- Invasive alien plants are those that do not occur naturally and have been introduced into a region. They grow fast and outcompete native flora for resources.
- Over the last few decades, multiple restoration efforts have been set up to conduct the scientific removal of invasive alien plants.
- These include uprooting, applying organic weedicides, controlled fire or cut and burn technique, and cut rootstock method.
- The efficiency of these different methods are varied, depending on the plant species.
Thousands of species of India’s flora are alien plants that arrived from other parts of the world during the colonial era. A majority were brought in intentionally to be used as ornamental plants and crops, while a handful arrived by accident with cargo and took root. Alien plants are those that do not occur naturally in a region but proliferate in the area they have been introduced into. They have had varied, complex and often destructive effects on ecosystems.
A study in September 2012 noted 1,599 alien plants in India. By 2023, more research has led to more records and the number had grown to 2,503, according to the book Guests Who Never Left. While many of these plants have remained within the confines of cultivation, a tenth of alien species expanded widely beyond cultivated areas via airborne or waterborne seeds or pollinators. These have become invasive alien plants (IAPs) – some of the most aggressive examples are Lantana camara, mesquite (Prosopis juliflora), and water hyacinth (Pontederia crassipes).
IAPs are generally fast-growing in nature and outcompete native flora for resources. They choke ecosystems, mostly offer no food and obstruct the movement of fauna. One study that sampled natural areas like forests, savannas and open ecosystems in 20 Indian states found that “53% of the plots recorded at least one of the 11 high-concern invasive plants that effectively invaded 254,880 sq km.”
Over the last few decades, multiple restoration efforts have been set up to conduct the scientific removal of IAPs, largely by nonprofit organisations in India, both inside and outside of Protected Areas. Some networks, like the Ecological Restoration Alliance-India, help foster collaboration in such efforts.
“Despite having a rich legacy of environmental policies, a legal framework exclusively to deal with invasive species is non-existent in India,” notes a 2021 paper that proposes policy interventions for IAP removal efforts. It also emphasises that “poor response capacities, lack of coordination between stakeholders, heavy dependence on biological resources and little public awareness” are major roadblocks that result in state inaction.
These include uprooting, applying organic weedicides, controlled fire (a traditional technique), cut and burn, and cut rootstock method. The efficiency of these different methods are varied, depending upon the species of IAPs. Aside from these, another method, though less commonly used, is the use of weed control mats which often prove to be challenging since they suppress native species and may be washed away in highly rain-fed areas.
How does uprooting work?
Uprooting can be an onerous process, as it is performed manually using knives, garden hoes, pickaxes, or similar tools. It is used to tackle woody stemmed invasives like L. camara, Acacia mearnsii, Cestrum aurantiacum and large shrubs like Solanum mauritianum. Harshavardhini Angappan works on IAP removal at Keystone Foundation, an organisation that works with indigenous communities in the Nilgiris on community-wellbeing and biodiversity conservation. She notes that regular maintenance for three years or so was needed following treatment after uprooting in intervention areas.
Uprooting must be carried out before fruiting season, when seeds emerge. However, some invasives like L. camara and S. mauritianum fruit year round, which means that timing isn’t a priority for these while planning removal. Plants are chopped before being uprooted. The woody branches are often used as firewood by local communities.
Keystone Foundation has tried and tested uprooting, along with several other methods in the Nilgiris district of Tamil Nadu. Resprouting (through vegetative propagation) did not occur after uprooting, though regeneration of seeds in the soil did. Regular maintenance aided in control of the regenerated seedlings. This method was ideal for areas where both invasives and native species thrived, since plants could be handpicked for uprooting.
Alolika Sinha is a conservation scientist who works with Aaranyak, an organisation that works on biodiversity conservation and education in northeast India. Sinha has led projects on several invasive management strategies in Manas National Park, Assam, and also experimented with uprooting. “We began a sampling study in 2013, and found that 30% of Manas’s grasslands were affected by invasives. Disturbances like cattle grazing, unregulated vehicular movements, etc. led to invasives spreading more,” Sinha told Mongabay India. Such activities degrade or fragment parts of the habitat, leaving the area open for IAPs to take root.
Sinha’s team published findings of the efficiency of their experiments in Assam with three treatments of the wind-dispersed invasive shrub Chromolaena odorata in one hectare plots – uprooting, cutting, and cutting and burning. Just before the invasives flowered in monsoon, 80% of the plot was treated while 20% was allowed to remain the same.
