Curly-leaf Pondweed

Curly-leaf pondweed is a rhizomatous perennial herb producing a flattened, branching stem up to a meter long. The submerged leaves are alternately arranged. The leaves are sessile, linear or oblong in shape, 25–95 millimetres (1–3+3⁄4 in) long and 5–12 millimetres (3⁄16–15⁄32 in) wide. The leaves may be bright green, olive green or (especially later in the season) fibrous and brownish and have noticeably serrated margins, a feature that distinguishes them from other pondweeds. The leaves usually have wavy edges but this is not always apparent, especially on new growth. Turions occur in leaf axils and at stem tips.

The inflorescence is a short spike of flowers emerging above the water surface. It flowers from May until October. The turions of the plant develop along with the fruits and germinate, leaving the newly sprouted plants to overwinter. Although quite variable, P. crispus is usually a straightforward plant to identify. Hybrids with various other pondweeds are recorded, but these do not usually closely resemble P. crispus. There are described hybrids with Potamogeton trichoides (P. × bennettii Fryer), P. perfoliatus (P. × cooperi (Fryer) Fryer), P. alpinus (P. × olivaceus Baagøe ex G.Fisch.), P. lucens (P. × cadburyae Dandy & G.Taylor), P. praelongus (P. × undulatus Wolgf.), P. ochreatus (P. × jacobsii Z.Kaplan, Fehrer & Hellq.) and P. friesii (P. × lintonii Fryer).

Potamogeton crispus is native to a wide range of countries in Asia (Afghanistan, Bangladesh, Bhutan, China, India, Indonesia (Sumatra), Japan, Kazakhstan, Korea, Kyrgyzstan, Laos, Mongolia, Myanmar, Nepal, Pakistan, Russia, Tajikistan, Thailand, Turkmenistan, Uzbekistan, Vietnam); Africa, the Middle East, Australia, and Europe. It has been introduced to the Americas and New Zealand.

Curly pondweed is widespread and common across most of its native range, growing in standing and slow-flowing water including small ponds and ditches. It is strictly a lowland plant and requires fine substrates in standing or slow-flowing calcareous water. However, it is tolerant of significant nutrient pollution, and this has allowed it to persist in intensively farmed areas where more sensitive pondweeds have declined. Its production of both seed and turions makes it relatively resistant to disturbance such as dredging, in contrast to some of the larger broad-leaved pondweeds, although cutting down to the sediment surface in the early growing season may inhibit turion production.

Potamogeton crispus is sometimes cultivated as a pond plant, and generally makes a good garden plant. Since it starts to die back rather early, it is recommended to cut it back in July after it has flowered. In common with other pondweeds of this group it roots poorly from stem cuttings and is best propagated by division of the rhizomes or from turions. As it has proved invasive in some areas, curly pondweed should not be grown outside its native range.