Large, glabrous perennial herb ± 0.9–1.8 m., tall, with a stout, solidly pithy stem; stem and branches terete, finely striate.
Fruit very strongly dorsally compressed, almost flat, broadly obovate to broadly elliptic, ± 10–17 × 5.5–12 mm., primary ribs narrowly prominent; stylopodia depressed, crenate-margined, shortly conical centrally; styles slender, recurved, ± 2 mm.; vittae 3 in each dorsal vallecula, but sometimes reduced to 2 or even 1 in some, or interrupted; commissure 4-vittate.
Basal leaves mostly 4–6, broadly oblong in outline, ± 35–85 × 15–35 cm., tripinnatisect with mostly 6–8 pairs of pinnae, the ultimate segments mucronate, narrowly linear, ± 6–35 × 0.25–1 mm., channelled above, the margins revolute below (frequently to the midrib); sheaths oblong, ± 10–24 cm., narrowing upwards and ± auriculate at the apex; petioles ± 10–18 cm.; lower stem leaves similar but smaller; median stem leaves sessile on the sheaths, deltoid, about as broad as long; inflorescence leaves mostly reduced to narrowly oblong, amplexicaul sheaths which are ± acuminate — the lower firm and green, sometimes with a rudimentary lamina, the uppermost thinner and submembranous; all leaves glabrous.
Petals bright yellow, ovate, glabrous, 1–1.25 mm., with an acute, incurved apex.
Umbels numerous, the lower often solitary and unisexual, the upper alternate or opposite, mostly 3 on a common branch with a larger central hermaphrodite umbel and 2 lateral males; terminal umbels in groups of 3 to 5, the central hermaphrodite and the lateral 2–3(–4) ♂; buds arising at anthesis from pairs of concave bracts, the hermaphrodite arising centrally and the males clasped in the bracts. Peduncle of hermaphrodite umbels 0–5.5 cm., of males 2.5–11 cm.; no involucre to either; hermaphrodite umbels with ± 10–25 rays 1.5–5 cm. long; partial umbels ± 10–18-flowered, pedicels 5–12 mm.; involucel absent; ♂ umbels with ± 13–25 rays 1–2.5 cm. long; partial umbels ± 10–25-flowered, pedicels 3–6 mm.; involucel absent or of up to 3 very narrowly linear, tapering bracteoles to ± 3 mm. long.