To assess the biotic and abiotic drivers of feral crop persistence, the occurrence and size of alien Brassica populations across an agricultural landscape in Canterbury, New Zealand, were surveyed over three years. Measures related to propagule input and site conditions were recorded and their role in explaining population occurrence and persistence assessed through GLMs and proportional-hazard models. Many Brassica populations were transient, with about 60% of populations disappearing within two years. New populations were founded at a rate that compensated for those that disappeared, and were more likely to occur along transportation routes and near seed companies, suggesting they established from seed spillage. Larger populations and those growing where habitat conditions were similar to those in which Brassica are cultivated had higher probabilities of survival. Without anthropogenic seed input to found new populations, Brassica spp. are unlikely to persist in this landscape beyond ten years. To avoid overestimating the extent of naturalised populations over time it is important to account for local population extinctions. The abundance of feral crops that occur as casuals in the landscape, along with other aliens that are maintained by external seed inputs, could be controlled by managing propagule sources. In themselves, casual populations are unlikely to facilitate gene flow or act as sources of further population spread.