Kerria japonica (L.) DC.
Synonyms: Rubus japonicus L.; Corchorus japonicus Thunb.
A deciduous shrub of bushy form 4 to 6 ft high, branches and twigs slender, supple, quite glabrous and glossy, forming a dense interlacing mass. Leaves alternate, 11⁄2 to 4 in. long, ovate-lanceolate, parallel-veined, the base rounded, the point long and tapering, glabrous above, hairy (especially on the veins) beneath, the margins doubly toothed. The leaves are much larger on the barren shoots of the year than on the flowering twigs. Flowers yellow, solitary at the end of short leafy twigs springing from the previous year’s shoots, 11⁄4 to 13⁄4 in. across. Petals normally five, obovate. Calyx green, 1⁄2 in. across, with five oblong lobes. Stamens numerous, yellow. Fruits not often produced in this country, but as seen on wild specimens, is a cluster of two or three nutlike bodies about the size of peppercorns, enveloped in the persistent calyx. Flowers in April and May.
cv. ‘Pleniflora’. – A double-flowered variety much commoner in gardens than the type, and remarkably distinct in growth, the branches being stouter, more erect, and the shrub of a gaunt and rather lanky habit, showing none of the dense twiggy character of the type. The flower is a rounded mass of bright yellow petals, 11⁄2 to 2 in. across. It is as hardy as the type, but is often given wall protection. In the vicarage garden at Bitton it was 12 ft high.
The kerria has long been cultivated in Japan, and its existence there was known as long ago as 1700, but the double-flowered form (the first introduced) did not reach England until 1804, when it was introduced to Kew by William Kerr, a plant collector sent out from that establishment the previous year to China. As the reproductive parts were wanting, its botanical affinities could only be surmised, and it was called “Corchorus japonicus”. Corchorus is a genus allied to the lindens. When the single-flowered typical plant was introduced in 1834 by Reeves, and blossomed two or three years later, it was seen to belong to the rose family, and was then named Kerria by De Candolle. This species is a native of China, and is only naturalised or cultivated in Japan. It was collected in flower and fruit by Wilson in W. Hupeh, China, in 1900, and earlier by Henry. Wilson also collected it in W. Szechwan, and Giraldi in N. Shensi.
The typical kerria is a beautiful shrub when in flower, and quite hardy, thriving in good loamy soil. It is easily increased by moderately soft cuttings placed in brisk bottom heat. All the kerrias are benefited by an occasional thinning out of old stems.
In addition to the well-known double-flowered form, the following is also cultivated:
cv. ‘Variegata’. – Like the type in habit, but scarcely so vigorous; its leaves are deeply and irregularly margined with white. It flowers more or less during the whole summer, but is scarcely so hardy as the type (K. japonica picta Sieb.). A cultivar with yellow-margined leaves is also known.