Potamogeton perfoliatus

Lore & Background

Potamogeton perfoliatus was one of the original species named by Linnaeus in Species Plantarum (1753). The specific epithet 'perfoliatus' refers to the leaf encircling the stem. Two subspecies have been described: subsp. perfoliatus occurs in the Old World, while populations on the eastern seaboard and Gulf States of North America are considered subsp. bupleuroides. Since many pondweeds, including P. perfoliatus, vary naturally in response to growing conditions, it is possible that these variants are ecotypes rather than genuine subspecies. The related P. richardsonii was formerly considered a subspecies of P. perfoliatus but is now classed as a separate species.

Perfoliate pondweed grows from a robust creeping perennial rhizome, producing round stems up to 3 m long. The submerged leaves are oval, translucent, stalkless, and clasp the stem (perfoliate), with a flat apex and 5–12 veins on either side of the midrib. Leaves vary considerably in colour, ranging from bright green to dark green, yellowish, olive, or brownish. There are no floating leaves. The stipules are delicate and usually fall off soon after the new leaf unfurls. Insignificant flowers are produced between June and September, and the fruits are 4 mm across and olive green in colour.

Perfoliate pondweed regularly hybridises with many other Potamogeton species, including P. crispus, P. gramineus, P. lucens, P. alpinus, P. berchtoldii, P. wrightii, P. maackianus, P. nodosus, P. richardsonii, P. epihydrus, and P. praelongus. A triple hybrid, P. gramineus × lucens × perfoliatus, is also known. Many of these hybrids are perennial and long-lived, sometimes occurring in the absence of one or both parents, so care should be taken with identification of unusual specimens.

Reader's Guide

Potamogeton perfoliatus is significant as a widespread and ecologically important aquatic plant. It grows in a wide range of freshwater habitats including lakes, rivers, streams, ponds, canals, and ditches, and does not tolerate drying out. The most robust plants tend to occur in water deeper than 1 m. Unlike most other broad-leaved pondweeds, it is reasonably tolerant of water flow and can use running waters to a significant extent. It is not especially sensitive to water chemistry provided the water remains reasonably clear and not too base-poor, occurring in diverse habitats from exposed Scottish lochs to lowland fenland rivers. However, at high nutrient concentrations it is vulnerable to shading from phytoplankton, epiphytic algae, and filamentous algae. It may also grow in brackish or estuarine habitats, notably in Chesapeake Bay, though elevated salt concentrations negatively affect both growth and flowering.

Waterbirds, fish, water beetles, and caddis larvae can all be important grazers, reducing its biomass or eliminating it locally. Perfoliate pondweed is less threatened than other broadleaved pondweeds, but it is listed as Vulnerable in Spain and the Netherlands and Endangered in Flanders. There are ongoing efforts to restore populations in Chesapeake Bay, where it was formerly a co-dominant species. It is the commonest of the Magnopotamion group of pondweeds, which are a characteristic floristic component of the protected Habitats Directive habitat type 'Natural eutrophic lakes with Magnopotamion'.

In cultivation, perfoliate pondweed is not widely grown despite being ecologically undemanding and attractive. It roots poorly from stem cuttings and is best propagated by division of the rhizomes. It is easy to grow in a garden pond or barrel but needs to root in a suitable substrate such as sand or aquatic compost. Keeping nutrient levels low helps prevent smothering by algae and invasive plants. It may also be established from seed, which should be sown on a suitable substrate in shallow water and left uncovered or covered minimally; seed buried more than 1 cm deep either does not germinate or the seedlings die. Seed typically germinates within about 10 days.

Did You Know?

• It is tetraploid, with a chromosome number of 2n=52.
• It regularly hybridises with at least 12 other Potamogeton species, and a triple hybrid is also known.