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Weeds can originate from adjacent habitats, dispersing naturally from non‐crop habitats as well as brought in from other crop fields by machinery or as contaminants of crop seeds. This constant dispersal may homogenise populations and transmit resistance genes. In this study, we examined (a) the genetic variation in quantitative traits and (b) the level of resistance to two widely used herbicides, amidosulfuron + iodosulfuron‐methyl‐sodium and florasulam + 2,4‐D, in eight population pairs of Sinapis arvensis from crop and adjacent non‐crop habitats across eight regions in Northern Algeria. Our quantitative genetic approach on phenotypic and phenological data of plants grown under controlled conditions from seeds of these populations showed similar levels of within‐population genetic variation and no genetic differentiation in quantitative traits between the two habitat types. This points to an important exchange of S. arvensis seeds and/or pollen between the two habitat types, but not among crop fields at the regional scale, as we found a strong regional effect for most of the plant traits for both habitat types that correlated with longitudinal temperature and precipitation gradients. We also found an absence of resistance against the two herbicides at a detection level of 10%. Florasulam + 2,4‐D showed a higher level and speed of phytotoxicity compared to amidosulfuron + iodosulfuron‐methyl‐sodium. Given our findings, we propose measures for more efficient management of S. arvensis, including minimisation of seed dispersal during transport of straw balls, crop rotation to avoid the build‐up of herbicide resistance, and management of populations in the close vicinity of crop fields. [ABSTRACT FROM AUTHOR]