Euphorbia albomarginata (formerly Chamaesyce albomarginata), whitemargin sandmat or rattlesnake weed, is a small low-growing perennial, in the spurge family (Euphorbia, Euphorbiaceae) native to desert, chaparral, and grassland habitats of southwestern North America, from southern and central California to Northern Mexico and Louisiana.

It can be easily identified by its small size, dusty green leaves, very flattened growth pattern, and the white circular margin around the edge of its burgundy centered flowers. It is one of four members of the former Chamaesyce genus that are native to the Santa Monica Mountains, in addition to three species that have been introduced there, most of which share to some degree or other the white margin on the flower. As with other typical members of the Euphorbia family, it has a white milky sap, and is poisonous. It is one of only 11 members of the Euphorbia native to California, and one of four native to the Santa Monica Mountains.

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