Rhynchosia volubilis Seeds | Twining Rhynchosia
LATIN NAME: Rhynchosia volubilis
COMMON NAME: Twining Rhynchosia
Plant Profile
Twining rhynchosia is a slender, herbaceous to slightly woody perennial vine of the pea family, with fine, twining stems that climb two to three metres through whatever is available. The leaves are trifoliolate, with oval to rhombic leaflets, softly hairy beneath, and the whole plant has a light, airy look that belies how quickly it can cover a fence.
The flowers are small, yellow, pea-shaped and borne in loose racemes from summer into autumn. They are followed by short, flat pods containing two round seeds, usually red-brown mottled with black, and those seeds are striking enough that the plant is sometimes grown for them alone. The pods shatter when ripe, throwing the seed a short distance, and the seed coat is hard enough to survive a winter in the soil.
Its range runs through Japan, Korea, China and Taiwan, in thickets, forest edges, roadsides and abandoned ground. It is a plant of disturbed and transitional places rather than of closed forest, and it moves into a site quickly after clearing, then persists as a small population in the undergrowth.
The root is used in Chinese and Japanese folk medicine, where it is known as Lu Dou or by related names, and it is that use, more than the flower, that has kept the species in cultivation. In the garden it is a light, quick climber for a rough fence or a trellis, and it is useful for covering ground that nothing else will.
Science & Biotechnology
The genus Rhynchosia is large and taxonomically difficult, with centres of diversity in Africa, Asia and the Americas, and the Asian species have been revised several times. Rhynchosia volubilis is separated from its relatives by the shape of the leaflets, the hairiness of the calyx and the markings on the seed, and molecular work has confirmed its placement in a group of East Asian twining species.
Like most of its genus it is a nodulating legume, forming root nodules with a range of rhizobia, and it is one of the species screened in work on the diversity of native legume symbionts in East Asian soils. That matters for restoration: a native legume that nodulates with local strains can fix nitrogen without the need to introduce an inoculum, which is one reason native species are preferred over exotic green manures.
The seed biology is typical of the phaseoloid legumes. The seed coat is impermeable, and germination in the wild follows scarification by fire, by abrasion in the soil, or by passage through the gut of a bird. In cultivation, scarification or a hot-water soak is needed for reliable results, and untreated seed can germinate over several years.
The medicinal literature is mostly Chinese and Japanese, and it concerns the root rather than the seed. Extracts have shown anti-inflammatory and analgesic activity in animal models, and phytochemical work has identified flavonoids and triterpenes, but the studies are preliminary and the plant is not a standardised medicine in any pharmacopoeia.
Uses & Benefits
In the garden the plant is a quick, light climber for a rustic fence, an arch or a bank where the soil is poor. It flowers over a long season, it feeds bees and small butterflies, and it is not aggressive enough to become a weed in the way that some introduced vines do. It is also a useful plant for a wildlife garden, since its foliage is eaten by the larvae of several native blue butterflies.
The root is the medicinal part, used in decoctions for rheumatic pain, for injuries and for fever in regional practice. It is sold dried in Chinese herbal markets, usually as slices of the root, and it is regarded as a mild remedy rather than a powerful one. As with any wild-collected medicinal, the identity of the material is a real problem, and adulteration with related species is common.
The seed is not eaten, and the pods are too small and sparse to make the plant a useful pulse. The plant’s agricultural value is as a forage and green manure in poor soils, where its nitrogen fixation and its tolerance of drought make it useful in mixed pasture and in the understorey of orchards.
For a gardener interested in native plants, the species is worth growing as a representative of the East Asian legume flora, and as a reminder that not every useful plant needs to be a crop. It is also a good subject for a school or a botany class, since the nodules are easy to see on the roots of a seedling grown in a jar of soil.
Growing from Seed
Scarify the seed by nicking the coat with a file or by soaking in water at about eighty degrees for a few minutes, then leave it in warm water overnight. Sow two centimetres deep in a loose, well-drained mix, and keep at twenty to twenty-five degrees in bright light. Germination is usually one to three weeks once the coat is broken.
Grow the seedlings on in full sun, and give them a support to climb from an early stage, since the stems are weak and the growing tip is easily damaged. Plant out once the root has filled the pot, into ordinary, well-drained soil. The plant fixes its own nitrogen, so no feeding is needed, and it should not be given rich compost, which produces leaf at the expense of flower. Cut it back in late winter to keep it within bounds.
Our Take
This is a modest plant, and it is worth being honest about that: a light yellow-flowered vine with a hard seed and a small medicinal root. Its virtues are that it is quick, native, bee-friendly and easy from seed, and that it will grow on a poor bank where nothing else wants to. Plant it on a rough fence and let it be.
Related Reading
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