1. Tall morning glory (Ipomoea purpurea)
An annual fast-growing herbaceous twining vine that can grow up to 2 – 3 m in length. Stems are slender, cylindrical, and covered in fi ne hairs. Leaves are thin, heart-shaped, and are arranged alternately. The showy flowers are blue, purple, or pink in colour. Flowers are funnel-shaped, with 5 fused petals. Between 1 to 5 flowers are borne in clusters in leaf axils. The fruit is a capsule ~1 cm in diameter with 3 – 4 black seeds. Individuals can produce up to 25,000 seeds per plant. Seeds are dispersed by wind, rain, or gravity. They can also be dispersed by people accidentally, mixed in with seeds of crops or flowers.
Impacts
As a vine it grows using other plants for support forming a dense canopy, shading other plants and competing with them for nutrients and water. The plant is an agricultural weed and is known to reduce crop yield. It can also become widespread both in disturbed areas and in forests.
Management
Seedlings can be easily uprooted manually. Small patches of the plant can be cut, but roots need to be dug out to prevent regeneration. Where the plant forms large patches, cutting is typically followed by herbicide application to prevent re-growth.
2. Water hyacinth (Pontederia crassipes)
A floating herb, which can gain up to 1 m in height, the species is fairly widespread. The striking mauve to purple flowers are in the form of spikes and each flower has around six lobes. The main lobe has a bright yellow diamond shaped patch which is quite prominent. The leaves are rounded and have thick swollen stalks. Common in freshwater bodies in the tropics.
Impacts
The plant spreads rapidly and replaces other aquatic vegetation, reducing water quality, affecting fish and other freshwater aquatic species. It also creates challenges for navigation, irrigation, hydroelectric power generation and tourism. Water bodies choked with water hyacinth have also become breeding grounds for mosquitoes and pose health risks. However, research has shown that the plant can remove contaminants from water and can be used in biotreatment of wastewater.
Management
Physical or mechanical methods have been found to be effective in controlling the spread of the plant. Biological control using fungus species such as Alternaria alternata has also been tested. In some regions, the plant has been harvested to make biochar for soil restoration.
3. Lantana (Lantana camara)
Lantana is a perennial woody shrub that can grow to be 2 – 5 m tall. It is a very adaptable plant, growing sparsely in dry environments, but forming dense thickets in wetter environments. The plant can also scramble up trees, like a climber. Lantana has spread across the country, from southern India to the Himalayan foothills, and can be found at elevations up to 2000 metres above sea level. Young stems are square and often have prickles. The leaves are opposite, green to yellow-green in colour, broadly oval, with a pointed tip. The leaf margin has rounded teeth, and the leaves are rough to the touch. The flowers are small, tubular, and range in colour from cream, yellow, orange, pink, purple, to red. They grow in attractive clusters, attracting butterflies and many other invertebrates. Fruits mature rapidly and change colour from dark green to black. Flowering and fruiting take place throughout the year. Fruits are widely dispersed by many birds, and mammals like the sloth bear.
Impacts
Lantana forms dense thickets in the forest understory, suppressing regeneration of native species and changing forest structure and composition. The plant adds to the fuel load, thus causing frequent and intense fires that can increase tree mortality. The plant is unpalatable, thus reducing forage availability for wild herbivores. In pastures, lantana displaces fodder species causing severe fodder scarcity. Lantana leaves are toxic to livestock. Lantana also serves as a host for sandal spike disease of sandalwood (Santalum album). Forest-dwelling communities perceive lantana-invaded forests as risky due to the dense thickets, which increase the chances of wildlife encounters. Lantana also suppresses regeneration of important plants like amla (Phyllanthus emblica), which people harvest from the forest.
Management
The “cut-rootstock” method, first attempted in Corbett Tiger Reserve, is considered the most effective form of mechanical removal. This involves cutting lantana individuals below the root collar, which prevents resprouting. Uprooting is less effective and more labor intensive. It brings buried seeds to the soil surface, resulting in increased regeneration from seed. Continued removal and establishment of canopy cover reduces the density and robustness of these shrubs.
4. Black wattle (Acacia mearnsii)
An evergreen tree that can grow to a height of 15 m. The trunk diameter can be from 10 – 45 cm. The bark of younger trees is smooth and greybrown in colour; in older trees the bark becomes brown-black in colour and develops cracks. The tree is branched with hairy branchlets. Leaves are dark olive-green in colour and are bipinnate with 8 – 12 pairs of pinnae, each with multiple pairs of leaflets. Flowers are spherical, creamy-white to pale yellow, and produced in clusters. They produce fruit pods that are constricted between the seeds.
Impacts
Plantations of black wattle have replaced large areas of shola grasslands and affected native biodiversity of these montane ecosystems of the Western Ghats. In the upper reaches of the Palni Hills, research shows that wattle plantations have decreased stream flow and reduced soil moisture. Bird abundance and diversity in these plantations are low compared to the neighbouring shola forests. Apart from regenerating from seed, the plant also spreads through suckers, forming dense thickets that crowd out other plant species.
Management
Mechanical removal by cutting the trees is currently practised. Young seedlings can be uprooted by hand after the wet season and larger plants cut at the beginning of the dry season. Removal efforts can be targeted at the invasion front in grasslands. Dead black wattle trees also should be removed carefully so that they do not act as perches for birds or as fire hazard. Sustained efforts across multiple years are required to exhaust soil seed banks as these seeds have long dormancy.
Excerpted with permission from Guests Who Never Left: Common Invasive Alien Plants of Peninsular India by R. S. Reshnu Raj, A. P. Madhavan, T. R. Shankar Raman, Divya Mudappa, Anita Varghese, Ankila J. Hiremath. Published by: Nature Conservation Foundation and Biodiversity Collaborative.
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