Lagerstroemia L. (Lythraceae) in Indochina (Cambodia, Laos, and Vietnam) comprises 21 species. Six species, namely Lagerstroemia densiflora W.J.de Wilde & Duyfjes, sp. nov., L. gagnepainii Furtado & Srisuko, L. kratiensis W.J.de Wilde & Duyfjes, L. micrantha Merr., L. lecomtei Gagnep., L. petiolaris Pierre ex Laness., only occur in Indochine. Lagerstroemia densiflora W.J.de Wilde & Duyfjes, sp. nov., from Vietnam, is new to science; it differs from the most similar Lagerstroemia duperreana Pierre ex Gagnep. by a compact and densely flowered inflorescence, the calyx tube smooth and unridged, the pseudopedicel at anthesis 3-4 mm long, in fruit 4-5 mm long, and the glabrous capsules with a smooth surface. For each species the original reference, typification, spot- characters and distribution are presented. There are two keys to the species, one for flowering and one for fruiting material.
Lythraceae, Lagerstroemia, Indochina, Cambodia, Laos, Vietnam, lectotypification, new species.