Thlaspi arvense and Camelina sativa have gained considerable attention as biofuel crops. But in some areas, these species, including C. microcarpa, are becoming rare arable plants because of agricultural intensification. Including them as crops could guarantee their conservation in agricultural systems. The fitness (i.e., vegetative and reproductive growth) of two populations of T. arvense, Teruel (Spain) and Minnesota (USA), and one for each C. sativa (Minnesota) and C. microcarpa (Teruel) was studied under different climates (semi-arid Mediterranean and continental temperate) over two seasons. Fitness of all species and populations was highly affected by the final plant density and the climatic conditions at each site and season. T. arvense from Spain and C. microcarpa showed more constant equilibrium in resource distribution patterns than those species or populations with American origin. The density variation between species, populations and seasons, which varied between 4 and 500 pl/m2, indicate the importance of emergence for crop establishment of these species. Moreover, results showed that environmental conditions significantly affected crop development, i.e. plants were always higher in Spain than in the USA, indicating that mild winter sites may be better suited for their production than others. The establishment of T. arvense as a crop in a semi-arid region like that of North-eastern Spain is unlikely without irrigation, while C. sativa could be a good option as a rotational crop regardless of irrigation. In the case of C. microcarpa, although it could be cultivated, its final densities (hence germination and emergence) must be improved in order to establish it as a crop.