Roadside reclamation involves standard revegetation practices that often fail under the adverse conditions imposed by subordination to the infrastructure construction schedule. We experimentally tested for seed and microsite limitations on roadslopes by assessing the effects of seed addition and habitat suitability upon plant cover and species richness. The relative contributions of topsoil seed bank, seed rain, and hydroseeding with standard or native seed mixtures were analyzed in relation to soil texture, fertility, and stability. In order to increase applicability, this research was fitted into the actual construction design and schedule of a highway in central Spain, which resulted in topsoil of varying quality, steep roadcuts and embankments (34°), and out-of-season hydroseedings. During the first 2 years following roadslope construction, there was an uneven but sustained increase in plant cover and species richness. Topsoil spread on embankments led to greater plant cover in a shorter time and to lower sedimentation rates at slope bases. The topsoil seed bank was extremely poor. Hydroseeding invariably failed, regardless of seed mixture and roadslope type. The seed rain provided seven times more seeds than hydroseedings, and was correlated with the distance to vegetation patches. Recruitment, however, was limited by microsite suitability, as the initial soil content in nitrate, total nitrogen, and organic matter explained up to 80% of variation in plant cover. In conclusion, when revegetation was performed outside the optimal season due to schedule constraints, measures aimed at overcoming microsite limitation were more cost-effective and enhanced roadside carrying capacity for local species.