Siamese Cassia (Tagayasan)

Senna siamea is native to Myanmar and Thailand and probably Cambodia, Laos and Vietnam, and is widely planted in the tropics, including southern China, as a street and shade tree; it has naturalized across Southeast Asia. Because it tolerates harsh, dry sites it is also used in reforestation, where trees are often cut young for fuel rather than grown into large timber. The tree reaches about 10–18 m in height and 30–50 cm in trunk diameter.

Genuine tagayasan shows a dark brown to blackish heartwood crossed by many fine, regular pale stripes, giving cloud- or arrow-feather-like figures on flat-sawn faces; the sapwood is white and of little use. Japan's wood information center notes that much wood now sold in its place is murasaki-tagayasan (a Millettia species of the same family but a different genus), which is not the same wood. The English name "Bombay blackwood" used by some retailers for tagayasan is recorded by The Wood Database as an old commercial name of East Indian rosewood (Dalbergia latifolia), a different species.

The wood reached Japan in the Edo period, when it was recorded as used for shamisen bodies; the scholar Kaibara Ekiken praised it as the finest of woods, and the written names 鉄刀木 ("iron-sword wood") and 鉄樹 refer to its hardness. The origin of the reading tagayasan is uncertain; the dictionary Daigenkai suggests it may come from the Philippine name tambulian. In Japan it has long been used for tokobashira (decorative alcove posts), fine furniture, small crafts, musical instruments, chopsticks and walking sticks, and was once a popular walking-stick wood.