Geranium arboreum is a large shrub that grows to heights of 20–40 decimeters. It has crowded internodes on its stems. The leaves are simple and alternate, with ovate to cordate blades measuring 4–7 cm long and 2.5–4.5 cm wide, featuring a green and moderately hirsute surface, which can be denser along the veins. The margins are sharply serrate with 8–14 teeth on each side, and the leaves typically have 5–8 veins. Petioles are 1–3 cm long, and the plant has subulate stipules measuring 12–14 mm long, which remain hirsute and persistent on the branches. Flowering occurs with usually 1–4 flowers in short, axillary cymes supported by 1.1–2 cm long pedicels. The flowers are bisexual and actinomorphic (radially symmetric), with a calyx of 5 lanceolate sepals, 12–16 mm long, and a corolla of 5 dull magenta petals that are narrowly obovate, with the upper three erect and the lower two reflexed. It has (5)10 stamens with pubescent filaments and dithecal anthers. The ovary is superior, 5-celled, with a stylar column that elongates in fruit to 35–40 mm long. Fruits are septicidal and elastically dehiscent capsules, with each carpel body about 3–3.5 mm long, and seeds that are 1 per cell; dark purple; narrowly obovoid; ca. 2.5 mm long; the surface conspicuously reticulate; reticulations elongate; endosperm usually scanty or absent; rarely copious and oily. It is found primarily in gulches within subalpine shrubland between 1,520-2,150 meters.