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Symphyotrichum is a genus of about 90 species of herbaceous plants in the family Asteraceae that were formerly treated within the genus Aster. The genus Aster historically included nearly 600 species in Eurasia and North America. A more recent treatment of the genus (Nesom, 1994) narrows its circumscription to about 180 species, almost all from Eurasia, with the North American species being moved to other related genera. The Eurasian species are characterized by larger sized and more symmetrically shaped chromosomes and the generally larger, 2-sided, glandular achenes. The North American species are made up of a number of distinct groups. By far the largest number of the North American species transferred from Aster to other genera by Nesom (1994) are now in Symphyotrichum, a genus originally established by the botanist Christian Gottfried Daniel Nees von Esenbeck and resurrected by Guy Nesom. The former Aster novi-belgii was transferred to Symphyotrichum novi-belgii, which is now the type species for the genus. Symphyotrichum novi-belgii is a North American species which has become naturalized in parts of Europe, thriving in ruderal and waste areas, and mainly spreading vegetatively (BFIS, 2010). According to Hoffmann (1996), S. novi-belgii is considered as one of the most frequently occurring aster species in central Europe. Moreover, it belongs to these few exotic herbaceous perennials which are extensively naturalised in Britain and “demonstrate the breadth of morphological and reproductive strategies inherent in highly invasive species” as stated by Hitchmough and Woudstra (1999). According to their studies, a small number of prairie and woodland edge species have proved to be aggressive, among them S. novi-belgii which is widely naturalised and significantly increasing in the UK (Rich and Woodruff, 1996). According to several studies, S. novi-belgii is characterized as highly invasive (Michalkova, 2004; Novakova, 2008). Stace (1991) and Clement and Foster (1994) support that S. novi-belgii is now established in Great Britain and many other European countries.
Several former and current Aster species and their hybrids, are commercially grown for cut flower production (Kadrnan-Zahavi and Yahel, 1985). Horticulturally they are all widely referred to as “asters” or “Michaelmas daisies”, reflecting their typical flowering period. The common name New York aster comes from its native status in this part of the USA. The species epithet novi-belgii means from New Belgium, a former name for the region between Virginia and New England in what became the USA.S. novi-belgii is native to eastern Canada and the northeastern USA. Perennial North-American asters such as S. novi-belgii have become common in central Europe since the 17th century when they started to be cultivated as ornamental plants. Today, they are widely spreading especially in alluvial sites in the lowlands (Jedlicka and Prach, 2006). S. novi-belgii is considered as a naturalized alien in the Czech Republic (Pysek, 2003). In Belgium, it is among the species found recently in Brussels (1991-1994) that are absent from 1940-1971 plant lists (Godefroid, 2001). As well as being widely distributed in Europe, it has been found in countries including Japan, Australia and New Zealand: see Distribution Table for details. Like many other species later recognized as being invasive in central Europe, North-American perennial asters were also used first as ornamental plants. They have been cultivated since the 17th century (Hoffmann, 1996) and today many species of this large genus, including a high number of cultivars, are used in horticulture. Only Symphyotrichum lanceolatum [formerly Aster lanceolatus] and S. novi-belgii are considered to be widely naturalized in central Europe (Meusel and Jager, 1992). In considering the earliest possible date of the arrival of Aster at Wicken region (UK), it should be taken into account that Asters were introduced from eastern North America from 1633 onwards, with S. novi-belgii being brought into cultivation about 1710 (Wheldon, 1919; Green, 1974). It was first observed in the wild in Belgium in 1865 (BFIS, 2010). (Source: www.cabi.org)