Rayless Arnicas

After much time off, I’m back to writing descriptions for my book and am trying to finish up the arnicas. Arnicas are a difficult bunch to sort out, and I’ve been struggling to gain some understanding of them for a while now. They are variable, can hybridize, and sometimes reproduce by a form of self-sowing called apomixis. Apparently this can lead to populations with different characteristics than the norm. This genus definitely has an independent streak and doesn’t like to follow the rules.

Rayless arnicas

Arnica discoidea (L) near Bradley Lake and the larger Arnica parryi (R) at Groundhog Mountain

Most arnicas have bright yellow daisy-like flowers, but in the Western Cascades, we have two species that are rayless—only disk florets are present. That gives them a button-like appearance. They are easy to tell from our other species but can easily be confused with each other. Neither Parry’s arnica (Arnica parryi) nor rayless arnica (A. discoidea) is common in our area. I’ve seen Arnica parryi in meadows several times as well as roadsides, but I’ve only seen A. discoidea along roadsides and only at 3 or 4 sites.

Arnica discoidea. The white pappus is most evident when it is going to seed but can still be seen in the fresh flower on the right, peaking out between the phyllaries and the florets.

Two definitive differences between these two species are with the pappus (the hairy things attached to the fruit) and the phyllaries (the bracts that hold the flower head). Arnica discoidea has white pappus, while that of A. parryi is straw-colored. The phyllaries of A. parryi are long and tapering, often flopping at the tips, and they reach out to the end of the florets. In discoidea, they are much shorter and blunter and don’t reach as far up the floret tube.

Arnica parryi has nodding buds and longer, narrower phyllaries and straw-colored pappus

The leaves in both species are variable but hairy and glandular. They have large basal leaves on long petioles and a few pairs of smaller leaves on the stems that become sessile as they get higher up the stem. The leaves of A. discoidea seem much wider (more triangular or heart-shaped) and more distinctly toothed. Those of A. parryi are longer and narrower and are either entire or have only small teeth.

For those who want to study these species further, here are the Flora of North America descriptions:

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