). Any type of sheeting must be removed and repositioned periodically (usually annually) to remove sediment (Hoffman and Kearns 1997).
Shade barriers have also been used, ranging from shading fabric to planting trees and stimulating algal growth (Madsen 1997). Shade barriers have proven inexpensive and effective in some circumstances, but like bottom barriers are nonselective and not universally applicable.
Mechanical cutting and harvesting: Much more than with terrestrial plants, mechanical control of aquatic plants seems to have captured the imagination of tinkerers and inventors. Mechanical harvesters can cut and remove large amounts of aquatic material. A major drawback of mechanical harvesting is that it is nonselective, removing native plants, as well as macroinvertebrates, semiaquatic vertebrates such as tadpoles and frogs, juvenile and forage fish, and even adult gamefish (Madsen 1997). Mechanical harvesting should be used only in water bodies that are already heavily infested with the target macrophyte.
Dredging: Dredging has successfully controlled curly pondweed and other aquatic macrophyte populations, though it is seldom used exclusively for this purpose (Tobiessen and Snow 1984, Madsen 1997). Dredging to 3.3 m (10.8 ft) greatly reduced the density and vigor of curly pondweed in a New York lake (Tobiessen and Snow 1984). Tobiessen and Snow (1984) attributed this to creation of areas too deep (light-limited) and cold for optimal growth of curly pondweed, though removal of most of the turion bank was probably a factor as well.
Significant problems can accompany dredging, including problems with dredge disposal, resuspension of silt (and any chemical contaminants it contains), high cost, and the potential for severe disturbance to sensitive aquatic communities. Dredging of a shallow southern Wisconsin lake decreased total plant biomass, increased abundance of species adapted to deeper water (including the invasive M. spicatum), and eliminated a few species that were apparently less adapted to disturbance, including two formerly common species (Nichols 1984).
Suction harvesting: A diver-operated suction dredge can remove smaller populations relatively cheaply and efficiently. This approach may have problems similar to standard dredging, though on a much smaller scale. Suction harvesting has been successful against Eurasian water milfoil (Myriophyllum spicatum L.), and can lead to an increase of native species (Madsen 1994).
As of 2000, six herbicides had been approved for use over water, with a seventh (tryclopyr) undergoing the registration process (Madsen 2000). In order to be registered for use over water, herbicides must not show any evidence of bioaccumulation, biomagnification, or persistence in the environment, and must be judged to have less than 1 in 1 million chance of harming "the environment", wildlife, or humans (Madsen 2000). Nonetheless they should only be used according to label instructions, and only when the benefits outweigh the risks. (It is legal and sometimes desirable to dilute them to a lower concentration than that recommended by the label, as described below.)
Chemicals have been used for decades to control (or attempt to control) aquatic plants. Recently, however, research has focused on adding small amounts of herbicide to the entire lake or other water body, instead of treating the plants directly. This work has been led a group of researchers within the (not always environmentally-friendly) US Army Corps of Engineers, who have done extensive work on the use of lakewide, low-level herbicide treatments in selectively controlling or eradicating invasive aquatic plants. Their work has led to some interesting and promising results.
Getsinger et al. (2001) treated 4 southern Michigan lakes that were heavily infested with Eurasian water milfoil with the systemic herbicide fluridone (Sonar) leaving 4 others as controls. Lakes were initially treated in May, when pondweed biomass was nearing its peak and turions had already begun to form. Fluridone was added to lakewater to achieve a concentration of approximately 5 micrograms/l in the top 3.05 m (10 ft) of water, with a follow-up treatment 2-3 weeks later. During the treatment year and the following year, lakewide, quantitative aquatic plant surveys were conducted in spring and again in summer, in all 8 lakes. Milfoil was reduced by 93-100% in the year of treatment, and 86% or more the year after treatment, in three of the four treated lakes. In contrast, curly pondweed was somewhat more abundant in all the lakes one year later. Getsinger et al. (2001) attributed this increase in curly pondweed to the decreased competition from milfoil, and/or a much warmer spring the second year, resulting in more rapid growth.
While these May fluridone treatments failed to control pondweed, Getsinger et al. (2001) point out that fall and early spring fluridone treatment has greatly reduced peak biomass and turion production of curly pondweed in Indiana and Illinois. They feel that mid-March to late-April treatments at concentrations as low as those effective on Eurasian milfoil (typically 5-10 micrograms/l), can also control curly pondweed, if applied early enough, before viable turions form.
Diversity of native plants increased from the first year to the second, in both the treated and untreated lakes, while cover increased or remained the same. Below 10 mcg/l most native species (including Elodea canadensis Michx., Vallisneria americana L., and two native Potamogeton species) are essentially unaffected. Some natives are adversely affected, though, and full recovery of the native plant community may take 2-3 years.
Another herbicide used as a selective control agent for curly pondweed is dipotassium salt of endothal (Aquithol), a contact herbicide. Skogerboe and Getsinger (2002) found that curly pondweed (and Eurasian milfoil) were controlled at 0.5-1.0 mg/l (to 0.00013 oz/gal) (active ingredient) in large microcosms. A number of native aquatic and wetland species were also tested at these levels - some, such as Illinois pondweed (Potamogeton illinoensis Morong.) and sago pondweed (P. pectinatus L.), were also severely damaged, while others, including coontail (Ceratophyllum demersum L.) and common cattail (Typha latifolia L.), were nearly unaffected. Illinois and sago pondweed showed some recovery 8 weeks after treatment, while curly pondweed (perhaps because its life cycle is not conducive to late-season growth) and milfoil showed very little to no ability to recover.
Drawdowns: Drawdowns have been used successfully for invasive plants, especially Eurasian water milfoil. They are only possible, though, where a dam or means to lower water levels exists. The major disadvantage of drawdowns is that they are extremely disruptive to native species and ecosystems (Madsen 1997). Several studies have found drawdown ineffective for curly pondweed (Cooke 1980). Lowering of water levels encouraged fruiting in at least one curly pondweed population (Hunt and Lutz 1959 in Catling and Dobson 1985).
Native plant restoration is useful as a cultural control, and is a goal of any aquatic weed management program (Nichols 1991). In lakes that have only recently been invaded, a propagule bank is generally present, but in lakes that have been dominated by invasives for many years, native plants may have to be reintroduced (Madsen 2000).
One of the first and most widely used biological control organisms used for aquatic plant control was the grass carp, Ctenopharyngodon idella (Madsen 1997). Use of these nonnative grass carp have had a number of problems, however, including forage likes and dislikes, feeding on nontarget plant and animal species, and escape to other water bodies, where they can cause ecological problems of their own. The use of sterile triploids has mostly solved this last problem (Tu 2003). They are also much less effective in northern climates than in the south (Stewart and Boyd 1994, in Madsen 1997). Grass carp are legally banned in Michigan, Wisconsin, and Minnesota (Nichols 1994, MDNR 2003).
At this writing we know of no host-specific biological control organisms being investigated for curly pondweed. However, its unique morphology and life history as compared to other pondweeds (Potamogeton spp.) suggests that curly pondweed is taxonomically somewhat distinct from these other species. It seems quite possible that certain insects or other organisms may have specifically adapted to the unique life cycle of curly pondweed, and would not be a threat to the other, generally perennial pondweeds. Organisms that would effectively control curly pondweed in North America may well be awaiting discovery.