Congress Grass
A Terrible Weed in the Plains of India
-Dr. Arvind Singh
Congress grass which is also known as ‘Carrot weed' in India is a versatile, annual herb. The scientific name of congress grass is Parthenium hysterophorus and it belongs to the Asteraceae family of the flowering plants. It is native of Tropical America, where it is known as 'Santa Maria Feverfew', 'White top weed' and 'Ragweed'. In India, it is vernacularly known as ‘Gajar ghas’ and ‘Chatakchandni’.
Congress grass was introduced to India in seed form as a contaminant of food grains imported from Mexico. It was the first time reported from India in 1956, growing as stray plants at waste places in Pune (Maharashtra). Within a decade it had spread so vigorously in the Plains of India that it became a problematic weed posing problem of its extermination.
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The worst effect of this weed is witnessed in the state of Karnataka, where several suicide cases have been registered following agonizing acute chronic respiratory troubles (asthma, bronchitis, hay fever, and the common cold) caused by the pollen grains of congress grass. Today this weed has been reported from entire India except for the hilly states.
Characteristics:
Congress grass is a tall-growing, deep-rooted, much-branched dicotyledonous plant species attaining a meter height at the fully flowering stage. The weed bears numerous small white flowers aggregated together to form the Capitulum inflorescence. At maturity, Capitulum is transformed into Cypsella type fruit which bears numerous seeds. The plant is mainly propagated through seeds.
Congress grass has the ability to produce approximately 154,000 seeds per square meter. A single plant can produce 620 million pollen grains and about 15,000-25,000 seeds. The seeds are fluffy, very small in size and light in weight easily dispersed by wind, water or through various human activities.
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The congress grass has the capacity to regrow from the cut or broken parts. The absence of natural enemies like insects and diseases are two important factors responsible for its rapid spread in India. The weed is often avoided by grazing animals.
Habitat and Ecology:
Congress grass is a very aggressive and robust weed. In Plains of India, this weed grows along roadsides and railway tracks, on uncultivated lands and wastelands, in industrial areas and on the sides of the open drainage system and irrigation canals, besides invading agricultural crop fields.
Congress grass enjoys a monopoly in its growing habitat due to allelopathic property. The weed releases growth inhibitors in the soil which suppresses the growth of other herbaceous vegetation in its vicinity. Thus it grows in dense pure stands over a large area by replacing the native plant species.
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In India, this weed starts appearing in the middle of spring i.e. at the end of February in flushes and continues to appear till the onset of winter. The luxuriant growth of congress grass is seen during July and August, whereas in winter it remains dormant by forming rosette appearance.
Harmful Effects of Congress grass:
Congress grass is a great health hazard to human beings and livestock populations. The pollen grains of the weed are allergic causing bronchitis, asthma, rhinitis, conjunctivitis, hay fever, sinusitis, common cold, pain in the neck and even depression in human beings. The common allergens found in congress grass are parthenin, coronopilin, tetraneuris and ambrosin. Frequent contact with the weed causes dermatitis, eczema, and gangrene. This weed is harmful and even fatal to the livestock population grazing in the congress grass infested area. These animals suffer from itching, fall of body hairs, discoloration of skins, inflamed udder, etc.
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As a weed of agricultural crops, congress grass has a marked effect on the growth and colonization of Rhizobia (nitrogen-fixing bacteria) in pulse crops. This may reduce nodulation up to 60% thus substantially reducing the nitrogen-fixing ability of the pulse crops. The presence of congress grass pollen grains inhibits fruit set in eggplant, tomato, chilly, beans, and maize. Besides ill effects, it also causes several other problems like blockage of common pathways and reduction in the aesthetic values of parks, gardens and residential colonies.
Control of Congress grass:
The congress grass defies all control measures however, the following methods can be followed to keep down this robust weed at bay.
Herbicides like Atrazine, 2, 4-Dichlorophenoxyacetic acid, Paraquat, Glyphosate, Diuron and Dalpan should be used to suppress the weed up to some extent as these are found partially effective against the weed. The plants Cassia sericea, Cassia uniflora, Cassia tora, Cassia auriculata, Amaranthus sinosus, Hyptis suaveolens, and Tephrosia purpurea have been found to replace the weed.
Hence they can be used as replacement species to eliminate the weed. Congress grass growing in agricultural fields should be destroyed early in the June month by ploughing the field before the setting of flowers. Burning should be done during early November when the whole plant dry and seeds mature.
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Presently biological control is the most effective way of controlling this weed through Mexican beetle (Zygogramma bicolorata). This insect was introduced to India from South America in 1984. Detailed host-specificity tests under quarantine conditions confirmed the safety of beetle to cultivated crops in the country. Both adults and larvae of this beetle are capable of feeding on the leaves of congress grass.
Larvae after hatching start to feed on soft growing leaves, first attacking the terminal and auxiliary bud and later the leaf blades, thus checking the plant growth and flower formation. Adults voraciously defoliate the plant. The beetle cut immature flowers in an effort to chew soft tissues beneath the flowers. The completely defoliated plants start to show the symptoms of dieback and gradually get killed.
Conclusion:
It can be concluded that congress grass is a terrible weed posing threat to biodiversity, crop yield, and human and livestock health in the Plains of India. Therefore, it is necessary to control the growth and spread of this weed by employing all possible control methods to avoid its harmful effects.
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Author Bio:
Dr. Arvind Singh is M. Sc. and Ph. D. in Botany with an area of specialization in Ecology. He is a dedicated Researcher having more than four dozens of published research papers in the journals of national and international repute. His main area of research is the Restoration of Mined Lands. However, he has also conducted research on the Vascular Flora of Banaras Hindu University-Main Campus, Varanasi (India). Furthermore, he is also an active science writer having more than 10 dozens of published science articles in different periodicals of national repute. His email address is: 
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