Cities worldwide are grappling with a growing mental health crisis. While urban green spaces offer promise as a cost-effective solution to improve well-being, the question of how much green is truly beneficial has remained unclear. A new study published in Nature Cities by researchers from The University of Hong Kong (HKU) provides a definitive answer: moderate amounts of urban greenery are key to maximizing psychological benefits, challenging the assumption that “more green is always better.”
The World Health Organization estimates that one in eight people globally lives with a mental disorder, yet treatment remains inaccessible for most. Urban greening has gained traction as a potential solution due to its proven links to reduced stress, anxiety, and depression, alongside enhanced cognitive function. However, previous studies yielded inconsistent results, failing to establish clear targets for urban planners. This new research fills this critical gap by analyzing decades of global data to reveal the optimal “dose” of greenness for mental well-being.
Led by Professor Bin Jiang, the team conducted a rigorous analysis spanning 69 quantitative studies published between 1985 and 2025. They examined data from five continents, encompassing over 500 datasets and representing various types of green spaces viewed both from street level and from above. The meta-analysis confirmed a consistent inverted-U shaped relationship: mental health benefits increase with rising greenness up to a moderate threshold, plateau at that point, and then decline, potentially becoming detrimental beyond this point.
Striking a Balance: The Optimal Green Thresholds
The findings reveal specific thresholds for both eye-level greenness (what people experience while navigating the city) and top-down greenness (approximated through satellite imagery). For street-level views, benefits peak at 53.1% green coverage, with a highly beneficial range between 46.2% to 59.5%, and a non-adverse range from 25.3% to 80.2%. Top-down perspectives show a similar pattern, peaking at 51.2% with a highly beneficial range between 43.1% to 59.2% and a non-adverse range up to 21.1% to 81.7%. These findings align with established theories like the Yerkes–Dodson Law, which suggests that optimal performance (in this case, mental well-being) occurs at moderate levels of stimulation.
Practical Implications for Cities: Planning for Mental Well-Being
This research offers a powerful framework for urban planners and public health officials. Rather than prioritizing relentless greening, cities can now target these specific thresholds to maximize mental health benefits while optimizing resource allocation. Eye-level greenery along streets and public spaces emerges as particularly crucial, justifying priority in design considerations. The established thresholds also allow planners to set minimum levels of green coverage to safeguard mental well-being and avoid diminishing returns beyond a certain point. This targeted approach supports more equitable distribution of urban land and maintenance resources.
“This work demonstrates how environmental interventions can address critical public health challenges,” explains Professor Peng Gong, Vice-President and Pro-Vice-Chancellor (Academic Development) at HKU and a member of the research team. “It provides much-needed evidence to achieve sustainable development goals related to health, well-being, and sustainable cities.”
Professor Jiang emphasizes that the most significant contribution lies in establishing the generalized curvilinear relationship between greenness and mental health outcomes. He further underscores two key takeaways: first, debunking the myth that “more green is always better,” highlighting the potential downsides of excessive greening; second, demonstrating how moderate green spaces are sufficient to provide optimal benefits while preventing resource overallocation. This delicate balance is especially relevant for densely populated cities like Hong Kong, where maximizing green space often clashes with other pressing urban needs.
Professor Chris Webster, Chair Professor of Urban Planning and Development Economics at HKU, summarizes the study’s dual impact: “We have provided robust evidence for a curvilinear relationship that ends decades of fragmented findings. Second, we translated this pattern into practical threshold values that directly inform greening guidelines and landscape design standards.”
By offering clear direction on achieving optimal mental health benefits through urban greening, this research empowers cities to make more informed decisions about resource allocation and prioritize the well-being of their citizens.
