Marsh aster (Symphyotrichum lanceolatum var. hesperium) is the most common and conspicuous aster at lower elevations in the Boise Front area.  The relatively tall plants with openly branched inflorescences can form large dense patches in wetlands, around the edges of ponds, and along rivers and streams.  The majority of populations are in the Greenbelt and South Boise sections of the Boise Front, with some scattered occurrences extending into the Foothills and along major streams on the backside of the Boise Ridge (e.g., Mores and Grimes creeks). Marianne Williams Park, Hubbard Reservoir, and Eagle Island State Park are particularly good places to enjoy marsh aster’s contribution to the late-summer/fall floral display.

Ray flowers of marsh aster range from light purple to nearly white, sometimes in the same or adjacent populations.  An openly branched inflorescence with numerous flowers is characteristic of this species, though some populations will have a narrower inflorescence, possibly due to hybridization with other species.  The key diagnostic feature involves hairs on the branches, which are arranged in distinct vertical lines rather than evenly scattered around the stem.  Phyllaries are generally relatively narrow and pointed, in contrast to the blunt, graduated phyllaries of S. ascendens.

In most older floras and other references, marsh aster was known by the scientific name of Aster hesperius.  Beginning in the mid 1990s, however, new research and reevaluation (e.g., Nesom 1994) resulted in the dismemberment of the unwieldy large genus Aster into several segregate genera, including not only Symphyotrichum but also Eurybia and Eucephalus in the Boise Front area.  Classification within Symphyotrichum is made difficult due to significant genetic and plastic variation within species, further complicated by interspecies hybridization.

Nesom, G. L.  1994.  Review of the taxonomy of Aster sensu lato (Asteraceae: Astereae), emphasizing the New World species.  Phytologia 77: 141–297.