Potamogeton × griffithii

Potamogeton × griffithii is a sterile hybrid pondweed between red pondweed Potamogeton alpinus and long-stalked pondweed Potamogeton praelongus. It occurs in clear, oligotrophic, low to moderate alkalinity lakes. It was discovered in 1882 in Llyn Anafon, North Wales by J.E. Griffith, a Victorian botanist, and was named after him by the noted pondweed expert Arthur Bennett. Although intermediate in character between the parents, Potamogeton x griffithii is unlikely to be confused with either. It has long, branched stems to at least 1.7 m The leaves are sessile, sometimes slightly clasping the stem but shortly petiolate leaves may also be present. The stipules are persistent, open, colourless with a milky tinge, and opaque when dry. Floating leaves may be produced and are relatively narrow, 85–105 x 13–25 mm. Turions are absent. The inflorescences are 10–20 mm long and have inconspicuous greenish flowers on robust peduncles 45–190 mm long. Fruits are not produced. Potamogeton × griffithii is relatively easy to identify. It is larger than P. alpinus, and the branching habit immediately distinguishes the two. The leaf tip is also more distinctly hooded. The leaves are relatively longer than P. praelongus and P. perfoliatus, and mostly do not clasp the stem. The stems are also markedly more slender than P. praelongus and do not develop the characteristic 'zigzag' pattern caused by the stem changing the angle of growth at each node. ==Distribution==

Potamogeton × griffithii is a rare hybrid. As well as its type locality in Wales, it has also been found in a lake in the north of Ireland, and in two lakes in western Scotland. ==Ecological requirements and conservation==

In Llyn Anafon, Potamogeton x griffithii grows in clear, oligotrophic water with some basic influence, 0.75–3.2 m deep, among beds of Chara virgata Potamogeton × griffithii is classified as Vulnerable by both the British and Welsh Vascular Plant Red Lists due to the low number of sites in which it occurs. ==Cultivation==

Potamogeton × griffithii is not in general cultivation, though like both of its parents it is an attractive plant. A patch has been established in a wildlife pond in Bangor University's Treborth Botanic Garden, where it grows well on a sandy substrate in about 1 m depth. In common with other pondweeds of this group it roots poorly from stem cuttings and is best propagated by division of the rhizomes. Good water quality is needed to reduce the risk of turbid water or growth of blanket weed. ==References==