Alien plants can come in in various ways. Well known examples of inadvertent introductions are, for instance, weedy species that are introduced as contaminants in cereals (so-called grain aliens) or pasture species that are introduced in sheep wool (so-called wool aliens). Likewise, the nursery trade is also responsible for the introduction and spread of weed species. Other sources may be bark or timber, granite, coal, sand, etc. but these vectors are obviously of a lesser importance in Belgium than are cereals these days or was wool in the past.
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National Botanic Garden of Belgium, Bouchout Castle, December 2, 2011
At the occasion of the six-monthly meeting on December 3 and 4, 2011, of the consortium of representatives of national networks of botanic gardens in the European Union (BGCI) a mini symposium was organised at the National Botanic Garden of Belgium.
Powerpoint slides and/or a short presentation-based text of some of the lectures can be consulted on the internet; see the links in the program below.
Chinese Mugwort, Artemisia verlotiorum Lamotte, is known in Belgium since 1937 when it was found on rough ground in Brussels (Matagne 1938). It remained very rare and poorly known for quite a long time but it is obviously increasing in recent times (Verloove 2003). Artemisia verlotiorum, like our native A. vulgaris L., belongs with Artemisia section Artemisia (“the Artemisia vulgaris-complex”), a large group of closely related and still insufficiently understood species with many representatives in the Asian Far East.
Rumex is a surprisingly poorly known genus in Belgium, especially with regard to its non-native representatives. In the past years several different taxa have been reported as “new” although some of them were already collected more than a century ago (for instance Rumex stenophyllus) (see Verloove 2008).
Leonurus cardiaca subsp. villosus, a long forgotten and probably neglected xenophyte in Belgium
Dipsacus is a genus of ca. 15 Old World species, mainly distributed in Eurasia and the Mediterranean region (some in Africa and Asia). All species are more or less alike. However, a few are distinguished by shortly petiolate cauline leaves (not sessile and connate as in most other species) and globose flower heads. These are sometimes segregated as a distinct genus, Virga Hill. (see for instance Štepánek & Holub 1997) but this point of view is not followed by other recent taxonomists nor is it supported by molecular phylogenetic studies.
Bromus section Ceratochloa is a taxonomically complex assemblage of several different closely related taxa, all of New World origin. Especially Bromus carinatus and its relatives are notoriously difficult in terms of identification: some authors tend to accept a single variable species, others are convinced that several species merit species rank.
Petrorhagia (Ser.) Link is an Old World genus with ca. 33 species, ranging from the Canary Islands to Kashmir. One species, Petrorhagia prolifera (L.) P.W. Ball & Heywood, is native in Belgium although it often occurs in highly artificial habitats (by railway tracks, in gravel pits, on coalmine heaps, etc.) where it is most likely introduced. A second species, Petrorhagia saxifraga (L.) Link, has been recorded in Belgium as an ephemeral escape from cultivation.
Return of the aliens in 2011
In the past years the importance of garden escapes (deliberately introduced non-native plants that run wild) has steadily increased in Belgium (see Verloove 2006). "Genuine" aliens (unintentionally introduced non-native plants), on the contrary, have much decreased lately. Wool aliens are no longer seen since the 1970's in Belgium while grain aliens still occur but often in small amounts and confined to a restricted number of localities (mainly in port areas). An obvious explanation for this decline is lacking but one might think of the following:
In August 2011 I came across a dense population of a species of Lathyrus that I know since at least 25 years. It grows by a disused railway track and in adjacent grassland near Kortemark (prov. West-Flanders, Belgium; IFBL D1.26.41). It has always been referred to as Lathyrus latifolius L. and is mentioned as such on waarnemingen.be as well (see for instance http://waarnemingen.be/waarneming/view/55774177).