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Last year, while searching the southern part of the Kii Peninsula for the elusive yellow-flowered toad lily (Kii joro-hototogisu, Tricyrtis macranthopsis), I came across an unusual tree, a “new discovery” that made my day.

A shower-of-gold tree (Cassia surattensis) in full bloom almost seems to advertise a farmer’s roadside stand amid the forests of the Kii Peninsula.

Those of you who may be familiar with that part of the area will know it is heavily forested, and the predominant color is thus green. As I was driving along the banks of the Koza River, however, I saw a tree completely smothered with bright yellow flowers. I hadn’t the foggiest idea what it was. The setting was also unusual: this yellow-flowered tree almost covered a drink machine, and beside it was a farmer’s stall. The tree with all its bright flowers acted as a sort of natural neon sign!

Needless to say, I dug out my camera and took some pictures. It’s a good rule to follow: When in doubt, take pictures and then check these against guidebooks on the flora of Japan, available in libraries all over the country. Searching for the name of a plant is like a detective story. You gather bits of information from different books to build up the complete picture.

When traveling around Japan, be it in Takarazuka, where I live, or even in Tokyo, I get great satisfaction “discovering” unusual plants. We all know that Japanese gardens are small, but do we know what grows in them? Not all gardeners are bonsai enthusiasts. There are many true do-it-yourself gardeners, so perhaps there are some unusual trees or other plants in the area where you live.

The tree I had seen growing in south Wakayama, it turned out, was a shower-of-gold tree (koba-no-sena, Cassia surattensis). This evergreen tree was introduced to Japan sometime during the early years of the Meiji Era, and especially in southern Kyushu is grown as an ornamental. The Japanese name means “small-leafed senna.”

C. surattensis, a native of tropical Asia, grows to heights of 2-7 meters. Its leaves are paripinnate (pinnate, without the terminal leaflet), with four to seven pairs of small, oval, smooth-edged leaflets per leaf. These plants have sugar-secreting glands; in C. surattensis they are located between the lower two leaflets. In Okinawa it blooms from April to May and again between October and December. Each yellow flower is 3.5-4 cm wide, with five shiny golden petals, borne in terminal umbels. Seeds are produced in pods 10-18 cm long.

Until recently the genus Cassia was huge, with an estimated 400-600 species. Recently, taxonomists supported the division of the genus into two genera based on the shape of the stamens. The new genera are Cassia and Senna. Senna has 350 species of trees and shrubs, mainly confined to the tropical and warm-temperate regions of the world. Cassia now has 30 species distributed through the tropics. What’s the difference? In Cassia, the stamens are curved, and in Senna, they are straight. It is the curving stamens that give the flowers an exotic appearance.

Most species of Senna and Cassia are cultivated for medicinal properties, particularly their powerful laxative effect, and only a few species are grown for ornamental appeal. Alexandrian or Tinnevelly senna (Senna alexandrina) is a perennial shrub native to Egypt and the Sahara, and in the Sudan, where it is extensively grown on the poor sandy soils of the Northern Province. Its leaves and pods are used to produce some of the commercial senna.

The purgative principles of senna are widely distributed elsewhere in the plant kingdom and are derivatives of anthraquinone, an organic compound of greatest importance in the dye industry. The principles occur in combination with glucose as glucosides, derivatives of glucose in which one hydrogen atom in the molecule is replaced by an organic compound. The term glucoside is applied generally to such compounds of all sugars. Senna and Cassia have two principle glucosides, sennoside A and B.

Common preparations of senna are a hot-water infusion of leaves and extract of the pulp surrounding the seeds. Pharmaceutical preparations contain about 2.5 percent of the sennosides and are often mixed with coriander (Coriander sativum) to prevent griping.

The true shower-of-gold tree is Cassia fistula. Other colloquial names include pudding-pipe tree, purging cassia and Indian laburnum. Often grown in the tropics as a street tree, it is reckoned to be the most ornamental of all cassia species. It grows up to 10 meters, with flowers in racemes, each flower about 5 cm across.

The seedpods, up to 60 cm long, produced by the shower-of-gold tree are often sold in health-food and natural-medicine stores as “Manna.” The fruit contains about 80 yellowish seeds embedded in a sweet-tasting pulp, palatable to monkeys, bears and other forest animals. The pulp is a laxative and is used against persistent constipation. A decoction consisting of the ground-up shell of the fruit with saffron, rosewater and sugar is said to aid childbirth.

Several species of senna are used locally to cure other diseases. Ringworm senna (S. alta), obviously, is used to treat ringworm. This tree grows throughout the tropics. Senna emarginata from Central America is used to treat insect stings. Root extracts of Senna occidentalis and S. serica are used to cure dropsy, a condition when watery fluid collects in the body.

The shower-of-gold tree is propagated by seed, if available, or else by stem cuttings. Garden centers in Japan normally sell plants when in flower; last year I saw this plant for sale in small numbers. If you do get hold of one, be sure to plant it where it will get the most sun and not be exposed to cold winds or heavy frosts.

You can also grow cassia as an indoor plant, if there is adequate light. All cassia species require soil with good drainage.