Michaelmas daisies are perennial, horticultural stars of the late summer and autumn but they are not a single species, or even a single genus. The current common name refers to the late flowering, around Michaelmas (29th September), whilst a former common name (starwort) and the scientific name refers to the shape of the flower heads.
European and North American Michaelmas daisies have been a feature of British gardens since the sixteenth century, and have been grown in the Oxford Botanic Garden since 1648. An early North American introduction was apparently made by John Tradescant the Younger. Generations of selection and hybridisation by horticulturalists, among Michaelmas daisies from across the globe, produced thousands of cultivars, a multi-million pound horticultural trade and complex patterns of relationships that are difficult to disentangle.
At the beginning of the twentieth century, the genus Aster contained more than 500 North American and Eurasian species. Since 1753, when the Swedish botanist Carolus Linnaeus first formally named the genus based on a European species, it had become a taxonomic 'dustbin'. Botanists placed morphologically similar species, that they could not readily be put into other genera, into Aster. This state of affairs changed in the 1990s when detailed scientific investigations of the morphology and DNA of species from across the geographic range of the genus were published. These studies showed North American members of the genus form a group that is evolutionary distinct from Eurasian species; scientific names had to change to reflect this new research.
Despite perceptions, botanists are reluctant to change scientific names, particularly for economically important plants, because of the confusion that may be caused. One reason generic names change is when new taxonomic research shows convincingly they comprise more than one thing; Aster is a case in point. The genus Aster was shorn of almost all of its North American members. The rules of naming plants mean the name Aster must refer to the Eurasian plants; today the genus comprises approximately 180 species. On-going research shows even the current concept of Aster may be too broad, and that additional pruning may be necessary to accommodate evolutionary relationships among Asian species. The North American Michaelmas daisies are now split among approximately a dozen genera.
The consequences of such research for botanists and horticulturalists who use scientific names have been profound, but most gardeners still use Michaelmas daisy and aster as if referring to the unshorn genus.
Li W-P et al. 2012. Phylogenetic relationships and generic delimitation of Eurasian Aster (Asteraceae: Astereae) inferred from ITS, ETS and trnL-F sequence data. Annals of Botany 109: 1341-1357.
Noyes RD and Rieseberg LH 1999. ITS sequence data support a single origin for North American Astereae (Asteraceae) and reflect deep geographic divisions in Aster s.l. American Journal of Botany 86: 398-412.
Picton P and Picton H 2015. The plant lover's guide to asters. Timber Press.