About Potamogeton zosteriformis Fernald
Scientific name: Potamogeton zosteriformis Fernald (synonym: Potamogeton compressus). This species is commonly called grass-wrack pondweed. It produces a strongly flattened, robust, branching stem with a maximum length of up to 90 cm. It grows annually from turions and seed, forming bushy plants that branch near the water surface. Its leaves are long and rather grass-like, measuring 85–240 mm long and 3–6 mm wide, and are colored olive-green or dark green, sometimes with a reddish tinge near the surface. Each leaf has two veins on either side of the midrib and ends in a blunt point. Unlike the transparent leaves of most pondweeds, grass-wrack pondweed leaves look rather opaque because they contain fibrous strands called sclerenchymatous strands. This species has no rhizomes and no floating leaves. Its inflorescences are up to 6 mm long, holding 4–6 flowers, and grow on a short peduncle that is 5–20 mm long, occasionally longer. The fruits measure 3.1–4 mm by 2.1–3 mm. Grass-wrack pondweed is relatively easy to distinguish from most other pondweeds by its combination of strongly flattened stems and sclerenchymatous strands in its leaves. In Europe, the similar species P. acutifolius can be told apart by its sharply pointed leaves, less branched growth habit, and flower spikes with only 2–6 flowers on peduncles up to 20 mm long. In the Far East, the similar species P. mandschuriensis is an altogether smaller plant, with leaves 1.5–2.3 mm wide, 8–14 sclerenchymatous strands, stems 0.8–1.5 mm wide, and fruit 2.8–3.8 mm in diameter. This is a diploid species with 2n=28. Hybrids have been recorded with P. acutifolius (named P. × bambergensis Fischer), P. oxyphyllus (named P. × faurei Miki) and P. trichoides (named P. × ripoides Baagøe). P. × bambergensis may be reasonably frequent where the two parent species coexist, but like many fine-leaved pondweed hybrids, it is difficult to identify reliably without genetic techniques. Potamogeton compressus is native to Europe (found in Austria, Britain, Denmark, Finland, France, Germany, Ireland, Netherlands, Poland, Russia, Spain, Sweden, Switzerland, Ukraine) and Asia (found in China (Yunnan), Japan, Kazakhstan, Mongolia, Russia). In North America, P. compressus / zosteriformis occurs in northern USA and Canada. Isolated populations exist in the Balkans, Oregon and the Rockies. There is considerable uncertainty about the exact distribution of this species in Siberia and Canada. Grass-wrack pondweed grows in still or slow-flowing, lowland, calcareous, often rather nutrient-rich water bodies such as river backwaters, ponds and slow-moving streams. It usually grows on fine substrates such as sand, silt, clay or peat, and is most commonly found in water less than 1.5 m deep, though Japanese populations have been reported growing in 5 m of water. It can also colonise artificial habitats such as canals and drainage ditches, as long as these are not heavily used by boats. Grass-wrack pondweed is intolerant of turbid water and prefers some shade. Like its close relative Potamogeton acutifolius, it rarely grows in lakes. Its shallow root system is intolerant of disturbance, making it vulnerable to wind action, boat disturbance and uprooting by fish. Potamogeton compressus is a rather early succession species, and tends to be outcompeted unless its habitat is regularly disturbed. As a result, its populations are often transient. Most reproduction in the wild appears to be asexual via turions, which likely means populations have limited ability to recolonise if they are lost. Turion production is not prolific, with wild plants typically producing only 4–5 turions. Flowering and fruiting seems to be more frequent in shallow water environments with fluctuating water levels, such as ditches. Grass-wrack pondweed is threatened in many parts of its range, especially in Europe: it is Extinct in the Czech Republic, Critically Endangered in Flanders, Endangered in Germany and England, Vulnerable in the Carpathian region and Wales, and Near Threatened in the Netherlands. In North America it is listed as Endangered in Maryland and New Jersey, Threatened in New Hampshire, and Rare in Pennsylvania. In Britain, P. compressus is a Biodiversity Action Plan priority species and is the subject of conservation action including translocation efforts. These population declines probably reflect widespread damage to riverine landscapes across lowland Europe, and in particular the loss of features such as back channels, oxbow lakes and floodplain ponds as rivers are channelised and modified for flood defence and agricultural purposes. The largest British populations are in disused or rarely boated canals, which cannot be the primary habitat for grass-wrack pondweed and do not provide a sustainable long-term habitat. However, in the short term, canal populations are an important reservoir for this species. Competition with the introduced species Elodea canadensis (Canadian pondweed) and E. nuttallii (Nuttall's water-thyme) may also be problematic. It is possible that the widespread reintroduction of beaver across Europe may help to stop or reverse the decline of grass-wrack pondweed, because beaver ponds may be a suitable habitat for this species. Potamogeton compressus is not currently in cultivation. It could probably be cultivated in rather silty ponds, as long as these are regularly cleaned out to prevent more competitive plants from excluding it. Cultivation experiments conducted for conservation purposes have successfully grown plants to maturity from turions planted in late winter, but adult plants are more difficult to establish due to their limited root system and fragile nature. High mortality was also observed in cultivation due to snail predation.
