Timing Is Everything
Curly-leaf pondweed control requires a fundamentally different timing strategy than management of warm-season aquatic weeds like hydrilla and water hyacinth. Because curly-leaf pondweed grows in cool water and senesces naturally in summer, management interventions that miss the spring growth window are largely ineffective. The key principle: treat in early spring before the plant reaches full canopy and produces turions. Late treatment — after turions have dropped to the sediment — has little effect on the population in subsequent seasons because the turion bank is already replenished. All aquatic weed control activities require state permits.
Permits Required: Aquatic herbicide applications require permits from your state's department of natural resources or environmental protection agency. Contact your state agency before beginning any management program.
Herbicide Treatment: Timing Window
The optimal window for herbicide treatment of curly-leaf pondweed is early spring, while plants are actively growing and before turion development is complete — typically March–May in the northern U.S. (earlier in warmer climates). Within this window, treatment before turion production in the leaf axils is most effective at reducing the following season's population by preventing turion bank replenishment. Several herbicides are effective:
Endothall (Aquathol K, Hydrothol 191)
Endothall is the most commonly used herbicide for curly-leaf pondweed control. It is a contact herbicide absorbed through leaf surfaces, causing cell membrane disruption and rapid tissue death within days. Application in liquid form by broadcast spray or injection is standard. Water temperature affects efficacy — minimum water temperature of 10°C (50°F) is recommended for adequate uptake. Endothall is applied during the spring treatment window and is favored for curly-leaf pondweed because its relatively rapid degradation minimizes effects on native warm-season plants that are just beginning to grow at the same time. Water use restrictions (swimming, drinking water withdrawal, irrigation) apply after treatment — check label requirements and coordinate with state permit requirements.
Triclopyr (Renovate)
Triclopyr is effective against curly-leaf pondweed as a broadleaf-active systemic herbicide. It is used in some treatment programs, particularly where curly-leaf pondweed co-occurs with other broadleaf target species. Application timing and efficacy considerations are similar to endothall treatment.
Fluridone (Sonar)
Fluridone is a broad-spectrum systemic herbicide that is effective against curly-leaf pondweed at concentrations maintained for 60–90 days. Because curly-leaf pondweed begins growing when water temperatures are low (fall through spring), fluridone applications for this species may need to begin earlier in the season than for warm-season species. Fluridone affects most aquatic vegetation and is most appropriate when multiple invasive species need simultaneous management. It is most suitable for enclosed water bodies where concentrations can be maintained.
Turion Reduction Strategy
Long-term management of curly-leaf pondweed requires depleting the turion bank in the sediment over multiple years, analogous to the tuber bank challenge in hydrilla management. Turions can remain viable in sediment for several years, meaning that even completely effective treatment in a given year does not prevent re-establishment if the turion bank remains intact. Multi-year treatment programs that consistently apply early-spring herbicide before turion production each year gradually reduce the turion bank. Research has documented meaningful turion bank reductions after 3–5 years of consistent early-spring treatment in some Minnesota lakes.
Winter Drawdown
In regulated lakes and reservoirs, winter water level drawdown — reducing water level in fall so that the littoral zone sediment is exposed to freezing temperatures — can kill turions in the exposed sediment. Curly-leaf pondweed turions are less cold-tolerant when exposed to freezing and drying than when protected by water. Effective drawdown requires exposing the turion-bearing sediment zone (typically 0–1 meter depth) to at least several hard freeze events (below -5°C). Refilling in spring before the growing season replenishes the lake. Like all drawdown programs, this requires regulatory permits, cooperation among lakefront property owners, and careful attention to effects on fish spawning habitat.
Mechanical Control Limitations
Mechanical harvesting of curly-leaf pondweed is possible during peak spring growth. However, unlike treatment programs that kill turions before they are produced, harvesting after turion drop (late May–June) has minimal effect on the following season's population — the turion bank is already replenished. Harvesting before turion drop (April–May) may reduce turion production somewhat by reducing the plant's resource base, but harvesting efficiency must be very high to meaningfully affect the turion bank. Mechanical harvesting is most appropriate for providing immediate navigational access in heavily infested areas while longer-term herbicide programs are implemented.
References
- Pullman, G.D. & Crawford, G. (2010). A decade of curly-leaf pondweed management. Lake and Reservoir Management 26:221–232.
- Anderson, L.W.J. (2011). Potamogeton crispus management. Journal of Aquatic Plant Management 49:60–64.
- Madsen, J.D., et al. (2012). Curly-leaf pondweed control options. Journal of Aquatic Plant Management 50:23–29.