Under what conditions do organizations adopt greening measures? Economic and political factors like wealth, democracy, decentralization, and corruption can influence the shape of sub-state greening behavior. We examine the content, extent, and determinants of port decisions to take on a set of 22 greening measures in the busiest 209 seaports in the world over the period 2005-2020 using data we collected. Greening measure adoption varies widely. We find that most measures are adopted by fewer than half the ports examined and are most frequently undertaken without regulatory mandate or incentive. We generate and statistically evaluate a set of hypotheses about state-level factors that could contribute to the likelihood of port greening. Our findings suggest that voluntary adoption is most likely when the political conditions for mandatory regulation would otherwise be strong—specifically, decentralized democracies with locally affected populations. These conditions may create incentives for ports to act preemptively rather than face regulation.