Is "Excess Greenery" Counterproductive? Urban Greenery Isn't Just "The More, The Better" — What is the "Optimal Amount" to Maximize Mental Health?

Introduction—Why Question "Usage and Dosage" Now

As the mental health crisis in urban areas becomes increasingly severe worldwide, "greening" has gained attention as a cost-effective and scalable public health intervention. The conventional wisdom has been "the more green, the better." However, the latest meta-analysis carefully examines this "common sense" and demonstrates that there is an "optimal amount" to maximize the effect. In other words, green spaces are similar to medicine, working best when used in the right dosage, and any excess or deficiency can be ineffective or even counterproductive.

Research Outline—Integrating 40 Years of Evidence

Researchers from the University of Hong Kong's Landscape/Geography and Earth Sciences departments screened approximately 88,000 academic papers from 1985 to 2025, extracting 69 dose-response curves linking mental outcomes (such as stress, anxiety, depression, cognition) and the "dosage" of greenness (like visible greenery, canopy coverage). They statistically refitted these curves and compared multiple models, such as linear and quadratic, concluding that an **inverted U-shape (maximum effect at the curve's peak)** was the most consistent. Evaluations were rigorously conducted using AIC/BIC, p-values, and adjusted R², following the PRISMA guidelines for systematic reviews.

Key Findings—Similar Optimal Points for "Eye-level Greenness" and "Top-down Greenness"

The study particularly presented thresholds from two perspectives.

• Eye-level Greenness: The proportion of greenery visible during daily commutes.The optimal point is around 53%, with a high-benefit range of approximately 46-60%, and a range where disadvantages are unlikely to occur is roughly 25-80%.

• Top-down Greenness: The proportion of canopy and vegetation seen from satellites or aerial views.The optimal point is around 51%, with a high-benefit range of approximately 43-59%, and generally resistant to harm at **20-82%**.

While numbers fluctuate by region and measurement methods, the pattern of peaking around "moderate" levels and then gradually decreasing or plateauing was consistently observed in data worldwide.

Why "Too Much Green" Becomes Ineffective

Theoretically, this is akin to the psychological Yerkes-Dodson Law (performance is best at moderate levels of stimulation) or the biological concept of hormesis (mild stress promotes adaptation, but excess is harmful). Excessive greenery in urban spaces can lead to anxiety from reduced visibility, safety concerns, humidity, pests, pollen, leaf litter burden, overcrowding of amenities, and opportunity costs of land (trade-offs with residential, commercial, public facilities). Conversely, a lack of greenery similarly increases stress and heat load.The moderate range offers the best "stress-benefit" balance.

SNS Reactions—A Mix of Surprise and Realism

This study spread across various SNS platforms from the day it was released. The surprise at results contradicting the intuition that "more green is better" and the clash with **realistic concerns about maintenance, security, and costs were striking. Starting from posts by science media (e.g., threads or posts by overseas psychology and behavioral science media), interpretations such as "eye-level greenery on streets is effective" and "just increasing parks is not enough" were shared. Meanwhile, there were also brakes like "should not be used as an excuse to reduce greenery" and "regional differences cannot be ignored." The implementation section in the latter half of this article considers these pros and cons.

Japanese Context—Compatible with "Green View Rate" Insights

In Japan, there is accumulated research and guidelines on the relationship between the proportion of green in the field of view ( green view rate) and comfort and satisfaction. Reports indicate that the peak of preference and comfort appears around a green view rate of 25-50% in outdoor environments, and administrative surveys show that residential satisfaction improves as the green view rate increases.The finding that "about half" of eye-level greenery is effective aligns with local experiential knowledge. Additionally, the global north-south disparity in green exposure and equity discussions pointed out by international research are important when considering the urban imbalances in Japan (differences between station fronts and residential areas, coastal and inland heat risk differences).

Implementation Checklist—Incorporating "Eye-level Greenness" into Design

1) Measure: Estimate greenery coverage from a pedestrian's perspective. Easily calculate the green view rate from street view images or on-site photos.

2) Prioritize Streets: First, enhance greening of sidewalk verges, medians, and building front yards. Form continuous "green corridors" with rows of street trees.

3) Ensure Visibility: Avoid creating blind spots with branch height and species arrangement. Use shrubs around intersections and crosswalks, and prune trunks to clear sightlines.

4) Diversity: Avoid single species, and distribute pollen, allergen, and disease risks. Mix evergreen and deciduous to mitigate seasonal bias.

5) Water and Soil: Balance water retention and drainage with infiltration zones and rain gardens. Use permeable paving as a standard to ensure oxygen supply to root zones.

6) Maintenance: Standardize leaf cleaning, pruning, and pest management. Balance maintenance costs and attachment with citizen participation (adopt programs).

7) Equity: Visualize the "imbalance" of greenery and prioritize allocation to schools, care facilities, and high heat risk areas. Increase accessible greenery.

8) Optimize from Above: Coordinate canopy coverage (top-down greenness) to correspond with around 50% eye-level greenery, combining pocket parks, street trees, and private green spaces.

Common Questions and Limitations

• Direction of Causality: Caution is needed in interpreting causality in observational studies. While this integration re-estimates curves from different methods to verify consistency, context dependency remains.

• Is "50%" Universal?: Optimal points subtly shift due to differences in climate zones, street widths, building heights, security, and culture. Use the **range (high-benefit zone)** as an indicator and fine-tune with local feedback.

• This is Not About "Reducing Green": Addressing deficiency zones remains urgent. Concerns about excess can be mitigated through design for visibility, safety, and maintenance.

• Latest Corrections: Note that there may be author corrections issued for the paper. Interpretations of figures and tables may be updated accordingly.

Conclusion—Incorporating "Green Prescriptions" into Standard Street Design

Approximately half of the greenery at pedestrian eye level—incorporate this "usage and dosage" as a KPI in urban design to avoid valleys of deficiency and excess. Continuously balance the placement and maintenance of greenery, visibility and safety, equity, and costs with data and on-site wisdom.Green is like medicine. That's why design it to be effective.