Egeria densa Planch.
Vigorous submersed perennial. Stems branched, brittle, c. 3 mm diam. Leaves very dark green, in whorls of 3 below, in whorls of 4 or more above, 15-30 × 4 mm, linear, lax, abruptly acute to acuminate. Male flowers obvious, usually 2-3 per spathe in axils of upper leaves; sepals green c. 4 mm long, petals white, c. 7 mm long; stamens 9, golden. Female flowers and fruits not known in N.Z.
N. North Auckland; Auckland; Hawkes Bay - Napier; Wellington - near Foxton. S. Canterbury - Lake Georgina.
(S. America)
First record: Mason 1960: 382,388.
First collection: Waikato River, near Elbow, [near Tuakau], B. E. G. Molesworth, 10.2.1946 ().
FL. 11-1.
E. densa differs from Elodea and Lagarosiphon in the denser, more leafy habit, shorter stems, and leaves in whorls of 3-8, occasionally some leaves opposite lower down the stem.
It is the most restricted of the "oxygen weed" group, being known only from the northern half of the North Island, with the exception of the Foxton and Canterbury occurrences. However, it is apparently the most common aquatic weed in the lower Waikato River area, and is also established in several of the upper Waikato River hydro-electric lakes, where unsuccessful eradication attempts were made in the late 1960s. In the Waikato River it is associated with Elodea canadensis, Lagarosiphon major, Ceratophyllum demersum and a number of less troublesome aquatic weeds.
It is not known whether its present restricted distribution has resulted because successful growth elsewhere in New Zealand is inhibited by some factors, or merely because the plant had no opportunity to spread further south and establish. French investigations have shown, however, that while the minimum temperature for satisfactory growth of all three "oxygen weeds" is 10 C, the optimum temperature for Elodea canadensis is 16 C and for Egeria densa and Lagarosiphon major it is 18 C: the maximum for Elodea canadensis is 18 C, for Lagarosiphon major 20 C and for Egeria densa 25 C (Mason op. cit. p.393).
