The golden shower tree blooms when spring warms up to summer, producing bunches of bright yellow flowers that are pollinated by buzzing insects. Seen here are common emigrants (Catopsilia pomona), which are among the many butterflies that come for its nectar and also lay eggs on the tree.
The golden shower tree or Indian laburnum (Cassia fistula; sarakonrai in Tamil, kanikonna in Malayalam and amaltas in Hindi) belongs to the family Fabaceae and is native to the Indian subcontinent and parts of Southeast Asia. It is popular as an ornamental tree in gardens and avenues and has been introduced to many tropical areas around the world. It blooms when spring warms up to summer, producing a profusion of bright yellow bunches of flowers drooping from its branches. The tree is often bereft of leaves during this time, with the flowers painting the entire canopy a golden yellow. Individual flowers emerge from short stalks around a long central axis, with older flowers at the base and younger ones at the tip.
Over a dozen butterfly species lay their eggs on the golden shower tree, whose larvae feed on its leaves and transmute between life stages. Butterflies, honey bees and carpenter bees feed on its nectar and are important pollinators of the plant. There is division of labour among stamens within each flower, where some stamens have pollen that can fertilize flowers, while others carry non-viable pollen to attract pollinators. These flowers are fertilized with the help of buzz pollination, with carpenter bees (Xylocopa species) acting as efficient movers of pollen. These bees produce high-frequency vibrations using their thoracic muscles, which are transmitted to the flower through physical contact. This causes the pollen within anthers (pollen carrying structures on the stamen) to move and jump around before being expelled through a small pore onto the buzzing bee.
Fertilized flowers produce long cylindrical fruit pods that contain horizontal compartments, each cell housing a solitary seed. Young pods are green in colour and ripen into a deep brown over many months. Younger fruit contain pulp that is eaten by many animals, who, in turn, disperse the seeds. “Reproduction is effected mainly, and perhaps entirely, through the agency of animals (monkeys, jackals, bears, pigs, and possibly others), which break open the pods to eat the pulp and thus scatter the seeds or swallow and disseminate them,” notes Robert Scott Troup, a colonial forester in his book The Silviculture of Indian Trees. Dispersal by animals also influences their spatial distribution in forests. Troup observes that golden shower trees occur in clusters in areas where monkeys are common, while having a scattered distribution elsewhere. The pulp present in young seed pods is used in traditional medicine.
The golden shower is the state flower of Kerala and is an integral part of mid-April Vishu celebrations marking the Malayalam New Year. Its flowering also marks the New Year in Laos where the flowers are a part of festivities. In recent years, people in Kerala are noticing changes in its flowering patterns, with missing blooms before Vishu and flowering at unusual times during the year. A citizen-science initiative called SeasonWatch is collecting information about seasonal changes in trees to understand baseline trends from tens of thousands of trees across India, including the golden shower tree. This data can help us identify unusual flowering events and test if such anomalies are associated with a changing climate.
On campus, one can see a golden shower tree near the main building and in front of the library. If you look closely, their leaves often have half-moon shaped slices missing from the edges, the handiwork of leaf-cutter bees, who use these to build their nests in tunnels and cavities. That is a story for another day.
