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A note about Yucca (Agavaceae-Asparagaceae) in the wild in Belgium

Yucca L. (Spanish-bayonet), originally confined to the southernmost United States and parts of Central America, is a popular genus in cultivation, especially in the hotter and drier parts of the world. In Belgium a few species are grown as ornamentals, indoor as well as outdoors. In recent years Yucca is increasingly recorded as an escape from cultivation (often merely as a garden throw-out) in Belgium but little is known about the exact identity of these escapes and about their persistence. Recent field observations have solved most of the problems that surround this genus.

The first subspontaneous occurrences of Yucca in Belgium were probably reported by Verloove (2002). Yucca filamentosa L. and Y. gloriosa L. were discovered along the Belgian coast, the latter being slightly less rare. Both were considered to be ephemerals and only solitary, often young and non-flowering plants were recorded. Hence their exact identity remained more or less uncertain.

Yucca gloriosa, Koksijde (Oostduinkerke), nature reserve Ter Yde, coastal dunes, July 2010, F. Verloove Yucca flaccida, Heist, nature reserve Baai van Heist, August 2010, F. Verloove
Yucca gloriosa Yucca flaccida

By now (2010) Yucca is still present in most of its original (coastal) localities in Belgium. The usual escape has rigid, broad leaves with entire margins (without filiferous threads) and eventually produces a distinct stem. Its fruits are indehiscent and usually pendent. These plants are now confirmed as Yucca gloriosa (incl. Y. recurvifolia Salisbury). The exact placement of the latter is critical. According to Hess & Robbins (2002) it merely is a variety of Yucca gloriosa, distinguished by more flaccid leaves, a shorter inflorescence and smaller, more erect fruits. Belgian plants correspond well with var. gloriosa. According to Cullen (1986) it probably is the most widely grown species in Europe. Guillot Ortiz & Van der Meer (2009) furthermore indicate phenological differences between both: Yucca gloriosa flowers in spring and Y. recurvifolia in autumn. All Belgian plants seen so far flower in early summer (end of June, early July; sometimes with a second flowering period in late autumn), which also rather corresponds with Yucca gloriosa s.str. Curiously, many standard floras dealing with horticultural plants (for instance Cullen 1986, Jäger & al. 2008) indicate autumn as the main flowering period for both which is definitely wrong (compare with Hess & Robbins 2002).

A second species of Yucca is stemless with leaf margins with conspicuous filiferous threads and flaccid, narrow leaves. It is much more rare and non-flowering plants were initially ascribed to Yucca filamentosa. In 2010 this species was recorded in flower, which enabled a more precise identification. Inflorescence branches were densely pubescent, a character of Yucca flaccida Haworth, a close relative of Y. filamentosa (and perhaps better considered as a variety of the latter). It is furthermore distinguished by its narrower and thinner, very flaccid leaves and smaller flowers. Moreover, its leaf threads are only slightly curled to almost straight (conspicuously curled in genuine Yucca filamentosa).

The species of Yucca that now occur in the wild in Belgium are opposed in the following couplet:

  • Leaf margins with conspicuous splitted-off filiferous threads. Leaf blade usually soft and limp, 10-40 mm wide. Fruit dehiscent, erect === 1. Yucca flaccida
  • Leaf margins entire without filiferous threads. Leaf blades usually rigid and stiff (more rarely flexible), 40-60 mm wide. Fruit indehiscent, usually pendent === 2. Y. gloriosa

Both Yucca flaccida (as Y. filamentosa) and Y. gloriosa were considered to be ephemeral or at most “persistent” in Belgium (Verloove 2002, Verloove 2006): once introduced they were thought to persist for some years but finally did not manage (yet) to reproduce and become naturalised. This surely still holds true for Yucca flaccida: at its (to our knowledge) unique Belgian locality in the nature reserve Baai van Heist it recently started flowering but no off-spring has been recorded so far. Yucca gloriosa, on the contrary, was already reported as abundantly flowering in 2001 (Verloove 2002). By now, the number of localities in coastal dunes has much increased. At present it is known, for instance, from Blankenberge, De Panne and Koksijde (especially Oostduinkerke). Most of its populations are in or near nature reserves or other vulnerable areas. At least on the northern verge of the nature reserve Ter Yde (Koksijde-Oostduinkerke, close to the Noordzeedreef) Yucca gloriosa now occurs with several tens of specimens (see pictures): tall and profusely flowering as well as juvenile ones (the latter obviously the result of self-sowing). At least in this locality Yucca gloriosa doubtlessly should be considered as naturalised. Additional cases of future local naturalisation along the Belgian coast are very likely.

Yucca gloriosa, Koksijde (Oostduinkerke), nature reserve Ter Yde, coastal dunes, July 2010, F. Verloove

Finally, Yucca is often considered to be an invasive environmental weed outside its native distribution range, especially in habitats that match with those at home (mainly sand dunes). The early detection of a locally naturalised population of Yucca gloriosa in Belgian coastal dunes is therefore important in view of the conservation management of the nature reserves in this area. A future removal of Yucca gloriosa from the Ter Yde nature reserve, now that it is still manageable, appears to be appropriate.


References

Cullen J. (1986) Yucca. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 1. Cambridge University Press, Cambridge: 273-276.

Guillot D. & Van der Meer P. (2009) El género Yucca L. en España. Bouteloua, Monogr. 2: 124 p. Online available at: http://www.floramontiberica.org/Bouteloua/Monografia_Bouteloua_02.htm

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.

Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.

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.

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