The Low-down on Those Darned Arnicas!

Originally published in the August 2022 issue of the WildFlower News

by Patsy Cotterill. Photos by author unless otherwise noted.

In the July issue of WN, a population of Arnica in photos taken in Nisku Prairie was wrongly identified as shining arnica, A. fulgens. In fact, it appears that the populations of Arnica in Nisku seen so far are all leafy or Chamisso’s arnica, A. chamissonis. There has been some confusion between the two species, including plants that have been transplanted into gardens. There are 11 species of Arnica in Alberta as a whole, although most of them grow in the mountains. We have two lowland species locally, the aforementioned A. fulgens and A. chamissonis, the latter being by far the more common and widely distributed across the province, including in the mountains. With Arnica having an excellent blooming year, now seemed like a good time to revisit some sites and clear up the confusion.

One of the pictures shown in the July issue of Wildflower News identified as shining arnica. It is leafy arnica, but it does show some characteristics of shining arnica, in not having particularly leafy stems, and single heads. Nisku Prairie, 2022.06.22.

In attempting to distinguish between related species it is important to look at the key characters that are considered to reliably separate them, such as are indicated in keys in plant guides and floras. Kershaw and Allen’s key in the Vascular Plant Guide, following the key in Flora of Alberta, separates the two on number of pairs of stem leaves: if the plant has 5-12 pairs, this leads to A. chamissonis; if it has 1-4 pairs, the key leads to A. fulgens. A problem can arise, however, when a specimen of leafy arnica (A. chamissonis) only has 4 pairs of leaves, especially if its lowermost leaves have withered by flowering time.

More reasonably, the key to Arnica in Flora of North America (FNA) separates out the two even more simply: if the leaves are mostly basal you quickly arrive at A. fulgens; leaves mostly on the stem (cauline) leads to A. chamissonis.

You don’t even have to count the number of leaf pairs on the stem. In the field, this character difference means that the two species look strikingly different. A. fulgens presents with straight, upright, bare-looking flower stalks, with only a pair of reduced stem leaves and the main leaves confined to prominent rosettes at the base. In contrast, the flower stems of A. chamissonis present as incredibly leafy, with large, toothed or toothless, stalkless leaves that often appear to weigh down the stem, causing it to bend over or become semi-prostrate. There are other differences too. A. fulgens usually bears a single composite flower head, whereas in chamissonis the top of the stem is branched and bears several large flower heads, making it the showier plant. Several other differences are less obvious. The FNA key notes that the basal leaf blades of A. fulgens have 3 or 5 subparallel veins, which are lacking in chamissonis. The tips of the involucral bracts in the latter should have discernible tufts of white hairs, although these are not always easy to make out on the generally hairy involucre.

L. Typical leafy arnica (Arnica chamissonis) in Nisku Prairie, 2022.07.03.

L.C. Shining arnica (A. fulgens) past flowering in Gibbons Badlands Prairie, 2022.07.09. The photo shows the reduced stem leaves, persistent basal leaves and single flower heads.

R.C. Basal leaves of shining arnica showing prominent parallel veins. Lowest basal leaves and the stem bases bear tufts of brown hairs. Gibbons Badlands Prairie, 2022.07.25.

R. Fruiting heads of shining arnica, with basal leaves dried up. Gibbons Badlands Prairie, 2022.07.25.

In addition, A. fulgens has tufts of brown hairs in its lower leaf axils and around its base, although seeing this character may require pulling the shoot out of the ground; A. chamissonis does not. This character is more important, however, in distinguishing fulgens from its sister and look-alike species, A. sororia, a denizen of Alberta’s more southern prairies.

So why all the confusion? Some of it has to do with the key: the strict cut-off between 1-4 and 5-10 pairs of stem leaves may not always hold. The fact that we don’t often find the two species together on the same site does not allow for a direct comparison. The two species do co-occur at Gibbons Badlands Prairie, although A. fulgens is by far the more common there, and I suspect this is because the badlands represent a more southern grasslands flora. If people do come across A. fulgens elsewhere in the Edmonton to Red Deer area I’d be interested in hearing about it.

Another source of local confusion may be that the population of leafy arnica in the middle field in Nisku Prairie is not typical. The population in the south field is very typical, leafy and multiple-headed, whereas that in the middle field has fewer leaf pairs and mostly single-headed flower stalks, and so approaches A. fulgens more in appearance. The basal shoots I have examined do, however, appear to lack any brown hairs, which should be present if the species is A. fulgens.

The literature gives no hint of hybridization, but FNA had the following of interest to say about the genus Arnica: “Arnica is circumboreal, predominantly montane, and exhibits maximum species diversity in western North America. It includes common and very widespread species as well as relatively uncommon, narrow endemics. Polyploidy and apomixis are common in the genus, resulting in considerable morphologic variability. Arnica montana from Europe has been used medicinally for centuries, and unsubstantiated claims have been made regarding the medicinal properties of some North American species.” Note the point about morphologic variability.

Incidentally, I’m reminded from the citations that some taxonomists at the University of Alberta worked on Arnica in the late 1980s and early 1990s: Steve Downie, William Gruezo, Steven Wolf and Keith Denford. Their specimens are in the U of A herbarium, indicating the importance of herbaria to the progress of taxonomy.

I should note that the populations in Nisku have appeared in considerable numbers this year after removal (by herbiciding) of invasive meadow foxtail (Alopecurus pratensis/arundinaceus), which occupies the lower, moister spots in the Prairie, microsites also favoured by Arnica. Without any further intervention on our part, these areas have been colonized by natives such as leafy arnica, woolly yarrow, narrow-leaved meadowsweet and even prairie onion over the last couple of years. It has been heartening to see!

Additional notes:

• Both species are rhizomatous perennials, with A. chamissonis having the longer rhizomes.

• In leafy arnica the broadly lanceolate, entire or toothed stalkless leaves occur in 4-10 pairs along the stem. Ray florets are yellow and the pappus is straw-coloured. The species also occurs in both the lowlands and the mountains; it can reach heights of up to 160 cm on the moist subalpine slopes near Cadomin, for instance. It is widely distributed across Canada (except the Maritimes) and the western U.S.

• Shining arnica has 3-5 pairs of stalked, narrowly oblanceolate to oblong, toothless leaves, mostly confined to the base of the plant, reduced in size and distantly arranged upwards. Ray florets are more orange-yellow and the pappus is lighter in colour. A grassland plant that also occurs in the foothills, it has a western North American distribution that reaches only as far as Manitoba in Canada.

References:

Flora of North America. Volume 21, page 366. Arnica.

Flora of North America. Volume 21, page 372. Online. Arnica chamissonis.

Flora of North America. Volume 21, page 370. Online. Arnica fulgens.

Kershaw, Linda and Lorna Allen. 2020. Vascular Flora of Alberta. An Illustrated Guide. Self-published. Kindle Direct Publishing.

Moss, E.H. 1983. Flora of Alberta. 2d ed. (ed. J.G. Packer). University of Toronto Press, Toronto.