Yucca flaccida Haworth (N-Am.) – A frequent garden plant, sometimes seen as a garden throw-out in ruderal areas or on dumps. A single clone is known since at least 2001 from the nature reserve Baai van Heist. Since then it has been repeatedly recorded in several additional localities in the Belgian coastal plain. However, it is by far most frequent on sandy soils in the Kempen. At least in these two areas, where it is found in climatologically favorable circumstances, Yucca flaccida has recently been able to establish itself. In some localities, e.g. in Lommel, it forms rather impressive stands. An up-to-date overview of Belgian localities is available here: http://waarnemingen.be/soort/view/197670. On a global scale, however, this species is less widely spread in Belgium than Yucca gloriosa.
The exact identity of the representatives of series Filamentosae, currently found in the wild in Belgium, long remained uncertain. All known populations are fairly recent and long remained in the non-flowering phase. They were initially ascribed to Yucca filamentosa L. s.l. (e.g. Verloove 2002, Verloove 2006). Most plants seen so far are characterized by inflorescence branches that are pubescent and flaccid, narrow leaves with threads that are not conspicuously curled. These features are characteristic of Y. flaccida, a segregate of Y. filamentosa. Also, Yucca flaccida is a more mountainous species and thus less frost sensitive than genuine Y. filamentosa, a species from coastal plains (Hochstätter 2002). The presence of the latter in the wild in Belgium cannot be excluded, however. It has glabrous inflorescence branches, larger flowers (tepals 50-70 mm long, vs. 30-50 mm) and wider and thicker, more rigid leaves (Hess & Robbins 2002). Both these species have often been considered conspecific. However, molecular studies have shown them to be quite distant (Clary 1997; see also Ward 2011). These studies further revealed that Yucca flaccida is the only dry-fruited species that groups with the fleshy-fruited ones.
In suitable habitats Yucca flaccida builds fairly impressive stands, apparently as a result of strong clonal growth. In the absence of yucca pollinator moths in western Europe, sexual reproduction is prevented (see also Rentsch & Leebens-Mack 2014).
At a first glance, Yucca flaccida and Y. gloriosa are much reminiscent (except for their different leaf margins). Both are very different in habit (acaulescent vs. caulescent) but this is only obvious in mature plants. Young plants of Y. gloriosa lack the typical trunk that only develops with age.
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Selected literature:
Clary K.H. (1997) Phylogeny, Character Evolution, and Biogeography of Yucca L. (Agavaceae) As Inferred from Plant Morphology and Sequences of the Internal Transcribed Spacer (ITS) Region of the Nuclear Ribosomal DNA. Ph.D. dissertation. University of Texas.
Hess W.J. & Laurie Robbins R. (2002). Yucca. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 26. Oxford University Press, New York-Oxford: 423-439. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=135226]
Hochstätter F. (2002) Het geslacht Yucca (Agavaceae). 10. Serie Filamentosae. Succulenta 81(1): 36-43. [available online at: http://www.cactuspro.com/biblio_fichiers/pdf/Succulenta/Succulenta_2002.pdf]
McDaniel J.C. (1975) The typical Yucca filamentosa L. Cact. Succ. J. (USA) 47(3): 110-111.
Rentsch J.D. & Leebens-Mack J. (2014) Yucca aloifolia (Asparagaceae) opts out of an obligate pollination mutualism. Amer. Journ. Bot. 101(12): 2062-2067. [available online at: http://www.amjbot.org/content/101/12/2062.full.pdf+html]
Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.
Verloove F. (2006) Catalogue of neophytes in Belgium (1800-2005). Scripta Botanica Belgica 39: 89 p. [available online at: http://alienplantsbelgium.be/sites/alienplantsbelgium.be/files/tabel_2.pdf]
Ward D.B. (2006) A nomenclatural history of southeastern filiferous Yucca, with selection of a neotype for Y. flaccida. Castanea 71(1): 80-84. [available online at: http://www.bioone.org/doi/pdf/10.2179/04-30.1]
Ward D.B. (2011) Yucca filamentosa and Yucca flaccida (Agavaceae) are distinct taxa in their type localities. Castanea 76(2): 222-228. [available online at: http://www.bioone.org/doi/pdf/10.2179/10-017.1]