Symphyotrichum novi-belgii
(New York aster)

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.

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.

Globally, there can be a very few countries left where Asters have not already been introduced, but the risks of further introductions clearly remain very high, given the potential of this genus for a variety of uses and the lack of any wide-ranging prohibitions in its movement. Alluvial sites, abandoned fields and regions near flowing water should be taken into account as potential entries of these species. There is a high likelihood of deliberate introduction of S. novi-belgii because of its intended and wide use as an ornamental plant, and it can then be spread by fly-tipping of green waste (BFIS, 2010).

This species is not listed as a quarantined species, however it has a potential for further invasiveness and dispersal. Despite some limitations in growth, it is evident that S. novi-belgii tends to form large and dense colonies, which can persist over decades (Briggs et al., 1989), while it belongs to the most capable rhizomatous European aliens (Pysek, 2003). This combination of high seed production and intensive vegetative spread is generally highly effective in spread of species, and such plants were characterized as ‘superspecies’ by Huston and Smith (1987).

In its’ native North America, S. novi-belgii is found growing in slightly brackish and tidal fresh marshes, occasionally borders of salt marshes; inland marshes, shrub marshes, shores and other moist areas (Tiner, 1987). Haines (2011) says that in New England, var. novi-belgii and var. elodes are found on coastal and near coastal habitats plus inland riverbanks, fens, marsh edges and boggy shores, while var. villicaule is usually found on rocky river shores. According to the Online Atlas of the British & Irish Flora (2016), in the UK it is naturalized on hedge banks, railway banks, roadsides, rubbish tips and waste ground, and sometimes occurs on lakesides and in fen vegetation. In Belgium it thrives mostly in ruderal and waste areas, especially on rich and moist soils, while it is less often observed in riparian habitats than other North American asters (BFIS, 2010).

S. novi-belgii spreads intensively in central Europe, becoming naturalized especially in alluvial sites along large rivers. Other invaded habitats include various disturbed and neglected sites, such as urban abandoned areas, road and railway banks (Jedlicka and Prach, 2006). It often invades abandoned sites and regions around flowing waters (Beniak et al., 2015). The species spreads rapidly along rivers, and it is not unusual to see large and dense growths on riverbanks and roadsides (Michalkova, 2004). In Poland and particularly Silesia-Cracow Upland, it is starting to colonize areas of former mining activity (Jędrzejczyk-Korycińska, 2004). 

Genetics

S. novi-belgii has four botanical varieties: var. villicaule, var. crenifolium, var. novi-belgii and var. elodes (Flora of North American Editorial Committee, 2016). It sometimes hybridizes with Symphytrichum lanceolatum and S. lateriflorum (Flora of North American Editorial Committee, 2016). It has a sporophytic count of 2n=6x=48 (Labreque and Brouillet, 1996; Missouri Botanical Garden, 2015).

Reproductive Biology

Flowers are developed between June-August and October-November. Flowers are hermaphrodite and are pollinated by bees, butterflies, flies, beetles and moths (USDA, 2003). Its ability to flower depends on the following sequence of events: vernalization under winter cold, stem elongation and branching during the long days (LD, 16 h of light or more) of late spring and summer, flower initiation and flowering during the short days (SD, 10-16 h of light) of autumn (Wallerstein et al., 1992a; 1992b; Kristiansen et al., 1997). LD conditions inhibit flower initiation but induce shoot elongation and branching, while SD conditions induce flowering (Schwabe, 1985; Kadman-Zahavi and Yahel, 1985). Branched shoots, having been induced to flower under SD conditions, produce normal or abnormal terminal buds (inflorescence or involucre of bracts) when re-exposed to LD conditions. Since at least first-order branching is a prerequisite for flower initiation, rosette shoots will not flower under SD conditions or produce terminal buds following their transfer to LD conditions.

Simultaneously with flowering, a rosette type of growth develops from basal buds located below the soil surface, at the base of the flowering stem. In cold zones the rosette leaves freeze in winter, and growth resumes from the basal buds in spring. If the winter conditions are such that the rosette does not freeze (as in Israel), it will start to elongate in spring. Under Israeli conditions vernalization is not a prerequisite for elongation in S. novi-belgii (Kadrnan-Zahavi and Yahel, 1985).

S. novi-belgii produces large amounts of efficiently distributed seed, and is considered to be a self-incompatible species (East, 1940).

