Family: Fabaceae

The rainbow shower tree is a cross between Cassia fistula and Cassia javacana. Territorial forester David Hughes made the hybrid by hand-pollinating the two species in 1918. It quickly became a botanical sensation and was extensively shared. Unfortunately, the progeny varied widely as they were not true to seed, which is a frustrating problem due to the rare occurrence of seed production.

In 1960 The Surveyor General of Territory of Hawaiʻi, Walter Wall, perfected the grafting method for the rainbow shower tree. He distributed many trees to beautify our public spaces. As a result, Honolulu adopted the pink and yellow hybrid as its official city and county tree.

Today, many cultivars exist with various colors and fragrances. Grafting is the superior propagation method. Air layered trees have shallow root systems that easily blow over in the wind. A clean tree, it doesn’t litter the ground with sticky seed pods that stain or leaf debris, although some leaf and flower litter should be expected.

Energy is spent efficiently by allocating resources on flower and leaf production rather than on the abundant seedpods as other Cassias have. That means exceedingly showy flowers that bloom much longer than other ornamental Cassias.

Plant Uses:

• Fragrant
• Nitrogen fixer
• Ornamental
• Shade
• Specimen
• Woodworking

Plant Dangers:

• No dangers

High Risk Traits:

• Broad Climate Suitability – Adapted to tropical/subtropical climates; widely planted in the tropics.
  
• History of Introduction – Intentionally planted as an ornamental and roadside tree in tropical regions.
  
• Shade Tolerance – Can grow in shaded conditions
  
• Nitrogen Fixing – As a legume, it may improve soil fertility and potentially outcompete native species.
  
• Propagules Dispersed Intentionally by People – Actively planted in gardens and along roadsides, facilitating spread.

Low Risk Traits:

• Sterile Hybrid – Does not produce viable seeds, severely limiting natural reproduction.
  
• No Naturalization Evidence – No record of becoming naturalized beyond planted areas.
  
• No Weedy or Invasive Behavior – Not reported as a weed in agricultural, environmental, or disturbed settings.
  
• Limited Dispersal Mechanisms – No evidence of wind, water, bird, or animal dispersal.
  
• No Vegetative Spread in Nature – Does not reproduce via fragmentation in the wild.
  
• Non-Toxic – Not reported as toxic to humans or animals.
  
• No Allelopathy, Parasitism, or Thorns – Lacks traits that could negatively affect surrounding plants or animals.
  
• Does Not Form Dense Thickets – No evidence of forming monocultures or dense stands.