Developing countries are increasingly exporting fresh horticulture products to high-income countries. These exports increasingly have to comply with stringent public and private standards, as well as other quality and safety issues. There is an ongoing debate on the effect of private standards on the inclusion of small-scale farmers in export supply chains. With this paper, we contribute to this debate by providing robust evidence from the Peruvian asparagus export sector, and thereby addressing several important methodological shortcomings and gaps in the existing literature. We use a unique firm level dataset on 567 asparagus export firms from 1993 – 2011 and several methods, including fixed effects and GMM estimators, to estimate the causal impact of certification to private standards on companies sourcing strategy. We find that certification leads to vertical integration and significantly reduces the share of product that is sourced from external producers, with a larger effect for small-scale producers. When distinguishing between production and processing standards, and between low-level and high-level standards, we find that especially high-level production standards have a negative impact on sourcing from (small-scale) producers.