Anthropogenic release of CO2 is an important factor in the continuing rise in mean global temperature. Carbon capture and storage (CCS) offers a promising technology to capture and sequester CO2 in deep geological reservoirs. In view of the possible impact of leakage from CCS systems on vegetation, we examined the effects of elevated soil [CO2] on growth and yield in Zea mays L. Maize was exposed to elevated soil [CO2] by injecting CO2 at controlled rates using a purpose-designed field exposure facility. Measurements of soil [CO2] and [O2] revealed a strong negative correlation. Plants in a 40–90 cm diameter area centred on the injection point showed reduced growth and progressive development of severe stress symptoms during the gassing period. All above-ground vegetative (shoot, stem and leaf weight plant-1, chlorophyll content) and reproductive growth variables examined (mature cob and seed numbers plant-1) were negatively correlated with soil [CO2] and positively correlated with soil [O2]. Plants exposed to the highest [CO2] produced adventitious roots, possibly as an adaptive response to hypoxic soil conditions. Leakage from CCS transport or storage sites may have strong localised negative impacts on surface vegetation, the extent of which differs greatly between species.