The efficiency of uprooting in Manas aligned with experiments by Keystone in the Nilgiris. “Manual uprooting worked the best. The plots were 90% covered in invasives at the start of the experiment and at the end, native grasses had returned. Five square kilometres of Manas have been totally restored in this way,” said Sinha.
How do organic weedicides work?
The State Agricultural Extension Management Institute (STAMIN) of Tamil Nadu has formulated an organic weedicide made from cow urine, Terminalia chebula fruits, neem oil and a local, organic soap used as an emulsifier.
Local farming communities in the Nilgiris, who were trained by Keystone to craft this weedicide, found it effective for suppressing invasive grasses and herbs like Parthenium hysterophorus without affecting soil health. Sprayed on a bright, sunny day, the combined reaction between the sun and the salts in the substance can dry out and kill the invasives within a few days. Monthly use of the weedicide may yield best results. Compared to uprooting, the application of this weedicide requires far less effort, and seedlings do not emerge as well.
“The soap helps the salts stick to the plants, and doesn’t allow it to be washed away by the rains,” explains Angappan. “A local soap that is used to wash clothes was used to make the weedicide. We have found this method efficient in controlling invasives in Biligiri Ranganathaswamy Temple Tiger Reserve.”
This method, however, is being commonly used in agricultural lands to clear weeds and not so much for invasives.
How do controlled fires work?
Farmers of the Irula community in the Nilgiris use controlled fires traditionally in the dry summers to clear areas for growing crops like a wide variety of millets. Both woody and herbaceous invasives can be controlled by this method. They are first cut, dried and gathered in heaps before being carefully ignited. The resultant ashes are spread across farms to enrich the soil.
IAPs are seen emerging again only after three months. However, this method is not ideal as soil health may be compromised. Additionally, fires threaten ground egg-laying fauna, amphibians and reptiles. This method is ideal for dealing with impenetrable thickets of IAPs.
Fires are used for subsistence agriculture in northeast India as well. The cutting and burning method attempted by Aaranyak in Manas is similar. The top growth is sliced and the subsurface portion uprooted before burning. “However, there is a global scarcity of data on the efficacy of these practices in controlling C. odorata, improving native grassland ecosystems and habitat for endangered grassland dependent species,” states the paper led by Sinha.
How does the cut rootstock method work?
Invasives like L. camara are cut below the “collar” under the soil, where the root stops and the stem starts. This has worked well in the Nilgiris, suppressing re-emergence from the stalk. “We are planning to try this method to control the C. aurantiacum, which is mostly present in upper Nilgiris. It grows in the understory along edges of habitats where there is a good mix of sun and shade,” said Angappan.
Some experiments in a 2009 study across restoration sites in the central Indian and western Himalayan landscapes have also found that cut rootstock was “the simplest and most cost-effective Lantana removal method”.
What are the complexities of IAP removal efforts?
Angappan stresses that assessing the cost of IAP removal methods is difficult as there are countless variables that affect each plot. Some of these include colonisation of secondary IAPs, urbanisation leading to habitat degradation and further spread, the scale at which IAPs have already managed to spread, the persistence of seedlings, and high level of maintenance that is required in sites post removal. She adds that monitoring to see how the plot responds to interventions is important, followed by adaptive management.
Happy Valley in Kotagiri, Tamil Nadu is one such example, where L. camara and S. mauritianum were cleared by Keystone via uprooting and native trees were planted in their place. Plots were regularly maintained, and native plants were nurtured and initially fenced. “Over time, the natives took over with no Lantana or Solanum remaining in the understory. However, Cestrum, dispersed by birds, became a secondary invasive here since it thrives in the shade. Now we want to plant understory native species to outcompete this new invasive,” said Angappan.
How does one pick a removal method?
There is no hard and fast rule to picking one method over the other, says Angappan. While one can begin by referring to papers that explore the success rates of different methods, existing knowledge is still scant on the effectiveness of most methods, especially given the diversity of both habitats and IAP species. Keystone’s biodiversity team has been largely relying on trial and error, and finding that uprooting is most effective across species and landscapes.
“For the most widespread IAPs like Lantana and Solanum, we do uprooting and it works well for us. It checks off most of the boxes one considers while choosing a method,” says Angappan. She goes on to elaborate on the criteria – the chosen method must be cost and time effective, leave minimal impact on soil health, limit regeneration of IAPs, require minimal interventions after application of the removal method, work on sloped or steep landscapes, and leave minimal to no impact on native vegetation. “However, we have not experimented much with other methods to understand their effectiveness for removing IAPs.”
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Banner image: Youth of local communities living near Manas National Park engage in experiments to remove IAPs choking native grasslands in the park. Image courtesy of Aaranyak.
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