Allionia, commonly known as windmills or trailing four o'clock, is a genus of two species widespread in the western hemisphere. They are unusual in their blooms, which actually consist of three separate flowers appearing to be a single flower.
The plants are finely pubescent annuals or short-lived perennials, with trailing stems up to a metre in length, often threading through other vegetation. The leaves range from oval to oblong, under 4 cm long. The inflorescences are axillary, consisting of three red-purple flowers symmetrically arranged and superficially seeming to be a single flower 3–15 mm across. The individual flowers are bisexual, bilaterally symmetric, with a distinct oblique funnel. The 5–7 stamens may be somewhat exserted, along with the style. The fruits have five ribs.
The range of Allionia incarnata L. includes North America, the West Indies, Central America, and South America, while Allionia choisyi Standley is more restricted in North America, occurring in Arizona, Utah, New Mexico, and Texas.
The two species can be only be reliably distinguished by characteristics of their fruits, and even those may be found intergraded where the species' ranges overlap.
Linnaeus named the genus after Italian botanist Carlo Allioni (1725–1804).