Boehmeria grandis is a shrub, typically 1–2 meters tall, characterized by its watery sap. It has stout, angular branches covered in strigose hairs. The leaves are simple and opposite, with elliptic to ovate or sometimes broadly ovate blades, measuring 12–25 cm in length and 9–13 cm in width. These leaves have an acuminate apex and a truncate to subcordate base. The upper leaf surfaces are sparsely strigose between the veins, while the lower surfaces are strigillose to short–hirtellous along the veins. The leaves are thick, somewhat coriaceous, and rugose, with punctiform cystoliths, and have serrate margins. They exhibit a palmately 3-veined structure, becoming pinnately veined towards the apex, and are attached to 2–7 cm long petioles. It has interpetiolar stipules that are 5–6 cm long and strigose. The flowers are arranged in open, branched panicles 8–15 cm long, with clusters about 2–4 mm in diameter, consisting of unisexual (monoecious) and actinomorphic (radially symmetric) flowers. The calyx of staminate flowers is (3)4(5)-lobed, with lobes about 2 mm long, while the pistillate calyx is tubular, about 1–1.5 mm long, puberulent to short–hirtellous, and 2–4-toothed at the apex. The plant does not have a corolla. Staminate flowers contain (3)4(5) stamens with puberulent filaments that reflex elastically when shedding pollen. Pistillate flowers have a superior pseudomonomerous ovary and are 1-celled with a single basal ovule. The fruit is an achene, somewhat compressed, about 1–1.5 mm long, and adheres to the calyx, containing one seed. It is found along streambeds, on slopes, ridges, or valley floors, and in mesic valleys as well as mesic to wet forests and diverse mesic forests from 150-1,000 meters.