Tree of the Month ~ Rainbow Shower Tree

If you are looking for some dramatic beauty in your garden, you might want to consider planting a rainbow shower tree. Though the blossoms are lovely and colorful, the name rainbow is not really an accurate description. Instead, imagine a Japanese brush painting of yellow with steaks of pink and orange. Add colors like cerise and coral to the palette and you’ll likely conjure an image that closely matches the beauty of these flowers.
 
The rainbow shower tree is actually a sterile hybrid of two shower trees in the Cassia genus. The genus includes about 30 species. Several of the shower tree varieties can be found in local landscapes as well as in parks or along streets. Trees in the Cassia genus are members of the very large Fabaceae (Bean) family, known for their leguminous seed pods. The pods produced by some shower varieties can be a litter problem in public places. This issue makes the nearly pod-less rainbow shower a popular choice. In Honolulu it became so widely cultivated that the multicolored cultivar “Wilhelmina Tenney” was declared the official tree of the City and County of Honolulu in 1965.
 
The tree is scientifically named Cassia x nealiae honoring Marie C. Neal. She was a well-known Hawaiian botanist and author of an early botanical reference book, “In Gardens of Hawai’i.” In her 1928 original as well as her 1965 revision, she refers to the rainbow shower as the cross Cassia javanica x C. fistula. Her propagation advice is that it is best done by cross pollinating blossoms of the pink-and-white shower tree with blossoms of the golden shower tree and using seeds from the resulting cross. These trees are each lovely landscape trees hailing from India and South America respectively but they do produce littering seed pods. The original hybrid cross was done here in Hawaii around 1916 by David Haughs. The resulting sterile rainbow shower is usually the preferred species.
 
Many trees available in the trade today are actually Cassia x nealiae grafted onto C. fistula rootstock. Though air layering is also a successful propagation technique for the rainbow variety, it does not produce strong roots making the resulting trees subject to toppling in heavy winds.
 
Four distinct color variants have resulted from the original cross, they include ones that are predominantly yellow, white or gold as well as “Wilhelmena Tenny” which produces the streaked yellow and cerise flowers described earlier. The flowers on the rainbow trees are produced on long pendulant racemes that appear on branches that bear stems of inch-long dark green leaflets. The trees tend to bloom most of the summer, losing blossoms and sometimes leaves in the fall. When in bloom the tree can appear quite full and very colorful.
 
Though shower trees can get as large as 50 feet tall and equally wide, they can be judiciously pruned to control their size and their naturally irregular growth habit. Careful, professional pruning is recommended to keep these trees attractive in small spaces.
 
Rainbow shower trees can tolerate many soil types but prefer to grow in soil that drains well. The tree is fairly drought tolerant and can make a nice addition to a xeriscape garden. It is not, however, very salt or wind tolerant so should be grown away from the ocean and in areas of low wind.
 
The tree attracts few pests and diseases. Those that do arrive can usually be treated with pesticides including soap and oil mixes or other organic compounds based on the identity of the pest. It is always important to positively identify the pest and match the treatment to the problem.
 
This tree will thrive with occasional deep watering in dry times and regular fertilization with slow-release fertilizers that include micronutrients. Follow fertilizer package instructions for amounts and frequency of applications.
 
Rainbow shower trees can provide a very attractive and interesting addition to a landscape. If you have lots of room, plant several. For small gardens, be prepared to train a single tree to be size appropriate. Call around to local nurseries to locate trees, choosing smaller specimens that can be trained is best for small gardens. Whatever size you choose for your property, you will certainly enjoy the annual display of colorful blossoms.
 
Diana Duff, Plant Adviser, Educator, and Consultant; Lives in Manoa Valley

[Editor's note: Starting in 1912, and continuing through the 1960s, TOC planted hundreds of shower trees along Honolulu's streets, including Vineyard, Piikoi, Pensacola, Makiki, and Nehoa. Our Branches statewide planted them as well. In 1995, TOC sponsored the "Shower Tree Festival" at Kapiolani Park, which grew into an annual event to heighten public awareness of Hawai‘i's trees and natural beauty.]