The epithet means leaves (folia) resembling hyssop (Hyssopus).

Lesser Loosestrife, Small Loosestrife, Hyssop Loosestrife, Hyssop-leaved Loosestrife, Loosestrife.

Herb, annual, procumbent, decumbent or ascending, glabrous. Stems ± 4-angled, ribbed, to 60 cm long. Leaves mostly alternate (lower leaves sometimes opposite), sessile or subsessile, linear to narrowly oblong, oblong-lanceolate or elliptic, (0.5–) 1–2.5 (–4) cm long, 2–8 (–10) mm wide, base obtuse to rounded or cuneate, margin entire, apex acute to obtuse. Flowers solitary in upper leaf axils (rarely 2-flowered inflorescences occur, see Lepschi 2000: 275), subsessile; pedicel to 1 mm long. Hypanthium narrowly obconical, 3–6 mm deep, c. 1 mm wide, ribbed (each rib ending in a narrow appendage); appendages 4–6, narrowly triangular, c. 1–1.5 mm long. Sepals 4–6, shallowly triangular with apiculum of hairs, c. 0.5–1 mm long. Petals 4–6, spreading, narrowly obovate to oblanceolate, 1–4 mm long, pink, blue or purple. Stamens usually 4–6 or rarely 8–12 and dimorphic, included to scarcely exserted. Capsule cylindrical, c. 4–6 mm long, c. equal to hypanthium, enclosed in the persistent hypanthium (flora tube), dehiscing at summit by 2–4 valves; seeds numerous, to c. 1 mm long.

Flowers September–February (–March).

The status of this taxon on Lord Howe and Norfolk Islands is uncertain, however it is treated as native in all other jurisdictions in which it occurs except for Western Australia.

Native of southern Europe. Apparently native in eastern Australia, occurring in South Australia, Queensland, New South Wales, Australian Capital Territory, Victoria and Tasmania. In Western Australia it is regarded as introduced (Lepschi 2000; Wheeler et al. 2002; Australian Plant Census and Florabase, accessed 14 October 2024), though there are several early records for the state, viz. Albany, Dec. 1898, R. Helms s.n. (NSW), Interior of Western Australia, 1893, E. Merrall s.n. (MEL), fide Australasian Virtual Herbarium (accessed 14 October 2024); Lord Howe and Norfolk Islands is of uncertain status (although treated in Green (1994) as naturalised, because of the species' general distribution and often inconspicuous habit, Green suggested that it is possibly a native of the Islands). Note, it is not unknown for wetland and water plants to have cosmopolitan or disjunct (sometimes unusual) distributions due to the possibility of long distance dispersal be migratory waders.

Grows in moist habitats, including wetlands, banks and margins of watercourses and waterbodies (damp edges to very shallow water), floodplains, creeklines, earthen drains and dams, roadside ditches, wet weedy areas, damp pasture and grassland, damp areas in woodland/forest, riparian vegetation, depressions; including seasonally wet habitats and areas prone to flooding or waterlogging (e.g. wetlands, drains). Colonises disturbed sites. Occasional weed of irrigated crops.

Re pollination, honey bees, various butterflies and other flying insects observed visiting flowers at Ainslie, A.C.T. (P.G. Kodela, pers. obs., 13 December 2020).

W.A. (naturalised): 1.5 km SW of Bremer, K.R. Newbey 4639 (PERTH).

S.A.: Glen Osmund, D.E. Symon 12830 (CANB).

Qld: Stanthorpe, Dec. 1916, H. Wright (BRI).

N.S.W.: Mittagong, K. Mowle 166 (NSW).

N.Is.: Melanesian Mission Creek, P.S. Green 2447 (K); Emily Bay, R.M. Laing (CHR); Bloody Bridge, R.M. Laing (CHR); loc. id., P. Ralston 10 (A); loc. id., W.R. Sykes NI 651 (CHR); fide Green (1994: 200).

L.H.Is.: Erskine Valley, J. McKean in A.C. Beauglehole 5573 (MEL), fide Green (1994: 200).

A.C.T.: Murrumbidgee River, 2 km downstream from Kambah Pool, I.R.H. Telford 7348 (CBG n.v., NSW n.v.)

Vic.: Lake Bringalbert, H.I. Aston 2716 (MEL n.v.).

Tas.: Sundown Point, G.S. Hope ANU21545 (CANB).

Lythrum hyssopifolia has been confused with L. paradoxum and L. wilsonii; L. paradoxum differs from L. hyssopifolia in flower and stamen number as well as stamen exsertion (although there may be some overlap in the first two characters), while L. wilsonii differs from L. hyssopifolia in a number of vegetative and floral features (Lepschi 2000); see this reference for key and further details.

Green (1994) provides a treatment for Lythrum hyssopifolia on Norfolk and Lord Howe Islands.

G.M. Cunningham et al., Plants of Western New South Wales: 509 (1981); J.P. Jessop (ed.), Flora of Central Australia : 234, fig. 276 (1981); G.R. Sainty & S.W.L. Jacobs, Water Plants of New South Wales: 280–281, fig. 51 (1981); J.P. Jessop (ed.), Flora of South Australia 4th edn, 2: 891, fig. 461B (1986); T.D. Stanley & E.M. Ross, Flora of South-eastern Queensland 2: 118, fig. 15B (1986); N.G. Marchant et al., Flora of the Perth Region 1: 366, fig. 132 (1987); K.A.W. Williams, Native Plants of Queensland 3: 196–197 (1987); C.J. Webb et al., Flora of New Zealand 4: 815, fig. 78A (1988); H.J. Hewson, Flora of Australia 18: 103, fig. 33G–I (1990); N.G. Walsh & T.J. Entwisle (eds), Flora of Victoria 3: 911, fig. 185i–j (1996); S. Harris et al., One Hundred Islands: the Flora of the Outer Furneaux: 185 (2001); G.J. Harden (ed.), Flora of New South Wales 2nd edn, 2: 235 (2002); J.R. Wheeler et al., Flora of the South West 2: 662 (2002); G.R. Sainty & S.W.L. Jacobs, Waterplants in Australia 4th edn, 236–237 (2003); R. Barrett & E.P. Tay, Perth Plants 2nd edn: 320–321 (2016); F.J. Richardson et al., Weeds of the South-east 3rd edn: 348 (2016).

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Revised edition, 14 October 2024. Adapted from Flora of Australia Volume 18 (1990), a product of ABRS, ©Commonwealth of Australia. Last updated: P.G. Kodela, 9 November 2020, 14 October 2024.