Crofton Weed (Ageratina adenophora) is a globally widespread invasive plant species. Native to Central America, it has become a problematic weed in many regions, leading to various ecological and agricultural challenges.
Identifying Crofton Weed
Crofton Weed is a perennial herb that typically grows 1 to 2 meters tall. Its numerous, erect, cylindrical stems are initially glandular and hairy, becoming woody with age and able to root at the nodes if damaged. Leaves are triangular to diamond-shaped, oppositely arranged along the stem, with serrated or toothed edges.
Small white flowers appear in dense clusters at the tips of the stems, usually from late winter through spring. These flower heads are composed of many individual florets, giving them a fluffy appearance. The plant reproduces primarily by seed, which are equipped with feathery hairs that aid in wind and water dispersal. It also reproduces vegetatively from a short, pale yellow rootstock.
Where Crofton Weed Spreads
Native to Central America, Crofton Weed has successfully invaded numerous regions across the globe, including Australia, New Zealand, parts of Asia, Africa, and Hawaii. In China, for example, it was introduced in the 1940s and has since spread rapidly across six southwestern provinces.
This plant thrives in disturbed areas such as roadsides, forest edges, riverbanks, and agricultural lands. It prefers shaded, wetter conditions but can also grow in open, sunny sites. Its adaptability allows it to establish in a wide range of elevations, from 500 to 2500 meters in China, demonstrating its capacity to colonize diverse habitats.
Impacts of Crofton Weed
The invasion of Crofton Weed has negative consequences for both natural ecosystems and agricultural productivity. Ecologically, it forms dense clumps that outcompete native vegetation, leading to a reduction in local biodiversity and the formation of monocultures. This aggressive growth can significantly decrease the density of native herb communities, including forage grasses, making invaded sites less productive. The plant also releases allelopathic substances from its roots, which can inhibit the germination and growth of neighboring plant species.
From an agricultural perspective, Crofton Weed poses a serious threat to pasture productivity, particularly in eastern Australia where it is an aggressive weed in pastures. It is generally not eaten by cattle, which further contributes to reduced carrying capacity of invaded areas. Crofton Weed is also toxic to livestock, especially horses, causing an acute pulmonary disease known as “Tallebudgera horse disease” in Queensland and “Numinbah horse sickness” in New South Wales. This condition can be fatal if horses consume enough of the weed over an extended period.
Managing Crofton Weed
Effective management of Crofton Weed often involves an integrated approach.
Prevention and Physical Removal
Prevention is the most effective strategy, emphasizing early detection and rapid response to small infestations. Preventing seed spread, such as by cleaning vehicles and equipment from infested areas, is also beneficial. Physical removal techniques include hand-pulling or digging out individual plants and exposing their roots. Cutting the plant with a machete, followed by ripping it out or ploughing, can be effective, especially when followed by sowing desirable pasture species. However, the plant can establish a significant seed bank in the soil, requiring consistent follow-up work.
Chemical Control
Chemical control options involve the use of herbicides.
Biological Control
Biological control efforts have also been explored with some success. For instance, the gall fly Procecidochares utilis, originally from Mexico, was introduced in China in 1983 to control Crofton Weed. While this agent has shown some ability to suppress population growth and reduce spread by decreasing flower numbers and seed viability, its dispersal can lag behind the weed’s spread, and its effectiveness can be limited by native parasitoids.