Erect, much-branched, balm-scented, viscid shrub. Lamina pinnatisect to palmatisect with pinnatisect segments, segments narrow and adaxially grooved, apices acute, apices of teeth acute, sparsely strigose. Peduncles distinctly articulated, pseudo-umbels with 3-9 flowers each. Flowers relatively small, pinkish-purple, pedicel shorter than hypanthium. Inflorescence and floral characters indicate a very close relationship between P. denticulatum and P. glutinosum (Jacq.) L'Herit. The leaves of these two species are both viscid and they have the same balm scent. However, the leaves of P. glutinosum are less divided (pinnatisect) and the leaf segments consequently much wider than those of P. denticulatum. The leaves of P. denticulatum could also be confused with those of P. radens H.E. Moore. The differences between the leaves of these two species are tabulated under P. radens.
Erect, much-branched, balm-scented, viscid shrub, up to 2 m high and 1 m in diameter. Stems herbaceous when young but soon becoming woody, villous to hirsute and with numerous glandular hairs interspersed, dark green and sometimes flushed with purple, becoming brown with age. Leaves sparsely strigose but densely beset with glandular hairs, green; lamina pinnatisect to palmatisect with pinnatisect segments, base cordate, (40-)60-80(-100) x (45-)70-90(-110) mm, segments narrow and adaxially grooved, apices acute, margins irregularly denticulate, apices of teeth acute; petiole (25-)50-60(-90) mm long; stipules asymmetric-triangular, ±6 x 2-5 mm. Inflorescence: flowering branches with normal and smaller foliar leaves; peduncles 20-60 mm long, distinctly articulated at distal and proximal ends in infructescences, sparsely villous to villous and with numerous glandular hairs interspersed; involucral bracts lanceolate to ovate, acuminate, indumentum as on peduncles, 4-7 x 2-3 mm; pseudo-umbels with 3-7(-9) flowers each. Pedicel 1-2 mm long. Hypanthium 4-9 mm long, indumentum as on peduncles. Sepals lanceolate, acuminate, indumentum abaxially as on peduncles, green with a reddish-brown tint, 8-10 x 2-4 mm. Petals pinkish-purple; posterior two spathulate to obovate, apices obtuse to emarginate, with dark red to purple markings, reflexed at ±90°, ±20 x 6 mm; anterior three spathulate with narrow claws, reflexed at less than 90°, ±14 x 4 mm. P. denticulatum flowers between April and November with a peak in September, and occasionally as late as the following January.
As in the case of so many other representatives of the section Pelargonium, this species is confined to mountains or their immediate vicinity, where the microclimate is considerably moister than in the neighbouring lower-lying areas. It is usually found in ravines and near streams. Its distribution area receives most of the rainfall during the winter months. Exact rainfall and temperature records for these mountainous areas are not yet available.