Hyssopus officinalis L. (Medit.) – A rare and apparently decreasing, locally naturalized escape from or relic of former cultivation. Most records are from the 19th and the first half of the 20th century. Known at least since 1821 from old walls in the valley of river Meuse in the surroundings of Huy, Liège, Tihange, Visé, etc. Its presence has been confirmed in most cases for many decades although it may have disappeared from many of its former localities. It was recently confirmed from Anseremme and Dinant only. Exceptionally seen outside the valley of river Meuse. In Flanders, for instance, seen on old walls of a castle in Vurste (Gavere) in 1963 and also known from the ruins of the former Sint-Pietersabbey in Gent, along with Ruta graveolens and Salvia officinalis (Verloove 2002, Verloove 2006). These three species were formerly cultivated in the abbey garden and now persist on old walls. A remarkably similar recent record is available from the Netherlands (Holverda & al. 2009). Since 2005 also known from a foot of a wall, also in Gent (a single specimen). In Wallonia (outside the valley of river Meuse) still recorded, for instance, on an old wall in Dour in 2006 (with Aurinia saxatilis).
Genova E. & Daskalova T. (1995) Notes on the embryology and essential oil composition of two Hyssopus officinalis L. taxa. Phytologia Balcanica 1(2): 81-84.
Holverda W., van Moorsel R. & Duistermaat L. (2009) Nieuwe vondsten van zeldzame planten in 2005, 2006 en ten dele 2007. Gorteria 34: 1-40.
Husain S.Z., Marín R.D., Qaiser M., Silic C. & Petcovic B. (1990) A micromorphological study of some representative genera in the tribe Saturejeae (Lamiaceae). Bot. J. Linn. Soc. 103(1): 59-80.
Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.
Verloove F. (2006) Hyssopus officinalis. In: Van Landuyt W., Hoste I., Vanhecke L., Van den Bremt P., Vercruysse W. & De Beer D., Atlas van de flora van Vlaanderen en het Brussels gewest. Instituut voor Natuur- en Bosonderzoek, Nationale Plantentuin van België en Flo.Wer: 488-489.
Walter E. (1994) Ein verwildertes Vorkommen von Ysop (Hyssopus officinalis L.) in Oberfranken. Ber. Naturforsch. Ges. Bamberg 69: 19-26.