Euphorbia marginata

Euphorbia marginata is a small annual in the spurge family.
It is native to parts of temperate North America, from Eastern Canada to the Southwestern United States. It is naturalized throughout much of China.
The type specimen was collected in Rosebud County, Montana from the area of the Yellowstone River by William Clark during the Lewis and Clark Expedition.

Description

Snow-on-the-mountain has grey-green leaves along branches and smaller leaves in terminal whorls with edges trimmed with wide white bands, creating, together with the white flowers, the appearance that gives the plant its common names.
Snow-on-the-mountain has also been found to emit large quantities of sulfur gas, mainly in the form of dimethyl sulfide.