This is only the most recent of many studies into this, eg. https://pmc.ncbi.nlm.nih.gov/articles/PMC5663018/

> There is an urgent global need for accessible and cost-effective pro-mental health infrastructure. Public green spaces were officially designated in the 19th century, informed by a belief that they might provide health benefits. We outline modern research evidence that greenspace can play a pivotal role in population-level mental health.