Juncus tenuis

Juncus tenuis Willd. – The Juncus tenuis group is a taxonomically difficult species complex. It may be treated in two ways: either as one or two exceedingly variable species or as a complex of several closely related but specifically distinct taxa. Since Wiegand (1900) the latter taxonomic viewpoint surely prevails and the most recently published taxonomic revisions of the group (Brooks & Clemants 2000, Kirschner 2002b) unequivocally treat Juncus tenuis in a narrow sense and accept 12 additional species, including J. anthelatus, J. dichotomus and J. dudleyi. However, recent fieldwork by the author, in Belgium as well as in southern Europe (mainly Italy; see Verloove 2010) has revealed that the distinction of some of the aggregates is often critical (especially Juncus anthelatus and J. dichotomus are obviously linked by intermediates with J. tenuis s.str.). In the present treatment the Belgian representatives of this complex are therefore accepted at subspecific rank (see also Verloove & Lambinon 2011). More or less aberrant plants of Juncus tenuis s.l. should be studied carefully.

1. Leaf sheath auricles scarious to coriaceous (leathery), hardly projected beyond the point of insertion, much wider than long, 0,1-0,5 mm long, rounded at apex === 2

1. Leaf sheath auricles hyaline (translucent), longer than wide, ca. (1-) 2-6 mm long, usually acute towards apex (more rarely rounded) === 3

2. Auricles scarious, whitish, opaque (neither rigid nor shiny). Tepals not or hardly spreading in fruit === subsp. dichotomus

2. Auricles leathery and very rigid, yellowish to orange-brownish, shiny. Tepals spreading in fruit === subsp. dudleyi

3. Inflorescence tall, very diffusely branched with widely spaced flowers (internodes at least partially much longer than tepal length). Plant usually more than 70 cm. Capsules ca. 2-2,5(-3) mm long, nearly spherical, mostly less than ¾ tepal length. Longest ultimate branches of the cymes 30-50 mm long === subsp. anthelatus

3. Inflorescence smaller, not diffusely branched with congested flowers (internodes rarely longer than tepal length). Plant usually much smaller than 70 cm. Capsules mostly more than 3 mm long, ellipsoid, only slightly shorter than tepals. Longest ultimate branches of the cymes 10-20 mm long === subsp.  tenuis


Juncus tenuis


Juncus tenuis subsp. anthelatus

Juncus detailpictures

        subsp. tenuis            subsp. dichotomus           subsp. dudleyi

    Juncus_tenuis_tenuis           Juncus_tenuis_dichotomus          Juncus_tenuis_dudleyi

     subsp. tenuis            subsp. dichotomus           subsp. dudleyi

Subsp. anthelatus (Wiegand) Verloove et Lambinon (Verloove & Lambinon 2011) (syn.: J. anthelatus (Wiegand) R. Brooks, J. tenuis Willd. var. anthelatus Wiegand, J. macer S.F. Gray var. anthelatus (Wiegand) F.J. Hermann) (N-Am.) – Apparently first found on the muddy bank of an exposed pond in a private estate in Turnhout (Villa ‘Het Heiken’) in 1977 (as Juncus tenuis). Now, as a result of infrastructural works, probably gone (comm. A. Vermeijen). Discovered since 2007 at several, widely scattered locations in Flanders (mainly but not exclusively in the Kempen). Found, for instance, in shady road verges in Kapellen (a single specimen) and Bellem in 2007. Also seen on damp acidic soil in a military base in Hechtel-Eksel since 2008. In 2009 also observed in a newly created nature reserve in Mariakerke (Gent), possibly from an exposed long-lived seed bank. In 2010 furthermore discovered in nature reserve (former military training area) Klein Schietveld in Brasschaat. Doubtlessly overlooked elsewhere. Subsp. anthelatus surely is locally naturalised in Belgium, especially in the Kempen. Previous European records were available from the Netherlands (see van Ooststroom & Reichgelt 1964, recently confirmed). Recently also re-discovered in the British Isles (Wilcox & Tregale 2008).

Typical plants of subsp. anthelatus are very distinct: very tall plants with a diffuse inflorescence with widely spaced flowers and capsules that are much shorter than the tepals. However, Wilcox & Tregale l.c. demonstrated that anthelatus-like plants of genuine Juncus tenuis (subsp. tenuis) also occur. A lower taxonomic rank therefore seems more appropriate.

Selected literature: Wiegand (1900), Brooks & Whittemore (1999), Haines (2001), Wilcox & Tregale (2008), Verloove & Lambinon (2011).

