Identification of Oxytropis besseyi, lagopus and similar species

Image: Oxytropis besseyi from Bozeman, MT, observed by Matt Lavin: (© CC-BY-SA)

Identifying Oxytropis (locoweeds) can be a bit of a challenge. As with many plants, published keys often use characters that are hard to see on pictures, or they require fruit when there are only flowers, etc.

Oxytropis is one of the genera I regularly identify, and I am going to share here what I learned while going through Oxytropis observations from Montana. Oxytropis besseyi and lagopus were new species to me because I live in Canada where both species have very small and localized ranges.

Literature used: Flora of North America Vol. 11, Fabaceae (2023); Manual of Montana Vascular Plants (2012). Some of the characters presented here are not mentioned in the literature, specifically the shape of the wing petals. Most taxonomic work is done on pressed herbarium specimen where some of the three-dimensional characteristics of the flowers are no longer observable. Therefore, characters like these are easily missed, and they wouldn't be very useful identifying herbarium specimens. What I am putting forward here are hypotheses, and in science every hypothesis needs to be tested. Therefore, please let me know if you find a mistake or see something that can be improved. But now to the meat:

Introductory note: Oxytropis flowers are variable in shape and colour. It is very common to find misshapen or atypical flowers where petals are bent in ways that are not normal. Please take that into consideration when using the diagnostic characters below. Try to assess a number of flowers, if possible, and disregard the ones that fall out of line. It is also important to consider the stage of the flower. Flowers that are in the process of unfolding have the wing petals closer together than mature flowers. Withering flowers are often bent out of shape. The descriptions below apply to mature flowers. The literature uses the presence vs. absence of black hairs on the calyx to distinguish lagopus from besseyi. I have not been able to see black hairs on any of the images here on iNaturalist. This character probably requires higher magnification.

Oxytropis besseyi

  • wing petals spreading widely (usually at least 90°), openly exposing keel
  • upper margins of wing petals approximately as far apart as lower margins (petals not tilted)
  • calyx villous, predominantly with spreading hairs
  • only basal part of mature pod sheathed by calyx (which is at most slightly inflated in fruit)
  • racemes 3-22-flowered, subcapitate to slightly elongate (esp. in fruit)
  • most common in mountainous areas (valleys to subalpine), marginal in prairie zone, but reaches southern Saskatchewan
  • blooming later than lagopus at the same elevation (June 5 to Aug 10 in Montana based on iNat observations, n=69)


Oxytropis lagopus

  • wing petals either not spreading widely (< 90°) or else petals tilted inwards dorsally
  • upper margins of wing petals usually closer together than lower margins or else wing petals not wide spreading
  • calyx villous, predominantly with spreading hairs
  • mature pod usually fully enclosed by inflated, persistent calyx (var. lagopus), or rupturing slightly inflated calyx in var. conjugans and atropurpurea)
  • racemes 5-18-flowered, subcapitate to slightly elongate
  • most common in mountainous areas (valleys to subalpine), marginal in prairie zone, but reaches Milk River Ridge in southern Alberta
  • blooming earlier than besseyi at the same elevation (April 27 to Jun 29 [one outlier: Jul 23] in Montana based on iNat observations, n=70)


Oxytropis lambertii

  • wing petals convergent above (often more or less touching), ± concealing keel
  • lower half of wing petals more or less parallel to each other or tilted outward
  • calyx predominantly with decumbent hair (few spreading hairs) [unlike lagopus, lambertii]
  • only basal part of mature pod sheathed by calyx (which is not inflated in fruit)
  • racemes 8-45-flowered, more elongate than in the other species (esp. in fruit), flowers well spaced
  • leaflets often very narrow
  • occurs mostly in prairie zone in north-central U.S. but reaches foothills, extends east to Manitoba, MN and IA
  • blooming May 7 to Jul 8 in MT and SD based on iNat observations (n=217)


Publicado el enero 31, 2024 05:41 MAÑANA por matthias22 matthias22


@aspidoscelis If you have any feedback or suggestions on this as well it would be much appreciated! I am planning to add multiceps soon.

Publicado por matthias22 hace 5 meses

Another feature I've noticed for Oxytropis besseyi—the keel is quite short, indistinctly beaked, curving downward at the apex. I haven't checked all the others, but it seems like most of the similar species in Wyoming, at least, have longer keels with a well-developed beak that sticks straight out or curves a little upward.

Publicado por aspidoscelis hace 5 meses

I find leaf pubescence useful, but perhaps not easy to describe. Some taxa have disheveled pubescence on the leaflets, the trichomes more or less appressed and mostly ascending, but varying in direction, some of them somewhat spreading, often a bit wavy. Others have tidy pubescence on the leaflets, the trichomes strongly appressed, parallel, straight.

Disheveled: Oxytropis besseyi var. fallax; Oxytropis lagopus var. atropurpurea; Oxytropis multicaulis; Oxytropis lambertii.

Tidy: Oxytropis besseyi var. ventosa; Oxytropis nana; Oxytropis sericea.

I haven't gone through any of these too thoroughly to make sure they're consistent, and I don't know anything about the other vars. of besseyi & lagopus. Also, my knowledge of Oxytropis lambertii is mostly from New Mexico and Colorado, I'm less familiar with what it does to the north. There's also potentially useful variation in leaflet shape / orientation. The 'tidy' taxa generally seem to have more rounded apices and bases on the leaflets.

Publicado por aspidoscelis hace 5 meses

Also, for Oxytropis multicaulis, I think the combination of few flowers per inflorescence and inflorescences barely taller than the leaves works pretty well. Dorn's 'Vascular Plants of Wyoming' focuses on number of flowers per inflorescence, but depending on conditions any species will have occasional depauperate individuals where that won't work well.

Publicado por aspidoscelis hace 5 meses

I'm glad you've been going through these. I've been a bit confused by Oxytropis for a while, and wasn't really in a position to start figuring it out on my own.

Publicado por aspidoscelis hace 5 meses

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