Physiology and Phenology

S. novi-belgii sexually (generatively) reproduces by seeds and non-sexually (vegetative reproduction) by underground stems called rhizomes (Michalkova, 2004). Therefore, S. novi-belgii is characterized as a rhizomatous plant which forms clonal patches (Cornelius, 1990; Chmielewski and Semple, 2001). According to Labrecque and Brouillet (1996) it is considered as a morphologically variable species and this variation appears to have both genetic and environmental bases.

Germination experiments show that most asters produce a relatively high number of viable and easily dispersed seeds, which often germinate immediately after ripening and usually do not require any special treatment. Low temperatures interrupt the germination of achenes in winter (Baskin and Baskin, 1979). S. novi-belgii, however, is generally considered to produce only a small amount of germinating seeds (Hoffmann, 1996). In Belgian climatic conditions, seeds do not seem to be able to mature (BFIS, 2010). When grown as an ornamental, seeds should be sown fresh in the autumn or spring (Heuser, 1997), with spring-sown seeds being chilled to improve germination (USDA, 2003).

Invasive asters possess a high capability of vegetative spread, while in already established polycorms, the rate of vegetative spread of clonal plants may be reduced (Harper, 1977). The vegetative spread of asters was very much reduced in compact soil (Schmid and Bazzaz, 1990). Chmielewski (1995) demonstrated very high phenotypic plasticity among clones, thus any findings cannot generalized. However, it is suggested that in suitable climatic conditions, S. novi-belgii can spread easily by seed over a long distance and, after its establishment, can spread fast vegetatively, monopolizing the space. From eight clones of S. novi-belgii the percentage of immediately germinated achenes was between 83.3 and 96.6 %, while after storage at 20°C germination percentages of most clones were further increased (Jedlicka and Prach, 2006).

Rhizome fragments of S. novi-belgii clones of a length of 10 cm with developing basal leaves of terminal rosettes can be put 5 cm deep in order to give new plants. Some aster clones produced more than 100 new rosettes from one initial rosette within a year. Apparently, S. novi-belgii forms large polycorms in a relatively short time, however the role of vegetative growth has not been extensively studied. It should be noticed that their rapid vegetative spread and biomass production of S. novi-belgii can be significantly reduced by competition from established vegetation (weeds) as shown by Jedlicka and Prach (2006).

Environmental Requirements

This plant requires well-drained soil and prefers sandy, loamy and clay soils. It can grow on nutritionally poor soil, in semishade or no shade but prefers a sunny location (USDA, 2003). S. novi-belgii grows usually at lower altitudes in lowlands and hilly areas. It tolerates a wide range of environmental conditions and quickly colonizes recently disturbed sites.

Dowson (1923) found that different species of Aster (the then wider genus) varied in their degree of susceptibility to diseases, the most susceptible being varieties of S. novi-belgii. S. novi-belgii vars 'Rufus', 'Snowsprite' and 'Climax' were found to be extremely susceptible to Phialophora asteris (Burge and Isaac, 1974).

General

• Botanical garden/zoo

Human food and beverage

• Seeds

Medicinal, pharmaceutical

• Traditional/folklore

Ornamental

• Cut flower
• Potted plant

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

For the chemical control of S. novi-belgii as an invasive plant, several non-selective herbicides could be used (eg glyphosate, glufosinate, diquat), since it has not yet entered any crops. Some other herbicides such as picloram and 2,4-D have been used for the control of similar species like Chloracantha spinosa [formerly Aster spinosus Benth.] (Gonzalez et al., 2010). However, risks of herbicide resistance development should be also taken into account since biotypes of Aster squamatus were found to be resistant to the ALS-inhibiting herbicide imazapyr after 10 years of use in Seville, Spain (Osuna et al., 2003).

S. novi-belgii can be controlled by means of tillage, however ploughing should be done in depths more than 15-20 cm, because of the rhizomes of the plant which should either come in the soil surface (in order to be burnt by sun) or move to greater depths. It is recommended to avoid planting this species near rivers and wet habitats, especially near protected areas (natural reserves, Natura 2000 sites, etc.) and to put a rhizome barrier in order to limit lateral expansion. In Belgium, introduction is prohibited within and nearby protected sites and areas of high biological value and in the vicinity of watercourses (Moniteur Belge, 2013).

29/02/2016, Original text by:

Dr Ilias Travlos, Agricultural University of Athens, Athens, Greece