Hypericum calycinum L. (SE Eur., Turkey) – A common garden plant (although possibly often replaced now by one of its hybrids) but apparently rare or under-recorded as an escape from or relic of cultivation. Probably first collected in 1867 in Pailhe (Saint-Lambert), without further details. More often seen in the 20th century, for instance in 1956 on an off-ramp in Jamioulx (claimed as more or less naturalized in the vicinity of a plantation) and in 1985 in a ditch close to a castle in Durbuy (Petit-Somme). In 2009 also seen as a relic near a cave in woodland in Aarschot. A nice, apparently stable population was found in 2013 on a grassy, slightly shady talus slope of motorway E17 near Kortrijk. Since then more regularly encountered, usually as a mere throw-out, but also sometimes found on levelled soil, newly created talus slopes, etc., where ever transported soil is contaminated with rhizome parts of it.
Hypericum calycinum may have been largely overlooked or neglected elsewhere. A closer examination of extant populations might reveal other, closely related taxa, especially H. ×moserianum.
Adolphi K. & Kremer B.P. (2010) Hypericum calycinum (Immergrünes Johanniskraut) - eine neue Art in Deutschland. Flor. Rundbr. 44: 80-84.
Salisbury E.J. (1963) Fertile seed production an dself-incompatability of Hypericum calycinum in England. Watsonia 5: 368-376. [available online at: http://archive.bsbi.org.uk/Wats5p368.pdf]
Salisbury E.J. (1964) The seedling mystery of the Rose of Sharon (Hypericum calycinum). J. Roy. Hort. Soc. 89: 125-126.
Salisbury E.J. (1969) A note on fertile seed prodution by Hypericum calycinum. Watsonia 7: 24. [available online at: http://archive.bsbi.org.uk/Wats7p24.pdf]