Gladiolus crassifolius is usually a solitary, herbaceous, cormous perennial, 0.25–1.2m high, sometimes producing 2 or 3, rarely 5 stems per corm. The corm is 18–30 mm in diameter, tunics coriaceous (resembling leather) to more or less coarsely fibrous. Cataphylls (scale-like leaves at the base of the stem) are coriaceous and red to brown. The stem is erect below, usually flexed outward above the sheath of the uppermost leaf, simple or with 1 to 3, rarely up to 5 branches, 2 mm in diameter below the spike. Leaves are very few, 4 to 8, emergent at flowering time, sometimes reaching the middle of the flower spike and even shortly exceeding it, narrowly lanceolate to linear, 5–12 mm wide, the margins and midrib thickened.

The spike is more or less erect or weakly inclined, bearing 12–22 flowers, the branches with fewer flowers than the main axis. Bracts usually pale green or flushed purple, soft textured initially, becoming dry and membranous above in fruit, acute and attenuate. Flowers are pale to deep pink or light purple, occasionally orange-red or cream-coloured, the lower lateral and sometimes the lowermost tepals each with a dark band of colours across the lower half of the limbs, the dark band often edged with cream, unscented. Perianth tube indirectly funnel-shaped, cylindrical below, widening and curving outward near the apex, 9–17 mm long. Filaments are 12–14 mm long, extending 6–7 mm from the tube; anthers 7–8 mm long, pale lilac to purple, the pollen whitish to pale yellow. Ovary obovoid, 3–4 mm long; style arching over the filaments, usually dividing opposite the lower half of the anthers. Capsules obovoid, 9–14 mm long, 3-lobed above; the retuse seeds elliptic, broadly winged, 4–6 × 2.5–4.0 mm, the wing usually more or less transparent.

The flowering time of Gladiolus crassifolius is mainly late summer (February and March), occasionally later or earlier at higher elevations. In tropical Africa usually in April to June, but sometimes August to November, especially after fires.