The world's most famous holiday plant is, in its homeland, a leggy tropical shrub. Native from Mexico to Guatemala, Euphorbia pulcherrima is grown almost everywhere as a compact potted gift for its blaze of winter color — but that color is not flowers. The showy red, pink, white, or marbled 'petals' are bracts (modified leaves); the true flowers are the small yellow-green cup-like cyathia clustered at the center. The bracts color up only in response to long, uninterrupted nights, which is why poinsettias turn for the winter holidays and why a houseplant in a lamp-lit room often refuses to re-color. It is frost-tender and hardy in the ground only in USDA zones 9a-11b, where it grows into an open, erect, multi-stemmed shrub 3-12 feet tall. A persistent caution: it is MILDLY toxic — the milky white latex (sap) can irritate skin and eyes and cause mild stomach upset if eaten — but its deadly reputation is a long-debunked myth, not a real hazard.