“Myriophyllum propinum” is a misspelling of Myriophyllum propinquum; what is sold under either name probably belongs to Myriophyllum simulans.12 P. J. van der Vlugt introduced it from Australia in 1983, at first as M. propinquum; only after Orchard's description of M. simulans (1986) was it assigned to this species.256 It might equally belong to M. jacobsii, described in 2011; whether the same species is always traded remains open.6 True M. propinquum is native only to New Zealand and the Chatham Islands.14
M. simulans is native to eastern Australia, mostly growing emersed in marshes and on muddy banks.235 The aquatic form has light green, finely pinnate leaf whorls, the terrestrial form stiff, needle-shaped to linear leaves.25 It is regarded as demanding: when well supplied it grows fast, algae on the shoots inhibit it severely, and in low light the stems become bare from below.5
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