Subsp. dichotomus (Elliott) Verloove et Lambinon (Verloove & Lambinon 2011) (syn.: Juncus dichotomus Elliott, J. tenuis Willd. var. dichotomus (Elliott) A.W. Wood, incl. J. platyphyllus (Wiegand) Fernald, J. dichotomus var. platyphyllus Wiegand, J. tenuis var. platyphyllus (Wiegand) Cory, J. tenuis var. unicornis E. Mey., J. cognatus Kunth, J. albicans Fernald) (Am.) – A very rare but possibly overlooked taxon. Known from the military base of Elsenborn in Bütgenbach (discovered in damp tracks of military vehicles in 2007, as Juncus tenuis). In 2014 also discovered on the exposed banks of the Kraenepoel in Aalter. On the occasion of this discovery Belgian public herbaria were revised which yielded several additional records, the oldest apparently from Beauwelz in 1883 (Hoste & Verloove 2016). Since then also seen on bare sand in Sint-Denijs-Westrem near Ghent, probably emerged from an exposed old seed bank. Typical plants of subsp. dichotomus have very short leaf sheath auricles that are wider than long (see Verloove 2010, Hoste & Verloove 2016). This character is preferably assessed on the lowermost fertile shoots. Leaf sheath auricles of the upper (younger) leaves and of non-flowering stems of subsp. tenuis are often very short and much reminiscent of those of subsp. dichotomus! Moreover, with age (and often in the herbarium as well!) the long and pliable auricles of subsp. tenuis are readily damaged. In some populations of the latter even well developed leaf sheath auricles are relatively short (ca. 1 mm long) but still distinctly projected beyond the point of insertion and longer than wide. However, such plants rather tend towards subsp. dichotomus and might be the result of introgression. It therefore seems more appropriate to reduce Juncus dichotomus to subspecific rank under J. tenuis.

Since its first 'discovery' in Europe (Italy; see Verloove 2010) subsp. dichotomus has been recorded in several additional European countries, mainly in eastern Europe (e.g. Tikhomirov 2013) but also in the Netherlands (Stolwijk & Zijlstra 2016).

Selected literature: Verloove (2010), Tikhomirov (2013), Hoste & Verloove (2016), Stolwijk & Zijlstra (2016).

Subsp. dudleyi (Wiegand) P. Fourn. (syn.: Juncus dudleyi Wiegand, J. tenuis var. dudleyi (Wiegand) F. Hermann, J. tenuis var. uniflorus Farwell) (N-Am.) – A very rare but possibly overlooked alien. Discovered in 2009 in an old sandpit in the seadunes near Koksijde (nature reserve Fluithoek in Noordduinen), subsequently confirmed in 2010 and 2018 (two tufts). In 2018 also observed in a military base in Houthulst. In Koksijde subsp. dudleyi grows as a pioneer on bare sandy soil, along with numerous valuable native species (including the exceedingly rare Carex punctata). The site was deforested in 2006 and the topsoil was subsequently removed. This area was occupied by American military troops during and shortly after World War I. Juncus dudleyi doubtlessly germinated from an old seed bank that might be associated with these former military activities (see also Carex crawfordii in the very same nature reserve and C. vulpinoidea in a similar site in Oostduinkerke).

Subsp. dudleyi is possibly overlooked. It is distinguished from subsp. tenuis by leaf sheaths with small leathery, yellowish-brownish, rounded auricles up to 1-2 mm long (vs. leaf sheaths with larger membranous, translucent, acute auricles up to 6 mm long in). The plants in its unique Belgian locality are also conspicuously taller (up to 80 cm long stems) than usually is the case in subsp. tenuis. It is, by far, the most distinct of the subspecies currently found in Belgium.

It was recently also recorded in northwestern France (Lambinon & al. 2004) and is reported from several other European countries (including Austria, the British Isles, Germany and Slovakia; see Druce 1931, Podlech 1980, Kirschner 2002b). In the Netherlands subsp. dudleyi (and its hybrid with subsp. tenuis!) was discovered in abundance in 2009 in Overijssel (Corporaal & Schaminée, in prep.). According to these authors both taxa readily hybridise which also pleads for a lower taxonomic rank.

Selected literature: Druce (1931), Podlech (1980), Sell & Murrell (1996), Corporaal & Schaminée (in prep.).

Subsp. tenuis (syn.: J. bicornis Michaux, J. macer S.F. Gray) (N-Am.) – A common and widely naturalised alien, in various kinds of habitats, (semi-) natural- as well as (and increasingly) man-made ones. Known at least since 1823 but the original introduction is otherwise poorly documented. Sometimes supposed to have originally been introduced with American hay in Europe (Clement & Foster 1994). The first European records are usually claimed from Belgium and the Netherlands (Weeda & al. 1994) but Juncus tenuis was already found in the British Isles from 1795-96 onwards (Sell & Murrell 1996, Stace 1997). At present subsp. tenuis is found in large parts of Belgium but it is obviously more abundant in the northeastern part of Flanders (see up-to-date distribution map in Verloove 2006).

