Achyranthes bidentata Seeds | Ox Knee, Niu Xi
LATIN NAME: Achyranthes bidentata
COMMON NAME: Ox Knee, Niu Xi
Plant Profile
The greenish or pinkish flower spikes carry separate tiny flowers packed along a slender axis, each one reduced to a set of chaffy bracts and a few stamens or a single ovary. There is no scent and no nectar, because there is nothing to advertise. The pollen is dry and light, and the stigmas are broad and receptive, a combination built for a plant that lets the breeze do the work.
In the wild ox knee keeps to damp, half-shaded ground, and it will not thank you for a baking bed in full sun with dry soil. It dies back to a thick root in cold winters and pushes up fresh growth in spring, forming a clump that widens slowly rather than running.
Because the species tolerates shade and damp, it suits the awkward strip under trees where lawn thins out. The leafy growth can be cut in midsummer for fodder or compost, and the plant returns from the crown without complaint.
Space the plants about 30 cm apart and let them knit into a clump. A mulch of leaf mould each autumn feeds the root and keeps the ground cool. Lift roots for drying at the end of the second season, or later for a heavier crop, and slice them before drying so the interior dries evenly.
Young shoots are soft enough to attract slugs, and the one real hazard is a wet, airless bed, where the crown and root can rot away over winter. Keep the drainage open, avoid replanting ox knee on the same ground year after year, and the crop looks after itself.
Science & Biotechnology
The root runs two chemical lines at once. The first is a set of phytoecdysteroids, among them 20-hydroxyecdysone, inokosterone and cyasterone. The second is a group of triterpenoid saponins built on achyranthoside-type frameworks, accompanied in the same tissue by a family of polysaccharides.
20-hydroxyecdysone is the hormone that triggers moulting in insects, so finding it made in quantity by a plant is a genuine curiosity. Plants appear to use phytoecdysteroids as a defence: an insect that eats the tissue takes in a dose of its own moulting signal and develops at the wrong time. That line of work has since widened into a whole field of plant-insect chemical ecology.
Because root harvest is slow, cells of Achyranthes have been grown in suspension culture to study how the polysaccharides and saponins accumulate. Much of the pharmacology follows the old clinical use: fractions of the root are studied for their effect on bone-forming cells, a research direction that runs alongside the traditional reputation of the root for joints and bones.
Ox knee is grown mostly for its root, but the young shoots are eaten in parts of its range while they are still tender.
The plant belongs to a famous quartet. Around the old prefecture of Huaiqing in northern Henan, growers supplied four recognised roots and barks, and the four Huai medicines were ox knee, rehmannia, Chinese yam and chrysanthemum. Ox knee grown there was considered the benchmark, and the name the Henan root is still used in the trade to mark that origin.
Uses & Benefits
Two different plants are sold as ox knee. The Huai root is Achyranthes bidentata itself, slender and slightly curved, while the Chuan root comes from Cyathula officinalis, a relative in the same family with a coarser, branched root. They are told apart by shape and internal colour, and any grower selling into the trade should expect the buyer to ask which one is on offer.
Growing from Seed
Once a flower is pollinated, the bracts around it harden and their tips sharpen. The whole structure dries into a small spiny utricle holding one seed, and the spines catch in fur, feathers and trouser cuffs. The opposite leaves are oval and pointed, and the stems are markedly four-angled.
Sow onto a bed of loam and leaf mould that holds moisture without standing wet, in dappled shade or a spot with morning sun only. Scatter thinly and cover the seed very lightly, barely more than its own depth, then keep the surface damp with a gentle spray rather than a heavy hose.
Our Take
If the amaranth family interests you, the vegetable types are worth a row of their own: amaranth, Amaranthus tricolor gives you the same wind-pollinated flowers with a crop of leaves attached, and Celosia argentea shows how far the family will go in the other direction with colour. Growers working the Chinese medicinal roots often pair ox knee with milk vetch seed in the same rotation.
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