Plant more trees, cut more hospital stays. That’s the core message of a large study led by a team of international researchers that stitched together 11.4 million psychiatric hospital admissions from 6,842 places across seven countries over two decades.

The experts found that the more green space surrounds you, the lower your odds of ending up in the hospital for a mental health condition. These benefits keep stacking up as greenness increases.

There isn’t some magic threshold you have to cross. It’s a steady, graded relationship, which is exactly the kind of dose-response you want to see if you’re thinking about policy.

What “greenness” means in this study

The team used a satellite measure called the Normalized Difference Vegetation Index (NDVI), a well-worn metric that tells you how much photosynthetic plant matter is in a given area.

The experts linked those NDVI values with hospital admissions for all mental disorders combined and six specific buckets: psychotic disorders, substance use disorders, mood disorders, behavioral disorders, dementia, and anxiety.

The scientists also accounted for a raft of confounders, such as population density, weather, air pollution, socioeconomic indicators, and season.

The team stratified this data by age, sex, urbanization, and season. In short, they tried hard to make sure “more green” wasn’t just a proxy for “wealthier neighborhood” or “milder climate.”

More green space, better mental health

Across the pooled data, higher local greenness was linked to a 7% reduction in hospitalizations for all mental disorders.

The association was even stronger for some diagnoses: roughly 9% lower for substance use disorders, 7% lower for psychotic disorders, and 6% lower for dementia.

Those aren’t trivial numbers when you remember the scale of the problem – 1.1 billion people worldwide were living with a mental disorder in 2021, accounting for about 14% of global disease burden.

The researchers estimate that, in urban areas alone, greater exposure to greenness could potentially prevent about 7,712 psychiatric admissions each year.

The team also found clear seasonal patterns, which tracks with common sense: how people use and benefit from green space changes with heat, rain, daylight, and air quality.

Not every map looks the same

The study spans Australia, Brazil, Canada, Chile, New Zealand, South Korea, and Thailand, and the protective effect wasn’t uniform across them.

Brazil, Chile, and Thailand showed consistent benefits across most disorder categories. Australia and Canada were the outliers, with modest increases in hospitalizations as greenness rose.

That doesn’t mean trees are bad for Canadians; it probably reflects context. Where is the green? Is it parks, river corridors, and street trees – or lawns along highways and industrial buffer zones?

How safe and accessible are these spaces? What’s the urban form, the climate, the culture of park use?

When the researchers zoomed in on cities, a 10% increase in greenness corresponded to fewer admissions ranging from about one per 100,000 people in South Korea to roughly 1,000 per 100,000 in New Zealand. Local realities clearly shape both exposure and payoff.

Green space: A mental health intervention

This is an observational study. It can’t prove that greener neighborhoods directly cause fewer psychiatric hospitalizations.

The hospital records only reflect the most serious cases – people ill enough to need inpatient care – which means countless episodes of distress and outpatient treatment aren’t captured here.

In addition, combining hospital data from multiple countries is, by nature, a messy and complex task.

That said, the consistency of the overall pattern after adjusting for big confounders, the dose-response shape, and the scale of the dataset make the findings hard to shrug off.

At minimum, the work strengthens the case that greening isn’t just a climate or biodiversity play. It’s plausibly a mental health intervention, too.

How cities can act on this now

If you’re a planner or policymaker, this study nudges you toward very practical moves. Put high-quality, safe, genuinely accessible green spaces where people actually live, especially in dense neighborhoods that currently lack them.

Don’t just plant saplings for a press photo. Build shade canopies over sidewalks and bus stops, connect parks to transit, add lighting and all-season paths so spaces work in winter and heatwaves.

Because the strongest protective associations showed up in cities, target funds where the mental health return per square meter is likely highest.

Also, be picky about the type of green. A pocket park with trees, benches, and a water fountain will do more for people than a strip of turf beside a six-lane road.

Future research directions

“Green” is not a single thing. A pine forest, a mangrove fringe, a street tree canopy, a schoolyard meadow, and a shaded urban square are all green, but they invite different behaviors and deliver different microclimates.

Future research should tease apart which spaces drive which outcomes, and how quality, safety, and accessibility amplify – or erase – benefits.

Fine-grained exposure data (time spent, perceived restorativeness, social use) and natural experiments from new park installations could sharpen causality.

Given the seasonal patterns, studies in extreme heat and harsh winters are especially urgent, because those are the moments when good design can make or break whether green spaces are actually used.

You don’t need to move to a forest to see mental health benefits. More trees on your block, a better-maintained park within a short walk, a shaded route to the bus – these aren’t just nice amenities. They’re nudges that, at population scale, line up with fewer psychiatric crises.

The relationship seems to scale without an obvious ceiling, which is exactly what you want when you’re designing policies for messy, growing cities. Greener places tend to have fewer hospitalizations for mental disorders.

If you’re looking for a mental health strategy that also cools streets, filters air, boosts biodiversity, and makes daily life a little kinder, this is one you can plant and grow.

The study is published in the journal BMJ.

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