Although frequently penetrating in valuable and vulnerable natural communities subsp. tenuis is generally not regarded as a noxious invasive taxon.

Subsp. tenuis is poorly distinguished from subsp. dichotomus. Leaf sheath auricles are rarely as typical as usually illustrated in standard floras (for instance Brooks & Clemants 2000).

Selected literature: Skogen (1986), Verloove (2002), Verloove (2006).

Herbarium specimen of Juncus dudleyi

Herbarium specimen of Juncus tenuis

Herbarium specimen of Juncus anthelatus

Herbarium specimen of Juncus anthelatus


Brooks R.E. & Clemants S.E. (2000) Juncus. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 22. Oxford University Press, New York-Oxford: 211-255.

Clement E.J. & Foster M.C. (1994) Alien plants of the British Isles. BSBI, London: XVIII + 590 p.

Corporaal A. & Schaminée J. (2015) Juncus dudleyi Wiegand, een nieuwe rus uit Amerika, en Juncus x sallandiae Corporaal & Schaminée hybr. nov., zijn hybride met J. tenuis Willd. Gorteria 37: 52-63.

Druce G.C. (ed.) (1931) Plant notes, etc., for 1930. Bot. Exch. Club Brit. Isles 9(3): 258-287.

Hoste I. & Verloove F. (2016) Juncus tenuis subsp. dichotomus, lang miskend maar mogelijk sinds kort inburgerend in België. Dumortiera 108: 22-29. [available online at: http://www.br.fgov.be/DUMORTIERA/DUM_108/Dum_108_22-29_Juncus%20tenuis%2...

Kirschner J. (2002a) Species Plantarum. Flora of the world. Juncus subg. Juncus. Vol. 7(2). Australian Biological Resources Study, Canberra: VIII + 336 p.

Kirschner J. (2002b) Species Plantarum. Flora of the world. Juncus subg. Agathryon. Vol. 8(3). Australian Biological Resources Study, Canberra: VIII + 192 p.

Kobierski P. & Ryś R. (2015) Juncus anthelatus – nowy, obcy gatunek we florze Polski. Juncus anthelatus – a new alien species in the flora of Poland. Chrońmy Przyr. Ojcz. 71(1): 61-67.

Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.

Milanovic Đ, Maslo S & Šaric Š. (2018) Four neophytes new for the flora of Bosnia and Herzegovina. Botanica Serbica 42(1): 139-146. [available online at: http://botanicaserbica.bio.bg.ac.rs/arhiva/pdf/2018_42_1_709_full.pdf]

Olshanskyi I.G., Shevchyk V.L. & Senchylо O.O. (2018) Juncus dichotomus Elliott (Juncaceae) – new alien plant species for Transcarpathia. Studia Biologica 12(1): 135-139. [available online at: http://publications.lnu.edu.ua/journals/index.php/biology/article/view/7...

Podlech D. (1980) Juncaceae. In: Hegi G. (ed.), Illustrierte Flora von Mitteleuropa (3th ed.), Vol. 2, Teil 1. Verlag Paul Parey, Berlin-Hamburg: 347-416.

Sell P.D. & Murrell G. (1996) Flora of Great Britain and Ireland. Vol. 5. Cambridge University Press, Cambridge: 410 p.

Skogen A. (1986) Ballastsiv (Juncus tenuis): en nykommer biter seg fast i Norge. Blyttia 44(3): 97-102.

Stace C. (1997) New flora of the British Isles, 2nd ed.: XXVII + 1130 p. Cambridge University Press.

Stolwijk P.F. & Zijlstra O.G. (2016) Bijzondere vondsten FWT-FLORON 2015. Hypericum 13: 4-8. [available online at: http://www.fwtwente.nl/h13.pdf]

Tikhomirov N. (2013) Juncus dichotomus (Juncaceae) in the flora of the East Europe. Bot. Journ. 98(6): 767-771.

Van Ooststroom S.J. & Reichgelt Th.J. (eds.) (1964) Flora Neerlandica. Flora van Nederland: 1. Alismataceae – 15. Typhaceae. KNBV, Amsterdam: 242 p.

Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.

Verloove F. (2006) Juncus tenuis. 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: 510.

Verloove F. (2010) Juncus dichotomus (Juncaceae) in northwestern Italy, a xenophyte new to Europe. Willdenowia 40: 173-178.

Verloove F. & Lambinon J. (2011) The non-native vascular flora of Belgium: new combinations and a new variety. New Journ. Bot. 1(1): 38-42.

Weeda E.J., Westra R., Westra Ch. & Westra T. (1994) Nederlandse oecologische flora. Wilde planten en hun relaties, vol. 5. IVN, Amsterdam: 400 p.

Wiegand K.M. (1900) Juncus tenuis Willd. and some of its North American Allies. Bull. Torrey Bot. Club. 27(10): 511-527.

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith