This post will cover how to identify North American Hylodesmum to species using fruits, flowers, and leaves, and how all three species are distinguished from Desmodium.

Not so long ago, Hylodesmum was split from Desmodium (Ohashi and Mill 2000). Despite the inevitable eyerolling that comes with any taxonomic change, this split is a fairly nice one: Hylodesmum was already a distinct subgenus of Desmodium (called subgenus Podocarpium) and all three North American species kept their epithets in the transfer. And best of all, the three North American Hylodesmum species are distinct and easier to identify than many Desmodium species!

Flowers

A lot of iNat observations are just a macro shot of a flower without any other images of the rest of the plant. Most Desmodium can't be identified to species from just a flower, but they can be distinguished from Hylodesmum flowers reasonably well: Desmodium always has a pair of yellow-green spots on the banner (the top petal), often lined with dark pink/purple. Only H. nudiflorum has a pair of spots on the banner, and these spots are dark purple (or dark green in white-flowered mutants).

Note: Most species of Desmodium and Hylodesmum have pink/magenta flowers. Some species, like Hylodesmum pauciflorum, Desmodium ochroleucum, and Desmodium tweedyi always have white flowers, and Desmodium sessilifolium usually does too. However, an uncommon mutation can cause an individual's flowers to be white- don't count on flower color for ID!

Desmodium Flowers

Note the shape of the flowers and the pair of light yellow-green dots on the banner.

Desmodium paniculatum @tshahan | Desmodium ochroleucum @ciafre | Desmodium ciliare @astrobirder

Hylodesmum Flowers

Note the different shapes of the flowers, and the presence of two dark purple dots on the banner of H. nudiflorum. Also note the relative lengths of the peduncles- they are longest in H. nudiflorum and shortest in H. glutinosum, and they work well to distinguish the two when the fronts of the flowers aren't open or facing the right way. However, in such cases the dark spots are usually still visible through the banner in H. nudiflorum.

Hylodesmum nudiflorum @etantrah | Hyloesmum glutniosum @cecildomyiidae | Hylodesmum pauciflorum @zeebes

Fruits

Fruit segments are one of the characters used to distinguish Hylodesmum from Desmodium in most keys. The sinuses between the fruit segments in Desmodium are much shallower than in Hylodesmum. The overall shape of the segments also differ- Desmodium is more variable than*Hylodesmum*, but the lower edges never have the rounded scoop or swoosh shape that Hylodesmum segments have. With a sharp eye, Hylodesmum can be identified to species with just a fruit segment, but Desmodium fruits are generally only helpful for species identification when leaves and are also visible (and are sometimes necessary for keying).

Desmodium Fruits

Note how wide the connections are between the fruit segments, and how shallow the sinuses are between them.

Desmodium cuspidatum @chert_hollow | Desmodium marilandicum @jimbo225 | Desmodium paniculatum @jimbo225

Hylodesmum Fruits

Note how round the curve is in H. glutinosum segments compared with H. nudiflorum. Hylodesmum pauciflorum segments are intermediate between the two.

Hylodesmum nudiflorum @cjfarmer | Hyloesmum glutinosum @urban_botanist | Hylodesmum pauciflorum @abelkinser

Leaves

The two more commonly encountered species of Hylodesmum, H. nudiflorum and H. glutinosum both have leaves that appear to be whorled. This separates them easily from H. pauciflorum and all Desmodium species which are clearly alternate. H. pauciflorum can appear to be one of the upright Desmodium species early in the season (particularly D. glabellum and D. perplexum), however it grows in colonies and its stems tend to zig-zag. Vegetative identification to species is possible in both Hylodesmum and Desmodium; we'll cover vegetative Desmodium identification in another post!

Desmodium Leaves

Note how variable the leaflet shapes are. This is a small sampling of the variability among species in this genus.

Desmodium rotundifolium @ciafre | Desmodium perplexum @mertensia | Desmodium tenuifolium @whiteoak

Hylodesmum Leaves

Note that the terminal leaflets of H. glutinosum are very wide at the base and tapered at the tip, making them shaped like a Hershey's Kiss. Hylodesmum nudiflorum and H. pauciflorum terminal leaflets are more rhombic, though H. pauciflorum usually has a slight taper at the tip.

Hylodesmum nudiflorum @mollyhickey | Hylodesmum glutinosum @mabrybiggs | Hylodesmum pauciflorum @froggynomad

Literature Cited

Note: the treatment below also includes a key to all Hylodesmum worldwide- in case anyone is feeling adventurous.

Ohashi H., Mill R.R. 2000. Hylodesmum, a new name for Podocarpium (Leguminosae). Edinburgh Journal of Botany 57(2):171-188. https://journals.rbge.org.uk/ejb/article/download/1214/1105