We present a detailed study of the environments of a sample of nine low-power [Fanaroff–Riley type I (FR I)] radio galaxies, based on new and archival XMM–Newton observations. We report new detections of group-scale environments around three radio galaxies: 3C 296, NGC 1044 and 3C 76.1. As with previous studies, we find that FR I radio galaxies inhabit group environments ranging over nearly two orders of magnitude in bolometric X-ray luminosity; however, we find no evidence for a tight relationship between large-scale X-ray environment and radio-source properties such as size, radio luminosity and axial ratio. This leads us to conclude that radio-source evolution cannot be determined entirely by the global properties of the hot gas. We confirm earlier work showing that equipartition internal pressures are typically lower than the external pressures acting on the radio lobes, so that additional non-radiating particles must be present. We present the first direct observational evidence that entrainment may provide this missing pressure, in the form of a relationship between radio-source structure and apparent pressure imbalance. Finally, our study provides further support for the presence of an apparent temperature excess in radio-loud groups compared to the group population as a whole. For five of eight temperature excesses, the energy required to inflate the radio lobes is comparable to the energy required to produce this excess by heating of the group gas; however, in three cases the current radio source appears too weak to produce the temperature excess. It remains unclear whether the temperature excess can be directly associated with the current phase of active galactic nuclei (AGN) activity, or whether it is instead either a signature of previous AGN activity or simply an indicator of the particular set of group properties most conducive to the growth of an FR I radio galaxy.