Begonia nhatii V.T.Bui, K.S.Nguyen & C.W.Lin, sp. nov. § Platycentrum (Figs. 1 & 2)

Type:— VIETNAM. Lao Cai Province: Bao Yen District, Cam Con Commune, Bong 2 Village, remnants of primary evergreen broad-leaved forest on lowland silicate mountains, around point 22.231815° N, 104.262751° E, elev. 200–300 m a.s.l., 18 March 2025, Trieu Van Nhat, TVN-LC 008 (holotype HN!, isotype HN!).

Diagnosis: —The new species is most similar to Begonia gulinqingensis, but it can be readily distinguished by its leaf blades ovate to oblong-ovate (vs. suborbicular), sparsely scabrescent on the adaxial surface (vs. hirsute), outer tepal margins of staminate flowers often serrate occasionally entire (vs. always entire), pistillate pedicels shorter, measuring 2–4 mm long (vs. ca. 1 cm long), and the presence of one or two bracteoles at the top of the pedicel (vs. absent).

Description: —Perennial, creeping, monoecious rhizomatous herb. Rhizome olive green to pale brownish-red, up to 10 cm or longer, 3−5 mm thick, internodes 3−10 mm or longer, densely creamy white villous-hirsute; leafless erect stem seen only at anthesis, inconspicuous, 0.5–4 mm long. Stipules persistent, olive green, narrowly triangular or triangularovate, 5–13 × 3–5.5 mm, herbaceous, slightly keeled, reflexed, abaxially white villous, more densely along midrib, margin subentire to fimbriate, apex attenuate and aristate, arista ca. 1 mm long; stipules of erect stem ovate to narrowly ovate, 4–6 × 2–3 mm, abaxially sparsely puberulous-villous, margin ciliate, apex attenuate and apiculate. Leaves alternate, oblique, more-or-less horizontally; petiole terete, slightly grooved above, olive green to dark brownish-pink, 3−7.5 cm long, 3−5.5 mm diameter, densely creamy white to purplish-red villous-hirsute; leaf blade asymmetric, oblique, ovate to oblong-ovate, (6.5−)13−19 × (4–) 7–12.5 cm, broad side 3–7 cm wide, basal lobes cordate, 2.5–4 cm long, apex acuminate-acute, margin crenulate-denticulate with a line of red villous-hirsute; leaf chartaceous, succulent, adaxially emerald green to olive green, slightly corrugated, sparsely red scabrescent, subvelutinous, with blue to aquamarine iridescence; abaxially pale greyish-green; venation basally ca. 9 palmate, midrib distinct, 2 or 3 secondary veins on each side, other primary veins branching dichotomously or nearly so, tertiary veins reticulate; densely red villous-hirsute or pubescent on all veins abaxially. Inflorescences 1–3, borne on a leafless stem, arising directly from the uppermost 1 or 2 leaf axils, rarely from lower nodes; bisexual, cymose, 1 or 2 orders of branching, sometimes subumbellate; peduncle 4–5 cm long, brownish-red or dusky pink, erect, moderately densely white to pink villous; bracts creamy pink, hyaline, semi-persistent, narrowly ovate to oblong-ovate, abaxially very sparsely pubescent on midrib or glabrous, margin ciliate, apex acute, 3–7 × 2–4 mm. Staminate flower: pedicel pink, 10–17 mm long, red or pink villous-hirsute; tepals 4, creamy white, outer 2 orbicular to widely ovate, 13–16 × 12–16 mm, margin serrate, each tooth tipped with a puberulous hair, sometimes subentire, apex rounded, abaxially red puberulous and hirsute; inner 2 lanceolate-obovate, 10–14 × 4–6(–9) mm, glabrous, margin entire, apex rounded; androecium actinomorphic, ca. 5 mm diameter; stamens 75–90, yellow, filaments free, 1–1.5 mm long, anthers lanceolate-obovate, 0.8–1 mm long, 2-locular, apex obtuse. Pistillate flower: pedicel creamy pink, red or pink villous-hirsute; 2–4 mm long with 1 or 2 narrowly oblong-ovate bracteoles 3–3.5 × 1.5–1.8 mm, ciliate; tepals 5 (rarely 6), creamy white, outer 4 ovate to widely obovate, 6–12 × 5.5–9 mm, margin entire to sparsely serrate, sparsely pink or white puberulous or hirsute abaxially; inner 1(or 2) lanceolate-obovate, (4–)6–8 × (1–) 3–5 mm, margin entire, glabrous; ovary pale pink to creamy white, trigonous-ellipsoid, ca. 5 mm long, 2.5–3 mm thick (wings excluded), sparsely red puberulous or hirsute; 3- winged, wings subequal, 8–10 × 4–5 mm, pale creamy pink to creamy white, narrowly rounded-triangular, obtuse or rounded proximally, nearly truncate distally, margin entire, red hirsute; ovary 3-locular, placenta bilamellate; styles 3, 3–4 mm long, orange to yellow, free, stigmatic band U-shaped and spirally twisted. Capsule pendent on a stalk 4–5 mm long, capsule 8–13.5 × 8–12 mm (wings included), wings subequal, narrowly rounded-triangular.

