Symphyotrichum is a genus with ca. 90 species, native to the Americas and Eurasia. It is by far most diverse in North America where 77 species occur (Brouillet & al. 2006). Many American species are cultivated as ornamentals (so-called ‘asters’ or ‘michaelmas daisies’) outside their native distribution range. Several of these have become naturalized and are often considered environmental weeds. This genus is one of the many segregates of Aster L., a genus that was found to be polyphyletic (Nesom 1994). The new circumscription for the segregate genera now has become widely accepted, in the New as well as in the Old World (e.g. Brouillet & al. 2006, Jäger 2011, Tison & de Foucault 2014, etc.; see, however Stace 2010). Its application in horticultural taxonomy was inevitable (Shaw 2014).
Symphyotrichum is still very poorly understood in Belgium and the present account is a provisional one. Most species are much alike and – surely as a result of hybridization – more or less fade into each other. Therefore, the distribution and invasion history of Symphyotrichum in Belgium is insufficiently known. According to our current knowledge, Symphyotrichum lanceolatum and S. xversicolor appear to be the most widely dispersed species of the genus in Belgium, while all others are probably rare and often ephemeral or only very locally naturalized. Dirkse & al. (2014) demonstrated that in riparian habitats in the eastern part of the Netherlands, in addition to the widespread S. lanceolatum, at least two additional small-flowered species occur (S. ontarionis (Wiegand) G.L. Nesom and S. lateriflorum (L.) Á. et D. Löve).
The current treatment is chiefly based on recently observed Belgian populations. Some other taxa, some of unknown identity, have been observed in the past in Belgium (prior to 1950) but are no longer seen at present.
The number of cultivars, often of complex hybrid origin, is very high today and traditional taxonomy is no longer applicable to a large part of the asters currently sold in nurseries or garden centers (as cut-flowers or for planting) (Hetterscheid & van den Bergh 1996). Fortunately, taxa of Symphyotrichum usually found in the wild in Belgium are mostly “pure” species or at least species with a more or less well-defined hybrid origin. With the key beneath most populations will key-out although some will turn out to pertain to complex hybrid swarms.
In Belgium several species of Symphyotrichum are noxious environmental weeds. Reproduction is chiefly (probably even exclusively) non-sexual, the species being dispersed by strong clonal growth. Viable seed is probably only rarely (if ever) produced in Belgium. Symphyotrichum is usually introduced via garden waste or from transported or washed-up rhizomes. It seems to be invasive mostly in riparian habitats, where Symphyotrichum lanceolatum seems to be the usual species (see, however, Dirkse & al. 2014).
In addition to the species included in this account some others are worth mentioning.
Symphyotrichum parviflorum (Nees) Greuter (syn.: Aster parviflorus Nees) is an enigmatic small-flowered taxon. Historical records of this species were cited by Hoffman (1996) from Belgium (“Bei Verviers an der Vesdre”). However, Nees’ Aster parviflorus is now considered part of the Symphyotrichum subulatum complex (e.g. http://www.tropicos.org). The exact identity of the plants formerly found near Verviers is uncertain but it is clear that the name Aster parviflorus was misapplied for them. They may be mere small-flowered forms of S. lanceolatum (var. interior (Wiegand) Nesom??) (see also Dirkse & al. 2014). Finally, a peculiar low-growing ‘species’ of Symphyotrichum is often cultivated and has been recorded as a more or less established throw-out. It shares features of both S. dumosum (L.) Nesom and S. novi-belgii (L.) Nesom and possibly represents their hybrid. It was well-established between basalt rocks in Oostende (Spuikom) but this population was recently destroyed after infrastructural works (2013).
Up-to-date information on asters and related genera is available here: https://uwaterloo.ca/astereae-lab/research/asters.
1 Heads disciform (ligular florets absent or inconspicuous). Annual === 2
Heads radiate (ligular florets present, conspicuous). Perennial === 3
2 Ligular florets absent. Phyllaries subequal or sometimes the outer slightly longer, foliaceous (entirely green). Disc florets whitish turning pink. Heads usually (sub-) sessile === Symphyotrichum ciliatum
Ligular florets always present, white, ca. 20-30 in number. Phyllaries unequal, the outer much shorter, with central portion green and hyaline margins. Disc florets yellow, sometimes tinged with purple. Heads on peduncles 0.5–4 cm long === S. subulatum
3 Inflorescence branches, involucral bracts and leaves glandular hairy (with numerous multicellular hairs) === S. novae-angliae
Plants entirely eglandular (without multicellular hairs) === 4
4 Corolla lobes distinctly longer than tube (lobe/limb ratio 50-75%). Flowers small (ligule ca. 4-5 mm long), often congested and in more or less one-sided arrays === S. lateriflorum
Corolla lobes distinctly shorter than tube (lobe/limb ratio < 35%). Flowers usually larger, not in one-sided arrays === 5
5 Plants densely hirsute-hairy throughout (not hairy in lines). Ligular florets always white. Involucral bracts with apex involute, spine-like. Caespitose (plants appearing singly) === S. pilosum var. pilosum
Plants sparsely hairy (hairs in lines) to subglabrous. Ligular florets mauve, bluish, purplish or white. Involucral bracts flat at apex, never spine-like. Colonial (plants appearing in dense stands) === 6
6 Upper cauline leaves sessile but not clasping stem. Ligular florets usually white (drying bluish), less frequently pink or blue. Involucral bracts very unequal, 0,5-0,7 mm wide, the median green part very narrow === S. lanceolatum
Upper cauline leaves slightly clasping stem to distinctly amplexicaul. Ligular florets mauve, bluish or purplish, rarely white. Involucral bracts 0,7-1 mm wide === 7
7 Involucral bracts markedly unequal, the outer ca. ½ to ¾ as long as the inner, the whitish hyaline border extending to ca. upper 1/3 of the bract (central green patch confined to upper 1/3 of the bract). Leaves often amplexicaul, ovate, at most 5x as long as wide === S. xversicolor
Involucral bracts subequal, whitish hyaline border extending to upper ½ of the bract (central green patch often occupying the upper ½ of the bract or bract sometimes entirely green). Leaves only slightly clasping, lanceolate, 4-10x as long as wide === 8
8 Involucral bracts subequal, very loosely arranged, usually more than 0,7 mm wide. Leaves usually distinctly clasping, lanceolate === S. novi-belgii
Additional aliens: Symphyotrichum racemosum (S. Elliott) Nesom (syn.: Aster racemosum S. Elliott) (N-Am., garden escape) and S. subulatum (Michaux) Nesom s.l. (syn.: Aster subulatus Michaux; incl. S. squamatum (Spreng.) Nesom, syn.: A. squamatus (Spreng.) Hieron.) (Am., wool alien).
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