Woody vines, sometimes shrubby; stipules borne adaxially on petiole, from somewhat above base to near apex, usually long and subulate; leaves mostly opposite, sometimes ternate, usually biglandular distally on petiole or against base of midrib and often glandular on margin of lamina as well, the tertiary veins usually strongly parallel (scalariform). Inflorescences axillary, usually 1–several umbels of 4–many flowers, the umbels when 4-flowered often borne in a cyme; bracteoles eglandular; pedicels sessile. Sepals all eglandular or the lateral 4 bearing (6) 8 abaxial glands; corolla bilaterally symmetrical; petals mostly yellow or yellow turning orange or red, glabrous; anthers glabrous, with the connective abaxially broad and swollen; ovary with the carpels nearly distinct; styles inserted low on ventral face of carpels, the apex with a large internal stigma and dorsally rounded to prominently hooked. Samaras separating from a very low pyramidal torus; samara mostly butterfly-shaped with the lateral wings membranous, cleft to nut at apex and base, borne on upper edge of nut (lateral wings coriaceous and ± reduced in a few species, turned into many short winglets in 1 species); dorsal wing small, sometimes reduced to a crest or lost; intermediate winglets or slender projections rarely present. Chromosome number unknown.
At least 55 species, in diverse habitats but avoiding very dry vegetation types, from western Mexico to Paraguay and adjacent Argentina and southeastern Brazil; absent from the West Indies, except for the Lesser Antilles (Grenada, St. Lucia). [map] — Regional key to genera: Caribbean, Central America.
Hiraea is usually immediately recognized by the elongated stipules projecting from the adaxial edge of the distal half of the petiole. When the stipules are short or located in the proximal half, the genus is still distinguishable by the axillary umbellate inflorescences, the petals bright yellow or yellow turning red, and the butterfly-shaped samaras. It is common in this genus for some populations to have the sepals all eglandular and other populations of the same species to have the lateral four sepals biglandular. Most species fall into two distinct groups. In one the umbels comprise five to many flowers and there is only one unbranched umbel in each axil. In the other group each umbel contains only four flowers, but the inflorescence may be a short cyme of umbels or there may be several separate stalked umbels in the same axil; some species with four-flowered umbels have only one stalked umbel in each axil.
Drawings: view all; H. affinis,H. amazonica, H. andersonii, H. angustipetala, H. apaporiensis, H. barclayana, H. brachyptera, H. breteleri, H. brevistipulata, H. bullata, H. celiana, H. cephalotes, H. colombiana, H. cuiabensis, H. ecuadorensis, H. escobariae, H. fagifolia, H. faginea, H. fimbriata, H. grandifolia, H. holmgreniorum, H. kariniana, H. klugii, H. longipilifera, H. mortoniana, H. peruviana, H. quapara, H. reclinata, H. sanctae-marthae, H. singularis, H. silvae, H. smilacina, H. ternifolia, H. transiens, H. woytkowskii