Paratype:— Lao Cai Province: Bao Yen District, Cam Con Commune, Bong 2 Village, remnants of primary evergreen broad-leaved forest on lowland silicate mountains, around point 22.227556° N, 104.257639° E, elev. 300– 350 m a.s.l., 15 April 2025, Trieu Van Nhat, TVN-LC 011 (HN!).

Distribution and ecology:— Begonia nhatii is currently known only from two close localities on the same mountain, of Cam Con Commune, Bao Yen District, Lao Cai Province, Northwestern Vietnam. It grows on soil or silicate rocks nearby a seasonal stream in a steep ravine, at an elevation of 200–350 m a.s.l., under evergreen broad-leaved forest.

Phenology:— The flowering season of the new species lasts for over a month, beginning in mid-March. Fruit development is expected from November to early the following year.

Etymology:— The specific epithet is named in honor of Mr. Trieu Van Nhat, the discoverer and collector.

Conservation status:— As reported by Nhat, approximately 50 mature individuals inhabit an area of less than 2 km 2 along a ravine or a seasonal spring on a silicate mountain. Based on the current information, we tentatively assess its conservation status as Data Deficient (DD), in accordance with the Guidelines for Using the IUCN Red List Categories and Criteria version 16 (IUCN 2024).

To facilitate a more accurate conservation assessment, comprehensive field surveys should be conducted in lowland forested mountain regions surrounding Hoang Lien National Park, including Bao Yen and Van Ban districts (Lao Cai Province), Tran Yen and Nghia Lo districts (Yen Bai Province), and Than Uyen and Tam Duong districts (Lai Chau Province). These surveys are essential to obtain reliable data on population size, distribution range, and potential threats to the species.

Notes:— Begonia nhatii closely resembles B. gulinqingensis in its hairy rhizome and petioles, as well as its corrugated foliage with a greenish-blue iridescent upper surface. The two species are also similar in the number of tepals and in the trilocular structure of the ovary. However, B. nhatii differs from the latter in having an ovate to oblong-ovate (vs. suborbicular) leaf blade that is sparsely scabrescent (vs. hirsute) on the adaxial surface, outer tepal margins that are often serrate or occasionally entire (vs. always entire) in staminate flowers, shorter pistillate pedicels measuring 2–4 mm (vs. ca. 1 cm), and the presence of 1 or 2 bracteoles (vs. absent) towards the top of the pedicel.

The new species is also superficially similar to B. obsolescens Irmscher (1951: 37) in possessing ovate to oblong-ovate leaves, a sparsely short-hairy upper leaf surface, staminate flowers with four tepals, pistillate flowers with five tepals, and a 3-loculed ovary. Moreover, both species are distributed in north-western Vietnam. Nevertheless, B. nhatii can be readily distinguished by its persistent (vs. caducous) stipules, larger lamina 13−19 × 7–12.5 cm (vs. 1.8−8 × 1.5–6.2 cm), scabrescent (vs. puberulous) hairs on the upper leaf surface, and an actinomorphic (vs. zygomorphic) androecium.