Classification
 Nomenclature
Scientific Name:
Salvinia ×molesta D.S.Mitch., Brit. Fern Gaz. 10: 251 (1972)
Synonymy:
Holotype: Ruziruhuru River inlet, Lake Kariba, Rhodesia [Zimbabwe], D.S. Mitchell 1330, SRGH.
Etymology:
From the Latin molestus (troublesome), a reference to its weedy and invasive nature.
Vernacular Name(s):
Kariba weed; salvinia
 Description
Aquatic fern, forming free-floating dense mats, with leaves tightly overlapping. Floating leaves broadly elliptic to orbicular when young, becoming broader than long at maturity, 12–30 mm long, 15–50 mm wide, often conduplicately folded along the midrib; apex emarginate; margins entire, sometimes inrolled; base cordate, shortly stalked; upper surface light or brownish-green, becoming darker on the margins, densely covered in papillae each bearing 2 or 4 uniseriate hairs united at their distal ends (shaped like an egg-whisk); papillae up to 3 mm long. Submerged leaves branched, 25–300 mm long, densely covered in brown septate hairs 1–3 mm long. Fertile axes on submerged leaves 15–70 mm long, bearing 4–20 pairs of sporocarps arranged in 2 rows. Sporocarps spheroidal or ovoid, 2–3 mm long, 1–2 mm wide, densely hairy, subsessile, containing sporangia that are usually empty or produce only aborted spores.
 Recognition
Salvinia ×molesta is easily recognised by its floating habit, its dimorphic leaves arranged in whorls of three, and the presence of egg-whisk-shaped hairs on the leaves for buoyancy. When mature and opened out, the floating leaves are shaped like the wings of a butterfly. The plant is sterile, with sporocarps containing only empty sporangia, or aborted spores, and is probably of hybrid origin.
 Distribution
North Island: Northland, Auckland.
Altitudinal range: 0–50 m.
A rampant weed of tropical and warm temperate regions, recorded as a naturalised plant in lowland areas of northern New Zealand from near Kaitaia to Hamilton. There are additional records of the plant from garden ponds as far east and south as Wairoa (AK 245013).
 Habitat
Recorded as an aquatic weed on lakes and ornamental ponds, and in Eleocharis sphacelata swamp.
 Biostatus
Exotic
 First Record
Mason (1964, p. 235), Armiger (1964) – as Salvinia natans. Voucher: CHR 234901, 1961.
 Notes
Salvinia ×molesta has been declared a noxious weed in New Zealand. It is believed to be a hybrid of horticultural origin from South America, now widely distributed in Africa and other areas outside the neotropics (Mitchell 1972). It is a pentaploid with 45 chromosomes (Schneller 1981).
It is very similar to S. biloba (syn. S. herzogii) from South America (de la Sota 1962, 1995; Tryon & Tryon 1982). At least some forms of this latter species are also probably of hybrid origin (Schneller 1990), and are distinguishable from S. ×molesta only by their chromosome number (Schneller 1980). However, there are no records of S. biloba in horticultural cultivation, and infestations outside South America all seem to be referable to S. ×molesta.
It has been proposed that the earlier name, S. adnata Desv., should replace S. ×molesta (de la Sota 1995). However, Moran & Smith (1999) argued that S. adnata is of uncertain application, possibly attributable to either S. biloba or S. ×molesta. Desvaux’s type specimen is vegetative, and although de la Sota (2001) provided some characters for distinguishing S. biloba and S. ×molesta, he was unable to examine them in the type specimen itself. The name S. ×molesta should therefore continue to be used for the invasive plant of the Old World tropics.
 Bibliography
Armiger, L.C. 1964: A guide to the more common fresh water plants of the Auckland Province. Tane 10: 49–57.
Biosecurity New Zealand 2008: National Plant Pest Accord. MAF Biosecurity New Zealand, Wellington.
Biosecurity New Zealand 2012: Regional Pest Management Strategies Database. http://www.biosecurityperformance.maf.govt.nz/
Biosecurity New Zealand 4 Aug 2011: Unwanted Organisms Register. http://www.biosecurity.govt.nz/pests/registers/uor
Brownsey, P.J.; Perrie, L.R. 2015: Salviniaceae. In: Breitwieser, I; Heenan, P.B.; Wilton, A.D. Flora of New Zealand — Ferns and Lycophytes. Fascicle 10. Manaaki Whenua Press, Lincoln.
de la Sota, E.R. 1962: Contribución al conocimiento de las Salviniaceae neotropicales, III Salvinia herzogii nov. sp. Darwiniana 12: 499–513.
de la Sota, E.R. 1995: Nuevos sinónimos en Salvinia Ség. (Salviniaceae, Pteridophyta). Darwiniana 33: 309–313.
de la Sota, E.R. 2001: Sobre el tipo de Salvinia adnata (Salviniaceae, Pteridophyta). Boletín de la Sociedad Argentina de Botánica 36: 125–129.
Mason, R. 1964: Aquatic weed identification in New Zealand. Proceedings of the 17th New Zealand Weed Control Conference 17: 229–250.
Ministry for Primary Industries 23 Feb 2017: Unwanted Organisms Register. Version 02.01.02. https://www1.maf.govt.nz/uor
Mitchell, D.S. 1972: The Kariba weed: Salvinia molesta. British Fern Gazette 10: 251–252.
Mitchell, D.S.; Thomas, P.A. 1972: Ecology of waterweeds in the Neotropics. UNESCO Technical Papers in Hydrology. Vol. 12. UNESCO, Paris.
Moran, R.C.; Smith, A.R. 1999: Salvinia adnata Desv. versus S. molesta D.S.Mitch. American Fern Journal 89(4): 268–269.
Schneller, J.J. 1980: Cytotaxonomic investigations of Salvinia herzogii de la Sota. Aquatic Botany 9: 279–283.
Schneller, J.J. 1981: Chromosome numbers and spores of Salvinia auriculata Aublet s.str. Aquatic Botany 10: 81–84.
Schneller, J.J. 1990: Salviniaceae. In: Kramer, K.U.; Green, P.S. Pteridophytes and gymnosperms. Vol. 1. In: Kubitzki, K. (ed.) The Families and Genera of Vascular Plants. Springer-Verlag, Berlin.
Tryon, R.M.; Tryon, A.F. 1982: Ferns and allied plants. Springer-Verlag, New York.