While walking across some North Dakota prairie during the early parts of the summer there always seems to be a variety of interesting plants to see. But twogrooved poisonvetch, also known as twogrooved milkvetch, or silver-leafed milkvetch produces an odor that gives itself away. It is a member of the pea family or Fabaceae and is known to botanists as Astragalus bisulcatus.
Twogrooved poisonvetch can be found across the state on upland prairie. It is rather bushy, growing to around 2 feet high and wide. The leaves are pinnately compound with 15-35 leaflets, each about a half an inch to an inch long. The rose-purple flowers are produced in a dense terminal cluster (raceme) during roughly May through June. Later on, when in fruit, the fruits have two grooves running down their length, thus the “two-grooved” name.
Twogrooved poisonvetch grows on soil high in selenium and is a selenium accumulator. That can lead to poisoning of grazing animals, particularly cattle. Although poisoning may occur, livestock usually avoid it. I am not aware of any problems associated with the plant here in the Dakotas.
But back to that smell. One is more likely to smell this plant before seeing it, particularly earlier in the growing season. The smell has been variously described as musty, pungent, or metallic. But the most accurate description I have heard is that similar to urine or a mouse nest.
Once you have smelled it, you are not likely to forget it. And that can lead to a little fun on subsequent field trips with an uninitiated companion. When you get a whiff of that urine or mouse nest smell just exclaim “There must be some twogrooved poisonvetch growing nearby.” Sure enough, a quick look around a twenty foot radius or so, and the plant is certain to be found nearby.
“How did you know that?”
It should be noted that a couple other plants have a similar smell, for example creamy poisonvetch (Astragalus racemosa), but that is of a more scattered distribution in the state and less likely to be obsserved.
So make a point of finding some prairie to visit over the course of the summer. There will always be something interesting to observe. And keep your nose on high alert for this smelly selenium accumulator.