Joinvillea ascendens subsp. ascendens is a rare species of erect herb, typically growing to a height of 1.5–5 meters. It is equipped with a fibrous root system. Its leaves are simple and alternate, featuring narrowly elliptic blades that measure 45–80 cm in length and 4.5–16 cm in width. The leaves are plicate with 7–13 pleats and have auricles extending to about 10–40 mm long. The base of the leaf is cuneate, with surfaces having scattered bristles and the lower surfaces varying from sparsely to moderately pubescent. The margins of the leaves are entire, and the veins are parallel. The leaves are sheathing and do not have stipules. The flowers are relatively small and inconspicuous, forming in a pyramidal, 3–5–branched panicle 10–40 cm long. The flowers are bisexual, actinomorphic (radially symmetric), and have tepals as long as or twice as long as they are wide, often splitting with age. The outer tepals are ovate to orbicular-ovate, measuring 1.7–4 mm long and 1.4–2.2 mm wide, with a central vein and occasionally 1–2 inconspicuous lateral veins. The inner tepals are orbicular-ovate, 1.8–3.5 mm long and 1.6–2.6 mm wide. The stamens are distinct with dithecal anthers that open by longitudinal slits. The superior ovary is 3-carpellate with axile placentation and one pendulous, orthotropous ovule per cell. The styles are terminal and either distinct or connate at the base with plumose stigmas. The fruit is 3.3–5.8 mm in diameter, with styles that are not persistent on the fruit. It is typically found as widely separated individuals or small populations on ridges, in wet forests, and along intermittent streams between 300-1,250 meters.