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''Lythrum salicaria'' can grow {{cvt|1-2|m|ftin|abbr=on}} tall, forming [[clonal colonies]] {{cvt|1.5|m|ftin|abbr=on}} or more in width, with numerous erect stems growing from a single woody root mass. The stems are reddish-purple and square in cross-section. The leaves are [[lanceolate]], {{convert|3–10|cm|in|frac=2}} long and {{convert|5–15|mm|in|frac=16}} broad, downy and [[Sessility (botany)|sessile]], and arranged opposite or in whorls of three.
The flowers are reddish purple, {{convert|10–20|mm|in|frac=4}} in diameter, with six petals (occasionally five) and 12 stamens, and are clustered tightly in the [[axil]]s of [[bract]]s or leaves;, (a type of inflorescence called a [[verticilaster]]), there are [[Heterostyly|three different flower types]], with the stamens and style of different lengths, short, medium or long; each flower type can only be pollinated by one of the other types, not the same type, thus ensuring cross-pollination between different plants.<ref name=fnwe/><ref name=foc>{{cite web |website=Flora of China vol. 13 |page=282 |url=http://www.efloras.org/florataxon.aspx?flora_id=2&taxon_id=200014658 |title=''Lythrum salicaria''|year=2007|access-date=27 October 2022}}</ref><ref name=blamey>Blamey, M. & Grey-Wilson, C. (1989). ''The Illustrated Flora of Britain and Northern Europe''. Hodder & Stoughton. {{ISBN|0-340-40170-2}}.</ref> For instance, if the [[pistel]] is medium length, then the [[stamens]] will be long and short, but not medium.<ref>{{cite book | last= Meeuse Ph.D.| first=B.J.D. | date= 1961| title= The Story of Pollination | location= New York | publisher= Ronald Press Co. | page= 174- 175 with illust.}}</ref> The flowers are visited by many types of insects, and can be characterized by a generalized [[pollination syndrome]].<ref>{{Cite journal | doi = 10.1111/plb.12328| pmid = 25754608| title = Competition for pollinators and intra-communal spectral dissimilarity of flowers| journal = Plant Biology| volume = 18| issue = 1| pages = 56–62| year = 2015| last1 = Van Der Kooi | first1 = C. J.| last2 = Pen | first2 = I.| last3 = Staal | first3 = M.| last4 = Stavenga | first4 = D. G.| last5 = Elzenga | first5 = J. T. M.| url=https://www.researchgate.net/publication/273158762}}</ref>
The fruit is a small {{convert|3–4|mm|in|frac=64}} [[capsule (fruit)|capsule]]<ref>Clapham, A.R., Tutin, T.G. and Warburg, R.F. 1968. ''Excursion Flora of the British Isles''. Cambridge University Press. {{ISBN|0 521 04656 4}}</ref> containing numerous minute [[seed]]s. Flowering lasts throughout the summer. When the seeds are mature, the leaves often turn bright red through dehydration in early autumn; the red colour may last for almost two weeks. The dead stalks from previous growing seasons are brown.<ref name=fnwe/><ref name=foc/><ref name=blamey/>
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