August 29, 2022
It’s a patch of pale purple among late summer’s dry crested wheatgrass (Agropyron cristatum) and the buzzing songs of the insects. A couple of checkered whites (Pontia protodice), their wings tattered, are landing to sip nectar. It’s one of our purple asters: one of those beautiful plants that, through no fault of its own, gives botanists headaches. The problem is just that there are so many types of purple asters – identifying species can be difficult.
But the checkered whites, the striking orange and black tachinid flies, and the occasional bee visiting these flowers don’t seem too concerned about the botanist’s dilemma. The crested wheatgrass that dominates this site is another of our non-native grasses that typically forms simplified, low-diversity stands. And among what seems to be a biological desert of crested wheatgrass, this aster patch is a comparative oasis of activity.
To identify this aster, we’re going to have to take a closer look at it. This patch is in full bloom today, though a few of the flowerheads have already matured. Each of these mature heads is tipped with a silky tuft of hairs. The hairs will help the seeds disperse, carrying them to a new home on a breath of wind.
Like all of our asters, each daisy-like purple flowerhead is actually not just one flower, but many. In this case, there are flowers of two different types. Each purple, petal-like structure is an individual ray flower, the corolla flattened to resemble a petal but narrowing into a tube at the bottom. And within the sun-ray assembly of these flattened, lavender ray flowers are dozens of disk flowers. These don’t look at all like petals. Each disk flower is a yellow tube, surrounded by the tuft of hairs that will eventually carry the seeds on the wind. Within each tube is a yellow stigma, forked into two branches and protruding outwards. This is the structure that is ready to receive pollen as the tachinid flies and checkered whites brush past.
To identify an aster
I won’t bore you with all of the steps it takes to identify this plant. I’m using the Manual of Montana Vascular Plants, a dense and indispensable reference that contains step-by-step identification keys with almost every wild plant known from Montana. To identify purple asters, we have to pay close attention to the leaves and the flowerheads. We also have to look carefully at the involucral bracts, a series of leafy green structures that surround the flowerhead. The identification process also involves a lot of guessing and checking. Are the involucral bracts “strongly imbricate” – that is, strongly overlapping – with tips that are often purple? Or are they “somewhat imbricate” without purple tips? This helps us distinguish between asters in the genera Eurybia and Symphyotrichum.
When in doubt, identification is a process of elimination. Unsure about whether the involucral bracts are “strongly imbricate” or only “somewhat imbricate,” I start by reading about the species of Eurybia. None of them match the characteristics of the plants in front of me – so what I’m looking at isn’t a Eurybia. It must be one of our Symphyotrichum species.
Noticing the details
I continue through the ID process. Eventually, I arrive at one species that seems to match what I’m seeing: western aster (Symphyotrichum ascendens). And in the process, I’ve noticed a variety of intricate details about the plants in front of me. These details are somewhat technical, so feel free to skip over them. But I share them here to give an idea of the aster identification process:
- The involucral bracts are not glandular, nor do they curve back away from the flowerhead.
- These plants are rhizomatous, forming a loose patch connected by underground stems.
- The leaf surfaces are only sparsely hairy.
- Unlike some asters, the stem leaves do not clasp the stem with ear-like lobes.
- The basal leaves are oblanceolate and entire (without teeth on the margins).
- The hairs on the stems are not arranged in vertical lines from the leaf bases.
- The outer involucral bracts are definitely shorter than the inner bracts.
- The leaves have minute, stiff hairs along the edges.
Now it’s time to check my identification. Does this Symphyotrichum ascendens actually look like what I’m seeing? Is it growing in a habitat that makes sense? Western aster, Symphyotrichum ascendens, is described as a plant of mostly dry habitats like grasslands and roadsides. That corresponds with what I’m seeing here. I also take a look at Dr. Matt Lavin’s Flickr site. This is an excellent resource for information about many of our local plants, with high-quality photos and various field notes about habitat and ecology. And indeed, Dr. Lavin’s habitat notes also match with what I’m seeing. He describes western aster as a plant that’s typical of somewhat disturbed sites, such as along trails. And his photos of the plant correspond well with this patch.
The identification seems solid: this is western aster (Symphyotrichum ascendens). But the identification process has been much more than just slapping the right name on this patch of flowers. It’s been a process of getting to know them. Hopefully, from now on, I’ll be able to recognize western asters wherever I see them. I’ll be able to notice the patterns: where this aster grows on the landscape, how it changes through the seasons.
Symphyotrichum ascendens: not just any aster
From a distance, it might be easy to overlook western aster. These plants don’t shout for our attention: they’re just a pretty patch of late-summer native flowers, blooming quietly among a desert of crested wheatgrass. But up close, this plant has a story. The complex architecture of flowers, leaves, and involucral bracts allows us to recognize this species. It’s not just some unknown kind of purple aster anymore. This is western aster, a grassland species that, based on Matt Lavin’s observations, can thrive on rather-disturbed sites. If we see a purple aster in some other habitat -high up in the mountains, or growing along the edge of a stream – it’s probably a different species, with its own story and its own habitat affinities. (For example, compare this plant to the smooth blue aster, Symphyotrichum laeve, that I looked at two weeks ago in a Missoula yard. Smooth blue aster tends to grow in somewhat moister habitats than western aster.)
Five species of insects collected from Symphyotrichum ascendens for species identification this winter.
And among the crested wheatgrass at this edge of the Helena Valley, western aster is a place of gathering. It’s an oasis of flowers that brings together bee flies, checkered whites, and spiky orange tachinid flies. Today I’ve collected a few of these insects, so this winter I’ll be able to learn more about their stories, too. What are the lives of these diverse insects that have come together here, to feed on western aster nectar and to brush pollen onto the yellow stigmas?
Recognizing the patterns
Western aster and its insect visitors: it’s another handful of the late-summer stories of this landscape. And as we start to recognize this plant, to become familiar with the insects that visit it, we have another point of connection with the landscape around us.
So next time you’re out for a walk, keep an eye open for a patch of western aster, blooming among the grasses along the trail. If you want to be sure, grab a magnifying lens and a copy of the Manual of Montana Vascular Plants and give the identification process a try. And let me know which insects you’re seeing on these flowers!