The Cool-Season Invader
Curly-leaf pondweed (Potamogeton crispus) occupies an unusual ecological niche among aquatic invasive plants in the United States — it is most active and problematic when other aquatic plants are dormant or just beginning to grow. While most aquatic weeds peak in summer heat, curly-leaf pondweed grows through winter and spring, forming dense canopies in April and May before dying back in June and July when water temperatures exceed its thermal tolerance. This inverted growth pattern — growing in winter, declining in summer — creates management challenges that differ fundamentally from those of warm-season species like hydrilla and water hyacinth.
Native to Eurasia, Africa, and Australia, curly-leaf pondweed was introduced to North America in the 19th century, possibly through shipping ballast or early aquarium plant trade. It is now established throughout the northern United States and Canada, with its primary range in the upper Midwest, Great Lakes states, New England, and Pacific Northwest. It is absent or less common in the warmer Southeast, where summer temperatures are too high for it to persist. Its distribution closely follows the geographic range of its thermal tolerance — lakes with cool-water influxes or deeper zones where summer temperatures remain cool may have curly-leaf pondweed even in otherwise warm regions.
Why It Is a Management Problem
Curly-leaf pondweed causes management challenges through three main mechanisms:
Competitive Exclusion in Spring
Because curly-leaf pondweed begins growing in late summer/fall (from turions), grows through winter, and reaches peak biomass in April–May, it occupies the littoral zone at a time when native submerged plants are just beginning to grow. In many northern lakes, curly-leaf pondweed forms a dense surface-to-bottom canopy in early spring that shades and suppresses the emerging native vegetation. When curly-leaf pondweed dies back in summer, the native plants that should have occupied that zone may be too suppressed to fill it before algal growth takes over the newly opened space. This spring competition effect, repeated annually, can gradually reduce native plant diversity and abundance even in lakes where curly-leaf pondweed's summer die-off might suggest a less severe impact.
Summer Die-Off and Nutrient Pulse
When curly-leaf pondweed biomass dies back in June–July, it releases the nutrients stored in plant tissue back into the water column. This nutrient pulse — primarily phosphorus and nitrogen — occurs at the beginning of the peak summer growing season and can trigger algal blooms, particularly cyanobacterial (blue-green algae) blooms, that impair water quality throughout the summer. In lakes where curly-leaf pondweed biomass is large, this summer nutrient pulse can be a significant and measurable driver of warm-season algae problems. Research in Minnesota lakes has quantified phosphorus releases from curly-leaf pondweed die-off as a significant contributor to summer epilimnetic phosphorus concentrations.
Navigation and Recreation Obstruction
Peak curly-leaf pondweed density (April–June) coincides with the opening of the boating and fishing season across the northern U.S. Dense spring canopies obstruct boat navigation, foul motors and fishing lines, and create unpleasant conditions for swimmers and other recreational users. Because the plant naturally dies back in summer, some lake managers prefer to accept spring obstruction rather than undertake herbicide treatment — but this approach foregoes the opportunity to prevent the summer nutrient pulse and competition effects.
Identification at a Glance
Curly-leaf pondweed is identified by its distinctive crispy, wavy leaf margins — the leaves have a "crimped" or ruffled appearance at the edges resembling a slightly wavy piece of ribbon. Leaves are submerged, strap-like, 5–10 cm long, 6–12 mm wide, and have three prominent veins visible with backlighting. Color is typically reddish-green. In late winter and spring, look for the distinctive spiky turions — compact, somewhat spiny structures 10–20 mm long produced in the leaf axils and at the stem tips — which are the primary overwintering propagule. Turions in late spring and summer are highly diagnostic. See the complete identification guide for details.
Quick Management Summary
Effective curly-leaf pondweed management targets the plant during its spring growth window — before turion production begins in May or June. Early spring herbicide treatments (April–May in most northern states) target actively growing plants before turion drop and are significantly more effective than treatments applied after turion production. Registered herbicides include endothall (contact action, fast-acting), copper-based herbicides, and fluridone at whole-pond rates for systemic season-long control. Mechanical harvesting in spring can provide immediate navigational access in heavily infested areas but does not meaningfully reduce the turion bank. Multi-year management programs are typically required because turions persist in sediment for several years and re-establish populations even after apparently successful single-season treatment. Always consult your state DNR and obtain required permits before beginning any herbicide program. Early spring treatment combined with monitoring, prevention of new introductions through boater education, and long-term nutrient management offers the best chance of long-term population reduction. See the complete curly-leaf pondweed control methods guide for detailed guidance on timing, herbicide selection, and multi-year management planning.
References
- Catling, P.M. & Dobson, G. (1985). The biology of Potamogeton crispus. Canadian Journal of Botany 63:2108–2116.
- Bolduan, B.R., et al. (1994). Curly-leaf pondweed — nutrient release. Journal of Minnesota Academy of Science 59:3–8.
- Rogers, K.L., et al. (2018). Curly-leaf pondweed assessment in Minnesota. Lake and Reservoir Management 34:303–315.