l by Dr. Lalith Gunasekera
(November 01, Melbourne, Sri Lanka Guardian) When you travelling through mountains in Sri Lanka especially in tea growing areas you would notice some spectacular yellow booms along the road. When you travel further and nearby forests, you can see some yellow booms through the forest areas as well. This invasion is being happened in Balangoda, Knuckles, Haldummulla, Ohiya, Suriyakanda, Rakwana and few more areas in higher country. What is this “yellow booming” plant. The plant is Senna (Cassia) spectabilis originated in Central and South America. Local name for this plant is “Kahakona” due to the yellow flowers. It was introduced to Sri Lanka as a hedge plant in tea plantations and now spreading through native forest ecosystems in central mountains.
The aggressive growth habits of senna spectabilis, even in infertile soils have been noted. This plant is unable to fix atmospheric nitrogen (even though it belongs to lrgume) and that its extensive root system enables it to tap nitrogen from deep soil horizons. It has tendency to readily naturalize in disturbed habitats. It has become invasive in forest ecosystems and national parks in Kenya, Tanzania, Uganda, Venezuela, and Trinidad and Tobago as similar climatic conditions to Sri Lanka. This species has included in Global Compendium of Weeds and citations of references to this species as a weed.
Senna is a medium to large tree and reaches to 20 m in height but is often much smaller (7 – 10 m) with a spreading crown. Bole is short, tends to fork near the ground and is wide spreading with drooping, leafy branches. Bark smooth, grey with horizontal markings, rougher with age. Leaves alternate, up to 40 cm, compound with 4-15 (max 19) pairs of leaflets, each up to 7.5 cm, petiole 3-4 cm. Upper surface of the leaves are dull green and hair less. Lower surface is dull light green and soft hairy but sometimes hair less. Inflorescences are large, terminal, leafy panicles, 15-30 cm (max. 90 cm) long, which are branched and very large. The bright yellow flowers are 3 cm wide but appear in dense racemes up to 60 cm long and fragrant, Sepals orange – yellow. The cylindrical seedpods that follow are 30 cm long turning green to black with maturity. The pod has many cross walls 3 mm or less apart, the seeds in separate compartments. Seeds 2.5 cm each division, 50-70 sub orbicular, flattened, brown about 5 mm in diameter. Propagation mainly by seed dispersal through wind, water, machines, animals and humans
Some introduced plants and tree species have been known to remain in small localized populations for long periods of time but later turn into burgeoning populations of invasive. For key features are associated with invasive plants.
1. They show prolific seeding and early age of first reproduction.
2. Have unpalatable foliage.
3. Can easily establish in degraded environments.
4. Have an ability to regenerate profusely from direct seeds, stems or roots.
These features make them good competitors amongst other plant species and allow their survival and abundant establishment. Senna plant shows it all and marching along the Central Mountain Region in Sri Lanka. This plant can grow up to 2000m above the sea level and mean annual rainfall is 800-1000mm. It has the ability to grow in deep, moist, sandy or loamy soils but flourishes even in poor, black soils. Be watchful on this species and include in watch list of Sri Lankan invasive plant list.