As Parthenium hysterophorus L. is an invasive plant, the natural predators are absent in the naturalized areas. In addition to that, lots of factors make this invasive plant indestructible. Some of them includes: very short life cycle, produces large number of seeds as compare to native plants, fast germination rate and ability to undergo dormancy (can germinate even after 8-10 yrs), large number of seedbank in soil, can be spread easily by means of vehicles, farm machines, livestocks, other animals or even seed grains. They affect the native plants by competing for excessive resources and the space with the production of allelochemicals.

Parthenium hysterophorus produces large number of seeds. It has the ability of producing 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. Furthermore, Parthenium hysterophorus has the capacity to regrow from the cut or broken parts. Absence of natural enemies like insects and diseases are two important factors responsible for its rapid spread in India. It is also avoided by grazing animals. This 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.

How to control Parthenium hysterophorus?

Parthenium hysterophorus defies all control measures however, following methods can be practiced to keep down this robust weed at bay.

Herbicides like Atrazine, 2, 4-Dichlorophenoxyacetic acid, Paraquat, Glyphosate, Diuron and Dalpan can 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. Therefore, these plants can be used as replacement species to eliminate the weed. Parthenium hysterophorus growing in agricultural fields should be destroyed early in the June month by ploughing the field before setting of flowers. Burning should be done during early November when the whole plant dry and seeds mature.

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 die back and gradually get killed.

Good question but what makes Parthenium hysterophorus dangerous?

Parthenium is multiannual weed and produces numerous light weighted seeds every year which are disseminated easily by water and wind. Henceforth, it is difficult to eradicate or control. However, community approach may be helpful.

Parthenium is a problem to the environment as it spreads rapidly, and no single method alone has controlled this weed successful. We need integrated methods to fight against its invasions e.g.- biological + chemical + mechanical.

Until we deploy any control measure on community approach it will not prove helpful.

… 

i AM COMPLETLY AGRRE TO Fredrick Ojija biological + chemical + mechanical IS USEFUL

Production of large number of seeds, higher seed dormancy period and phenotypic plasticity

The serious consequences of parthenium weed invasion is colonizing agroecologically useful landscapes and other natural vegetation. It is difficult to eradicate because it reduces (suppresses) the regenerative capacity of the native plant species in the environment where it is growing. Further it is produces abundant seeds to invade more landscape around it. It is a invade landscape easily. It is better to cut parthenium plants before it flowers to reduce the seed bank on soil surface or in soil.