Hydrostachys polymorpha is a perennial herb that is securely attached to the rocks in waterfalls or rapids by numerous stiff, thread-like roots radiating from a round, discoid, compacted, woody stem. One to numerous fern-like, feathery leaves arise from this stem. The compound leaves are elliptical to lanceolate in outline, trailing or submerged in the water, and occasionally noticeable above the water surface.

The leaves are olive-green, about 15-75 cm long, subdivided into 2 or 3 pinnae, which are difficult to see because the small leaflets are curled up, giving the leaf a rough, warty appearance. The petiole and axis of the compound leaf are flat, covered with numerous wart-like or tongue-shaped outgrowths.

The male and female flowers are borne on separate plants, in fleshy, erect, stiff spikes that stand out above the water surface. The spikes are up to 30 cm long, with about 200 flowers borne in the axil of straw-coloured, fleshy bracts partially hiding the reduced flowers. There are no petals or sepals; the male flowers consist of a single stamen, while the female flower is a single ovary with two deep rose-pink styles extending beyond the bracts.

Very little is known about the biology of this plant. Thanks to Mr Mike Coke, formerly of the Natal Parks Board, several new localities were found in KwaZulu-Natal. From his observations and from information on herbarium sheets, it seems to flower mainly in May, when the rivers are receding. By the middle of August through to September, empty, stiff, dried, golden-yellow/greenish spikes can be seen protruding above the surface of the water. with numerous small fruits enclosed in the bracts. Cook (2004) assumes that the plant is wind-pollinated and that wind and water disperse the seeds, but this needs to be verified.