Vascular Plants of North Carolina
Account for Stiff Tick-trefoil - Desmodium obtusum   (Muhlenberg ex Willdenow) de Candolle
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Section 6 » Order Fabales » Family Fabaceae
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Author(Muhlenberg ex Willdenow) de Candolle
DistributionPresent over most of the state, perhaps being absent in the northern Mountains and close to the immediate eastern coastal areas. Not well collected, perhaps due to similarity to a few other species and perceived to be "difficult" to ID.

This is a fairly wide-ranging Eastern species, ranging north to southern New England and eastern KS, and south to northern FL and central TX.
AbundanceUncommon to infrequent in the eastern and central Piedmont and Sandhills, but in some places can be fairly common in the Sandhills. Rare to uncommon, and certainly overlooked, in the western Piedmont, southern half of the Mountains, and most of the Coastal Plain. This is an easily overlooked species, as most biologists do not bother collecting or keying out Desmodium and Lespedeza species; it is certainly more widespread in counties than shown on the range map below.
HabitatThe habitats are essentially the same as for most others in the genus -- dry to occasionally mesic woodland borders and openings, old fields, powerline clearings, and some types of sandy soil habitats. It is most frequent in sandy soil of open pine/scrub oak sandhills and flatwoods habitats.
PhenologyBlooms from June to September, and fruits from August to October.
IdentificationThis is a fairly slender Desmodium, ranging to 2-3 feet tall, but with few branches, mostly strongly ascending. The stem is quite hairy. The leaflets (3) are somewhat in-between the size of the very small-leaved ones (D. ciliare and D. marilandicum) and the bulk of the others, but it has several fairly distinctive characters. The leaf petiole is quite short, barely 1 inch long, shorter than the length of the lateral leaflets. Also, the terminal leaflet on this species is much larger and longer than the lateral ones (noticeable in the field); the terminal one is narrowly ovate to nearly oblong and about 2 inches long and half as wide, whereas the lateral ones are elliptical but barely 1-1.5 inches long. Also, the leaflets are fairly thick and rigid (and in fact the species was formerly named as D. rigidum). The leaflets are fairly hairy above and below. The inflorescence is typically a terminal panicle with small pink flowers, each about 1/5-inch long. This species has among the smallest pods and segments of any in the genus, with only 1-3 segments and each one only about 1/5-inch long. As mentioned above, this is almost an intermediate species between the very narrow and small leaved ones (D. ciliare and D. marilandicum) and the others; what stands out is the much larger terminal leaflet as compared with the "runts" on the side, and the general thickness and stiffness of the leaflets. If people would stop to look and key out the Desmodium plants they encounter, they would spot this species every now and then, and in the Sandhills would spot it somewhat frequently.
Taxonomic CommentsMany references formerly named this species as D. rigidum, though RAB (1968) did name it as D. obtusum, as do nearly all modern references.

Other Common Name(s)Obtuse-leaved Tick-trefoil
State RankS3? [S4]
Global RankG4G5
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B.A. SorrieRoadside, Thunder Road, Sept 2009. MoorePhoto_natural
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