Snow-on-the-Mountain, Mountain Snow Spurge - Euphorbia marginata
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Euphorbia marginata - Snow-on-the-Mountain, Mountain Snow Spurge.
Euphorbia is a large genus with around 2000 species worldwide - one of the 3 largest in the world, and nearly 140 in North America. This genus is characterized by its unique pseudoanthium - a floral structure which appears to be a single flower, but is actually an inflorescence of multiple flowers, many of which may be of reduced size. Members of the Aster family (Asteraceae) are the most well known of those with pseudoanthiums (also known as flower heads.) In Euphorbia the pseudoanthium is a cyathium - a cup shaped involucre which contains several to many small male flowers and a single female flower. Several Euphorbia species are also known for their colorful bracts, including the well-known Poinsettia (Euphorbia pulcherrima), and the species presented here, Euphorbia marginata.
Although Euphorbia marginata is native to the Great Plains, it is cultivated for its ornamental foliage - the white and green variegated floral bracts. It has escaped from gardens and has naturalized in the United States far outside its native range, although in some areas this annual may survive only as a waif. It flowers from summer into the fall. It has a reputation as being weedy in pastures, largely due to the fact that the milky sap makes the plant bitter, so cattle do not graze it. Since plants around it are grazed off, Euphorbia marginata has little competition. That bitter sap is also poisonous, and can cause notable allergic reaction similar to poison ivy, especially in folks who are alergic to latex.
Found in:
AL, AR, AZ, CA, CO, CT, DC, DE, FL, GA, IA, IL, IN, KS, KY, LA, MA, MD, MI, MN, MO, MS, MT, NC, ND, NE, NH, NJ, NM, NY, OH, OK, PA, RI, SC, SD, TN, TX, UT, VA, WI, WV, WY
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Distribution of Euphorbia marginata in the United States and Canada:
Map courtesy of The Biota of North America Program.
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| | Site: Kansas Roadside Date: 2016-August-24 | Photographer: Gerald C. Williamson
Nikon D7000
Tamron SP 90MM f/2.8 AF Macro | | The inflorescence of Euphorbia marginata are the flower cyathia (described later) subtended by opposite or whorled leaf-like bracts that have wide white margins; occasionally entirely white. The upper leaves may also have margins with a wide white streak. | | 
| | Site: Kansas Roadside Date: 2016-August-24 | Photographer: Gerald C Williamson
Nikon D7000 | | I had hoped to have a detail description of the parts of the cyathia, but I have not found a good description with photos showing each part. I am somewhat confused. Each cyathium is surrounded by 3 to 5 (usually 4, as in these photos) glands - the green, kidney-shaped features. These glands have white. rounded appendages that appear like petals. The cyathium will contain 30 to 70 male (staminate) flowers. These clearly must be the whitish, thread-like features within each cyathium. The cyathium also has a single female (pistillate) flower. The pistil of Euphorbia has three styles, each of which may be connate or 2-fid. I am unable to identify the two yellow balls held at the end of each of 5 white structures - if they are anthers, there would be many more. If they are the stigmata of the female flower, there should be 3 pairs rather than 5. If you can explain it, please leave a comment. | | Click on the photo for a larger image
| | Site: Kansas Roadside Date: 2016-August-24 | Photographer: Gerald C Williamson
Nikon D7000 | | The stems terminate with clusters of cyathia - which contain many male flowers and a single female flower. In this photo you can see the fruit developing. The capsule has three chambers, each containing a single seed. Birds will sometimes eat the seeds. | | Click on the photo for a larger image
| | Site: Kansas Roadside Date: 2016-August-24 | Photographer: Gerald C Williamson
Nikon D7000 | | Snow-on-the-Mountain is an erect plant that only occasionally branches below the inflorescence. It prefers full sun and well-drained soil, and can grow to about three feet tall; sometimes as high as 5 feet. | | Click on the photo for a larger image
| | Site: Kansas Roadside Date: 2016-August-24 | Photographer: Gerald C Williamson
Nikon D7000 | | The leaves of Snow-on-the-Mountain are alternate along the terete (rounded) stem, which is usually hairy with spreading hairs, becoming more glabrous with age. The leaves are entire, sometimes with white margins, and are glabrous on both surfaces. They are ovate to oblong (or similarly shaped) and up to about 3.5 inches long, with the tips acute or mucronate (sharp point on the end.) | | Click on the photo for a larger image
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References used for identification and information:
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