Résumé
This account presents information on all aspects of the biology and ecology of Baccharis halimifolia L. that are relevant to understanding its invasive behaviour. The main topics are presented within the framework of the new series of Botany Letters on Monographs on invasive plants in Europe: taxonomy, distribution, history of introduction and spread, ecology (including preferred climate and habitats, responses to abiotic and biotic factors, ecological interactions), biology (including physiology, phenology and reproductive biology), impacts and management. Baccharis halimifolia L. (Asteraceae), groundsel bush, is a broad-leaved shrub native to the coastal area of southeastern North America. Introduced for ornamental and amenity purposes during the nineteenth century, it has become naturalized in several coastal habitats, as well as in disturbed areas of western Europe. The shrub is now common on the Atlantic coast of Europe from northern Spain to Belgium and it is an emerging problem on the Mediterranean coast. Baccharis halimifolia is a light-demanding pioneer species that colonizes following disturbance but can then become dominant in natural habitats. The shrub can grow on a large range of soil types but prefers moist soils with high organic content and it is well adapted to poorly drained saline soils. In contrast to its native range, where it is in competition with other coastal shrubs, populations in the secondary range have almost no native analogues across most of its ecological niche except for Tamarix gallica in Mediterranean areas. Baccharis halimifolia reproduces sexually, but it has a high resprouting ability following mechanical damage or fire. Very high seed production, coupled with dispersal by wind and water, ensure a good colonization capacity of suitable habitats. The species shows a relatively high plasticity for both morphological and ecophysiological traits, which is probably the basis for its tolerance to a wide range of ecological conditions, including salinity and light availability. Baccharis halimifolia is host to a limited number of insects, both in the native and introduced ranges, but a number of highly specific agents have contributed to the control of this plant following its introduction to Australia. In Europe, B. halimifolia is considered an invasive non-indigenous plant and the shrub is the object of control programmes. Negative impacts include the addition of a new canopy layer in formerly open habitats (e.g. Juncus maritimus communities), which causes a strong decrease in species richness and herbaceous cover and poses a threat to some birds by modifying habitat quality, mainly in priority habitats and in many natural protected sites. Most efficient control methods are mechanical removal and herbicide application either on leaves or stumps. Due to the high cost of mechanical removal and the unintended effects of herbicide application on other species, alternative management methods such as controlled inundations and biological control could also be considered. The plant is legally prohibited in several countries and it is prelisted on EU Regulation 1143/2014.