by Dr. Lalith Gunasekera
( July 20, 2017, Mackay – Queensland, Sri Lanka Guardian) Himalayan knotweed (PERCICARIA WALLICHI) was originated in Himalayan mountain region of India and distributed as an ornamental plant species in United Kingdom, Ireland, Northern states of USA, Europe and New Zealand. Several years after its introduction to those countries, they realized the invasiveness of this plant as it was spreading very vigorously covering all other native plant species in absorbing available moisture in the area. After assessing all relevant risks characters of Himalayan knotweed, they declared it as a “noxious plant” under their legislation in their country or state.
Himalayan knotweed is a perennial plant growing erect up to 1-3 meters tall from creeping rhizomes (underground stem). The above ground stem has several segments joint together like a bamboo plant stem with branches. The stem is hollow (hole inside) and reddish brown in colour and hairy. Stipules are fused around the stem to form sheaths that are red-brown. Leaves are lanceolate, light green colour, 9-22 cm long, 2.8 – 7.8 cm wide, containing slight hairs on both sides. Leaf petioles are 3-20 mm long. Leaves are arranged alternatively on stems. Flowers are produced at the end of branches in clusters. They have white or pinkish petals which are oblong to ovate in shape. Flowering occurs from August to October each year. After pollination, flowers produced seeds. Seeds are brown, 2.1 – 2.5 mm long and 1.3 to 1.8 mm wide. Main aggressive feature of this plant is its underground rhizome system. They can grow up to 3 metre deep and 5–7 meter wide in the soil. The horizontal rhizomes can produce large number of shoots around the mother plant and forms dense stands. Very small piece of rhizome (about 1 inch) could be developed to a huge stand of Himalayan knotweed infestation. I have noticed this huge plant stands smothering native plants and water resources in and out of Nuwara Eliaya region.
Crofton weed (AGERATINA ADENOPHORA) is native to Mexico presently serious weed in Australia, New Zealand, India, Thailand, Jamaica, Fiji, South Africa, China and United States. There is no any records of reaching this species to Sri Lanka.
Crofton weed is a shrubby perennial with a woody rootstock and numerous upright stems. It usually grows 1-2 meters tall. Young drooping stems are soft and establish roots where they touch the ground. The leaves are bright green, diamond shaped, 50-70 mm long, 25-50 mm broad with the edges toothed and arranged in opposite pairs along the stem. Stems are usually purple and are covered stalked sticky hairs. Stem branch in opposite pairs. Flowers are white, in small, dense heads at the ends of the branches. Seeds are slender, 2 mm long, almost black with fine white hairs at the tip. This is not a very familiar weed to people in Sri Lanka.
Mature plant can produce between 10000 and 100000 seeds per year. Seeds are very light (25000 seeds/g) and highly viable and disperse by wind, water, animals and humans.
Crofton weed is densely overtopping ground covers and preventing native plant species from regenerating. It can invade a wide range of habitats and are especially happy in wetland areas where they compete with vulnerable native plant species. This plant can tolerate shade, damp areas and moist soil types. This species mainly found in central highlands of Sri Lanka (Nuwara Eliya, Horton Plains, Hatton, Talawakele).
Mist Flower (AGERATINA RIPARIA)
This plant introduced to Sri Lanka through Hakgala botanical garden in 1905 and used as a hedge plant in home gardens in up country home gardens. The plant has been spreading along the walking track towards to world’s end viewing area.
A native plant of Central and South America, Mist flower is a low growing sprawling perennial herb grow up to 40-60 cm tall. It produces numerous branching stems that produce roots at the joints where they touch the ground. Leaves are opposite, mostly 5-8 cm long and 2-4 cm wide toothed along the edges and tapered at each end.
White small flowers produced at the ends of the branches. Large number of black seeds produced with fine white hairs at the tip. Mature plants can produce up to 100000 seeds each year. They spread by wind, water animals and humans.
Mist flower is aggressive invader spreading into endemic forest areas and grasslands of Horton Plains and displacing native vegetation. It can quickly invade disturbed areas on mountain slopes and dominate riverine groundcover habitats excluding many native species and the native animals which were reliant upon those plants. This species is the most invaded invasive plant in Horton Plains as well as in knuckles mountains especially at Riverston area.
Mist flower has been listed as a potential invasive plant under the provisional list prepared by the panel of experts on invasive species in Sri Lanka.
To be continued…
Dr. Lalith Gunasekera is an Invasive Plants Specialist based in Mackay, Queensland