Kadua laxiflora is a subshrub characterized by erect or sometimes ascending stems that are strongly quadrangular or winged, measuring approximately 30–60 cm long and are glabrous. The leaves are simple and opposite, with elliptic to lanceolate blades that range from 6 to 22 cm in length and 1 to 7 cm in width. The leaves are often asymmetrical, with an acuminate to long-acuminate apex and an attenuate base. The surfaces are glabrous, typically paler on the lower surface, and the margins are entire. The lateral veins are few, widely spaced, and slightly raised on the lower surface with obscurely reticulate higher-order venation. Petioles are slightly winged and up to 1.5 cm long, with bases that connate into a sheath. Stipules are deltate, measuring 5–14 mm in length, adnate to the petiole sheath, and terminate in a slender mucro that is 2–11 mm long. The flowers occur in terminal, compound monochasial cymes that form a lax, paniculate inflorescence up to 30 cm long. The branches are lax or pendent, with the ultimate branches bearing small, scale-like bracts. The pedicels are quadrangular and approximately 3–4 mm long. The hypanthium is turbinate, measuring 1-1.5 mm long. These flowers are bisexual (perfect) and sometimes functionally pistillate, pollinated by insects. The calyx has 4-5 lobes that are narrowly ovate to lanceolate, 1.5–5 mm long, and widen somewhat in fruit. The corolla is greenish-white, fleshy, salverform, and glaucous with a 5–14 mm long tube. The lobes of the corolla are about 1–4 mm long at anthesis, flat, and strongly inflexed in the bud. A nectary disc is present. The fruit is a turbinate to subglobose capsule, 2–3 mm long and 3–4 mm in diameter, that dehisces loculicidally across the disk and slightly further septicidally. The seeds are dark brown, more or less cuneiform, irregularly bluntly angled, and minutely dark granular. They contain a well-developed oily endosperm, though the endosperm is occasionally scanty or absent. It typically grows scattered in mesic forests and potentially in wet forests.