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Major regime shifts in mires such as the fen–bog transition and the transition from non-forested to forested peatland are driven by ecohydrological changes. However, little is known about how the magnitudes and/or durations of hydrological shifts relate to these regime shifts. Here we analyse long-term water table data in conjunction with plant community data collected from primary mires on the Finnish coast of the Gulf of Bothnia. These ecosystems represent various stages of drainage: undrained, drained sites with developing tree stands, and unsuccessfully drained sites not supporting tree encroachment. The varying success of drainage provides an ideal field laboratory for investigation of thresholds of water table control on the successional trajectories of primary mire. Our data indicate a likely mechanism for the control of vegetation regime shifts in northern peatlands by water table, with time of year being as important a factor as the magnitude of change. Spring flooding rather than summer water table level appeared to be crucial for controlling state shifts in primary mire vegetation. As the effects of climate change on peatlands are most likely to be mediated by changes in hydrology and water table level, our study indicates a need for more thorough investigation of seasonal variability in the controlling factors.