From US wheat to India’s fields: How ‘gajar ghas’ became an invasive weed nightmare

It did not arrive as a dangerous pest or an exotic species deliberately brought into India. It came quietly, mixed with food grain at a time when the country was battling a severe shortage of food. More than seven decades later, that accidental arrival has become one of India's most persistent invasive-weed problems.

Parthenium hysterophorus, commonly known as gajar ghas, Congress grass or carrot weed, is now found across large parts of the country. The plant, native to tropical and subtropical regions of the Americas, is believed to have entered India with wheat imported from the United States in the early 1950s. ICAR says it was first noticed around Pune in 1955-56. (Indian Council of Agricultural Research)

From food aid to an ecological problem 

The story goes back to a period when India was struggling with food insecurity. Under the US food-aid programme, large quantities of wheat were shipped to India. Seeds of Parthenium apparently travelled along with the grain.

Once introduced, the weed found conditions remarkably favourable.

It grows rapidly, produces large numbers of seeds and can establish itself in wastelands, roadsides, railway tracks, agricultural fields and other disturbed areas. ICAR estimates that the weed has invaded around 35 million hectares of land across India, including the Andaman and Nicobar Islands and Lakshadweep.

The plant's ability to reproduce quickly is central to its success. A single Parthenium plant can produce thousands of seeds, while some documented estimates put seed production at tens of thousands per plant. Its seeds can also remain viable for years, making eradication particularly difficult. (Krishikosh)

Why this seemingly harmless weed is dangerous 

Parthenium is more than an eyesore along roads and vacant plots.

It competes with crops and native vegetation for space and nutrients, reducing agricultural productivity and threatening biodiversity. What began largely as a weed of wastelands has progressively moved into croplands, orchards, plantations and forest areas. (ISWS)

The plant also contains allergenic compounds that can affect people who come into repeated contact with it. Exposure has been associated with skin irritation and dermatitis, while airborne allergens can trigger respiratory and allergic problems. Animals can also be affected. (PubMed Central (PMC))

That combination — ecological damage, agricultural losses and health risks — has earned Parthenium a particularly notorious reputation in India.

An invader that is difficult to eliminate 

The problem is not simply removing the plants that are visible today. The weed's enormous seed-producing capacity means that fresh plants can emerge repeatedly.

India has therefore experimented with several approaches, including manual removal, herbicides and biological control.

One of the more notable biological-control efforts involved Zygogramma bicolorata, a leaf-feeding beetle introduced from Mexico. The beetle feeds on Parthenium and has been used as part of efforts to suppress the weed's spread. However, biological control is not a simple one-step solution, and researchers continue to examine other insects, fungi and microorganisms that could help manage the plant. (ICFRE)

The lesson hidden in the weed 

Parthenium's story is ultimately bigger than one invasive plant. It shows how an organism can cross borders unnoticed through global trade and food distribution, establish itself in a new ecosystem and become extraordinarily difficult to control decades later.

What arrived as an invisible contaminant in grain during a food crisis eventually became a nationwide ecological challenge.