Shower Trees (Cassia sp.) are a group of flowering trees in the pea family, Fabaceae, that are native to India and southeast Asia. Hawaii's most common shower trees are the rainbow shower tree (Cassia x nealiae), which are sterile hybrids of the next two most common cassia species; pink shower tree (Cassia javanica), and yellow shower tree (Cassia fistula). Cassia x nealiae is named after well known Hawaii botanist Marie C. Neal. In 1965 The Wilhelmina Tenney cultivar of the rainbow shower tree was made the official City and County flower of Honolulu. Cassia fistula is the state flower of Kerala in India, where it is known as Ahela or "Aehela" (ඇහැල). It is also both the national tree and flower of Thailand.
Shower trees are frequently planted along roadways and in parks and yards. They do not do well exposed to salt or wind, so they often do not reach large enough sizes to get good lumber out of before blowing over or having to be removed as hazards. Rainbow shower tree in particular does not have stable roots unless grafted correctly when young to root stock of cassia fistula, and easily blows over, which makes salvageable wood from the rainbow shower tree that much more prized.
Cassia species are commonly pollinated by bees and butterflies, particularly carpenter bees.
The wood is very durable. It has been used in construction at the famous site of the Buddha's footprint, Ehela Kanuwa on Adam's Peak, as well as to carve King Parakramabahu's throne, and Bahirawa Kalliya (each out of a single block of cassia wood). It is an important tree in Sri Lankan Buddhism and is commonly used to make temple drums.
There is little data on cassia wood properties, the only species we have found with official statistics is Pheasantwood (Senna siamea/Cassia siamea), which from our limited experience is slightly more dense than shower tree. Official stats on pheasantwood are as follows:
Janka Hardness: 1490 lbf (6,640 N)
Average Dried Weight: 50 lbs/ft3 (800 kg/m3)
Specific Gravity: .62, .80 (at 12%)