Flora of New Zealand Series

Canadian Pondweed

Vigorous dioecious perennial. Stems slender, branched, very brittle, to 6 m long, c. 1 mm diam. Leaves opposite below, crowded and in whorls of 3 above, 6-12 × c. 2 mm, oblong or ovate-oblong, subobtuse, translucent, margins minutely toothed. All N.Z. collections, with exception of 2 from Hawkes Bay, have been from female plants. Flowers 1 per spathe in axils of upper leaves; male flowers with broad green, purple-striped sepals and narrow petals; female flowers with small, green, purple-spotted sepals and white petals ± 2.5 mm long, on perianth-tube to 20 cm long. Capsules not present in N.Z.

N. Southwards from Kaipara Harbour; not known in Taranaki. S. Throughout, rare in Westland. In ponds, lakes, streams and rivers.

(N. America)

First record: Armstrong 1872: 285-6, as Anacharis alsinastrum [Bab.].

First collection: There are undated collections from the Avon River, [Christchurch] at CHR and AK but the earliest dated collection is "Near Featherston", K. W. Allison, 1916 ().

FL. 11-1.

Of the "oxygen weed" group E. canadensis has been established the longest, occurs the most widely, and colonises the deepest water. It differs from Egeria and Lagarosiphon in having the leaves opposite below and in whorls of 3 above.

It occurs and proves troublesome in all types of waterways, and, in addition, it has established in abundance in some lakes in both islands. The heavy stand in Lake Rotoroa, Nelson Lakes National Park is noteworthy: between 1965 and 1971 the plant formed a virtually complete marginal weed-bed to a depth of 8.5 m, with stems to 6 m high. Here, in terms of dry matter per m², the amount of weed herbage is significantly higher than that recorded for any other freshwater macrophyte community elsewhere in the world.

Colonisation may be rapid and effective, all suitable ecological niches being occupied, as happened in Lake Rotoroa; it has been suggested that once maximum density is reached the stands may decline in vigour and density; this appears to be the case in the nearby Lake Rotoiti. Such a decline allows the entry and established of other aquatics, as has happened in several North Island lakes.

Fig. 2. HYDROCHARITACEAE, oxygen weeds, upper part of stem; A: Elodea canadensis, Canadian pondweed, leaves in whorls of 3. B: Lagarosiphon major, leaves alternate. C: Egeria densa, leaves in whorls of 4 or 5. © All rights reserved. [Image: 26X5]