Poinsettias were known at least as far back as Aztec times, when in the 14th to 16th centuries they used them for practical purposes. The sap was used for controlling fevers and the bright red, flower-like bracts for dye. Franciscan priests began using the "flowers" ornamentally in the 17th century as part of their nativity processions.
Joel Roberts Poinsett, a U.S. ambassador to Mexico, is credited with first introducing poinsettias to the United States in the 1820s, after spotting one growing along the Mexican roadside. By the early 1900s they were being grown in southern California as landscape plants and for cut flowers.
The native poinsettia is straight and tree like, but grower observation, careful seed selection and a "helpful" disease that causes increased branching have led to the lush, full Poinsettias we now know as greenhouse grown, holiday houseplants.