A single plant species, baccharis halimifolia, has transformed the landscape of the Arcachon basin in Southwest France over the past few decades. Known as the "southern feverfew," this shrub was originally brought from the eastern coast of the United States in the late 17th century for its decorative appearance. In the 19th century, it was used by local foresters to help stabilize coastal dunes and prevent erosion. However, its rapid growth and dense thickets have led to the displacement of native plants, altering the natural balance of the ecosystem. The plant has disrupted the hydrological balance of salt marshes, which are crucial for maintaining water flow and salinity levels. Local sauniers, who harvest salt from these marshes, have found their product contaminated with baccharis halimifolia seeds. Despite being banned from sale in the European Union since 2017, the plant continues to spread along the Atlantic coast. Its fluffy achenes—small, seed-like structures—allow it to disperse easily through the wind, contributing to its proliferation. In addition to its ecological impact, baccharis halimifolia has caused health concerns for people living near colonized areas. Its pollen can trigger allergic rhinitis, a common allergic reaction that affects the nasal passages. The plant has been classified as an invasive species under EU regulation No. 1143/2014, which aims to restrict its sale and spread. However, control efforts have only partially mitigated its impact, as the plant has become well-established in the region. The ecological changes caused by baccharis halimifolia include a loss of biodiversity, increased soil leaching, and the collapse of marsh banks, which are vital for coastal protection. Similar issues are reported in other regions such as the Guérande peninsula, the Brière, and the Morbihan. Efforts are ongoing to restore native plant species in the affected areas, but the long-term effects of the plant's introduction remain a concern for ecologists and local communities.