It must be late summer, when you see Snow-on-the-Prairie! Driving along the roads to Hagerman National Wildlife Refuge, this plant with cool appearing green and white leaves actually does look like a light dusting of snow where it is growing en masse. This white flowering plant can blanket a prairie in no time at all, hence the name. Because livestock stay away from the poisonous sap that the plant emits, it doesn't take much for it to cover a field.
There are actually two plants, Euphorbia bicolor Engelm. and A. Gray, and Snow-on-the-Mountain, Euphorbia marginata Pursh;NPIN, the Lady Bird Johnson Wildflower Center Native Plant Database, notes that the two are often confused.
As members of the Spurge Family, both plants have a milky sap that is irritating to humans with sensitive skin, as well as to the eyes, and is toxic to cattle. A volunteer at the Refuge told me that beekeepers try to keep bees away from the plant, as it makes the honey "hot".
Poinsettias are members of the same family. Growing 1 - 4 feet tall, in poor soils, the plants multiply by throwing seed, described by Dorothy Thetford in Wildflowers-of-Texas. Thetford says, “This ballistic dispersal of seeds explains the scattered arrangement of plants on the prairie.”
Both plants are annuals in the spurge family. The actual flowers are tiny white blossoms, surrounded by the green and white bracts. The bract of bicolor (in photos) is narrower than that of marginata. According to Texas A & M Agrilife Extension Snow-on-the-Mountain grows mainly in Central Texas, as well as north to Montana and Minnesota and south to Mexico, and Snow-on-the-Prairie mainly in the eastern third of Texas. NPIN shows a range including Oklahoma, Louisiana and Arkansas. The bloom time is July – October. We'll take anything that even helps us think "cool" at this time of year!