This article analyses Bent Flyvbjerg’s ‘dark side of planning’ theory and proposes to increase its critical strength by including, together with ideas of rationality and power, two further theoretical tools: the Foucauldian concepts of governmentality and biopolitics. The potentiality of this inclusion is exemplified by the analysis provided about the influence of 18th-century colonial governmentality on the real rationality of public garden planning in the modern liberal cities of most western European colonising countries. It aims to show that Flyvbjerg’s concept of ‘real rationality’ can be usefully regarded as the product of a broad interpretation of biopolitical technologies, including the disciplining of non-human further than human life, which makes it possible to control the ‘uncivilised’ instincts of society through public garden planning. This article aims to suggest, that by digging deep into the hidden rationality of planning, even in those cases in which only the progressive face of power is apparently involved, a dark side of planning is unavoidably present in the form of a disciplinary power.