There is limited evaluation of whether green space interventions lead to more people using these spaces. This is partly due to the difficulty of collecting data on visits to green spaces. Mobile phone data may offer one solution, but are rarely made available to researchers. As such, there is less examination of whether these data are valuable. We accessed small area anonymised mobile phone GPS data for Liverpool (2021-2022). Using a case study of wildflower planting, we identified green spaces where wildflowers were planted. A synthetic control was then constructed to identify similar green spaces where planting did not take place. We then compared the number of visits post-intervention between the intervention and control green sites. Our paper also gives an empirical and methodological evaluation of the strengths and weaknesses of using mobile phone data, which can inform others on how to best use them.