Dioscorea NIPPONICA

Dioscorea NIPPONICA - Nipponese yam, Japanese yam, Dioscorea nipponica.

Dioscorea is a large genus in the family Dioscoreaceae, comprising several hundred species of climbing plants. Most of them are tropical, but several species extend far north into the temperate zone of East Asia and North America.

Plants are perennial, dioecious, with climbing herbaceous stems that sprout each season from a perennial rhizome or tuber. The leaves are alternate, petiolate, with characteristic arcuate venation.

Flowers are small, inconspicuous, pale green, unisexual: staminate flowers are gathered in axillary racemes of cymules, while pistillate flowers are in simple racemes. The fruit is a three-valved capsule with winged seeds.

Dioscorea nipponica is an herbaceous vine up to 4 m (13 ft) long. The rhizome is horizontal, woody, up to 2 m (6.6 ft) long and 3-4 cm (1.2-1.6 in) across, with stiff roots. Stems are glabrous, twining, dying back for winter.

Leaves are broadly ovate, 3-7-lobed, on long petioles. Flowering occurs in the second half of summer, and fruits ripen in late summer and early autumn. In nature, the species grows in open deciduous forests, forest edges, and shrub thickets of the Primorsky Krai, southern Khabarovsk Krai, and southeastern Amur Oblast.

Dioscorea seeds are flat, with a wide membranous wing; germination rate is low and declines quickly, so they are sown as promptly as possible. Prior to sowing, they are soaked for 12-24 hours in a weak solution of potassium permanganate or in a growth stimulator solution: SUPERthrive, HB-101, Aminozol, Epin, or Zircon. Stratification is mandatory: swollen seeds are held in moist sand at 3-5 °C (37-41 °F) for 2-3 months.

Sowing is carried out in spring, in a loose moisture-retentive substrate of peat and sand, to a depth of about 1 cm (0.4 in). The sowing is moistened with a spray bottle, covered with a transparent lid or placed in a greenhouse, and germinated in the light at a temperature of 18-22 °C (64-72 °F).

Seedlings appear after 3-6 weeks, unevenly. Trays are ventilated daily, and condensation is removed. Seedlings develop slowly during their first year, mainly building up a rhizome, so they are kept in pots and planted into their permanent location in the second year.

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A bright location or light partial shade suits the plant; in nature, it grows under the canopy of sparse forest and dislikes direct midday sun.

The species is winter-hardy: the rhizome endures temperate zone frosts under snow cover, while the aerial growth dies back for the winter. In the first year after planting, mulching young plants is beneficial.

Watering is regular: Dioscorea is moisture-loving, but does not tolerate stagnant water.

The substrate should be loose, moisture-retentive, rich in organic matter, slightly acidic or neutral. In container cultivation, drainage is placed at the bottom of the pot, and a deep container is chosen to accommodate the horizontal rhizome.

Feeding with organic and complete fertilizer is applied twice per season: in spring at the start of growth and in mid-summer.

Support is mandatory and installed immediately upon planting: stems are twining and will lodge without support. In autumn, dead shoots are cut back at the base.

Special notes: All parts of the plant contain saponins and are not edible. Only the rhizome serves as pharmaceutical raw material, harvested according to rules established for medicinal plants. In nature, the species is limited in numbers and is protected in several regions.