February 17, 2022

The exciting world of identifying asters

This journal entry was mostly copied from a post I made in September, 2020 in the Field Botanists of Ontario Facebook group.

Asters are one of my favorite groups of plants and late summer to the middle of fall is their peak season in most of North America. Asters are a paraphyletic group of plants in the tribe Astereae, which also includes the plants traditionally called "goldenrods" (Solidago) and "fleabanes" (Erigeron). Like butterflies and moths, asters, goldenrods and fleabanes can't be neatly divided into genetically distinct clades and there's crossover in their taxonomic treatment (e.g., the genus Oligoneuron includes species called both "aster" and "goldenrod"). This post is about plants traditionally called "asters", which were generally lumped in to the genus Aster in older taxonomic frameworks.

The genus "Aster" of most older field guides is now split into multiple genera distributed across various parts of North America. In Ontario, where I live, we have about 36 species in six genera:

  • Symphyotrichum (commonly called the "American asters") (23 or so species)
  • Oligoneuron (sometimes called "flat-topped goldenrods") (5 species)
  • Eurybia (sometimes called the "wood asters") (4 species)
  • Oclemena (2 species)
  • Canadanthus (1 species)
  • Doellingeria (1 species)

Some other North American genera formerly lumped into Aster include Dieteria, Boltonia, Ionactis and Sericocarpus. The former two genera are widespread in the western US and Canada and the latter two are widespread in the eastern US and Canada, but all four of those genera are apparently more enlightened than me and have chosen to avoid Ontario altogether. There is, in fact, one true aster left in North America, alpine aster (Aster alpinus), which only grows on the Arctic tundra and high peaks of the Rocky Mountains. If you are anywhere in North America other than those places, you do not have a "true" Aster, you have one of the related genera mentioned previously!

There are a lot of asters and identifying them requires careful examination of multiple parts of the plant, most often the involucral bracts ("phyllaries"), disc florets, cauline and basal leaves and the type of pubescence on the stem and other parts of the plant. For the vast majority of asters, one photo is not enough to positively identify it to species. More experienced botanists might be able to identify some observations by geographical range and "gestalt", but it's very difficult to be confident from just a single photo. Hopefully this post will help those of you trying to make sense of asters with what photos to take in order to have the best chance of identifying your plant to species.

Basic anatomy of an aster "flower"

Aster "flowers" are not true flowers but "composites" of many tiny florets called capitula or "heads" ( Figures 1 and 2). Incidentally, the composite nature of aster flowers is the origin of the historical family name Compositae, which is now called Asteraceae. The majority of plants in the tribe Astereae (i.e., asters and goldenrods) have two types of florets: ray florets with ligulate ("elongated") corollas and disc florets with five-lobed corollas (the exceptions are a small number of rayless species, such as Symphyotrichum ciliatum).

Above left: Front/facial view of a typical aster head; above right: side view of a typical aster head and peduncle. Photos by @wdvanhem.

Necessary photos for aster identification

The example below is an arrow-leaved aster (Symphyotrichum urophyllum) I photographed in September, 2020 outside of Cambridge, Ontario, Canada.

A high quality, identifiable aster observation should include at least the following eight photos:

Photo 1: Entire plant to show growth form, habitat, etc.

Photo 2: entire inflorescence.

Photo 3: close-up view of heads ("flowers") to see ray length and color, disc color, etc.; Photo 4: close-up of involucres, the single most important feature for aster identification.

Photo 5: cauline (stem) leaf; Photo 6: close-ups of adaxial (upper) and abaxial (lower) leaf surfaces to show pubescence, venation, etc.

Photo 7: mid-stem to show pubescence characteristics; Photo 8: base of plant to show basal leaves (if any).

Tagging some of iNat's top identifiers of asters:
@elizabeth1067 @tsn @sambiology @elacroix-carignan @seanblaney @popb25 @bouteloua @jayhorn

Posted on February 17, 2022 20:55 by wdvanhem wdvanhem | 6 comments | Leave a comment

May 20, 2018

Field Guide Exchange

For any iNaturalist junkies who are also on Facebook and who are interested in the free exchange of field guides and ultra-specific taxonomic keys, check out the Field Guide Exchange group:


@bouteloua @aaroncarlson @lincolndurey @tsn @mcaple etc....

Posted on May 20, 2018 12:57 by wdvanhem wdvanhem | 1 comment | Leave a comment


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