- Botanical name: Rehmannia glutinosa
- Common name: Prepared rehmannia root
- Literal name translation: cooked earth yellow
- Family: Scrophulariaceae, figwort family
- Part used in Chinese medicine: prepared root
- Major Chinese medicine actions:
- Tonifies blood, nourishes Liver and Kidney yin, tonifies essence
Photo 1: Rehmannia glutinosa; 05/2006; author Shizhao; permission under GFDL
Growing and Propagation
The plant is hardy to USDA zone 9, about -13 degrees F if the plants stay dry. The leaves are covered with soft hairs that make the plant susceptible to rot in warm damp winters, so in these climates, they are often grown in a greenhouse. It grows 6-12 inches high, preferring a warm location in sun to partial shade and soil that is loamy and moist but well drained. Flowers bloom from April to June, seeds ripen from May to July, and the roots should be harvested in November.
When propagating by seed, it can be sown in autumn or spring in a greenhouse. When seedlings are large enough to handle they are potted and grown for at least their first winter in a greenhouse. They can then be planted in the ground in late spring or early summer. Suckers and flower buds should be removed, leaving only one main stalk to grow.
Harvesting and Preparation
Rehmannia may be used fresh (Xian Sheng Di), dried (Sheng Di), or prepared (Shu Di). The processing of dried Sheng Di in the preparation of Shu Di requires special care and equipment. The dried root is combined with rice alcohol in a 7:3 proportion by weight. The root and alcohol are placed in a large double boiler and heated for eight hours, then allowed to cool overnight undisturbed. The next day it is cooked again in the same manner for another eight hours until the root is uniformly black inside and out, and again left to cool overnight. The roots are then taken out and dried while turning frequently until the material is 80% dry, then cut into pieces and dried completely.
For medicinal use, good quality Shu Di is thick, soft, and sweet. The preparation of the herb should leave it looking completely black in color.
Though they are not related, Rehmannia is sometimes called Chinese foxglove because of the similarity of the plant and its flowers to European common foxglove (Digitalis purpurea). In comparison, Rehmannia glutinosa is a somewhat smaller plant, with hairy leaves, and fewer flowers. It is native to northern China and Korea, growing on hillsides and waste areas, along highways and cracks in city walls.
Though they have been prolific in the wild, in recent times, most all Rehmannia is harvested from cultivated plants. The rootstock of vigorous plants found in the wild is dug up, cut into 1-2 inch lengths, and replanted in prepared beds. In mid-autumn, the roots are dug and buried in sand until the next spring. In April or May the roots are dug once again, cut into about 2 inch pieces and planted in permanent beds.