In 1984, Roger Ulrich published a short paper showing that patients recovering from surgery got better more quickly if they had a view of trees out of the ward window, compared with those who had only a brick wall to look at.

In hindsight, not a very surprising finding, but it galvanised a massive research effort that continues today. As a result, we now know that exposure to “green space”, i.e. trees, grass and flowers, has a positive effect on almost every measurable 21st-century ailment, from high blood pressure and diabetes to depression and cardiovascular disease.

And yet, the mechanism(s) of this effect remain elusive, as does the value of ­different kinds of green space. A new study, published in the International Journal of Health Geographics, asks whether gardens are good for you, using data from the 2001 and 2011 UK censuses. Thus, unlike some studies that have used rather small sample sizes, you can’t fault this one for scale: the entire urban population of England.

In both censuses, people were asked to rate their health on a three or five-point scale, so researchers combined answers to that question with land-cover data to ask: do people from neighbourhoods with larger private gardens, on average, report better health?

The short answer is yes; compared with those with the largest gardens, people surrounded by smaller gardens were more likely to report poor health, and the smaller the gardens, the bigger the effect. This result was not dependent on any of the obvious variables that might be linked to garden size, such as population density, air pollution, house prices or smoking; even if all those were controlled for, the effect of garden size remained.

Even more interesting was that parks and public gardens did not have the same effect – living in a neighbourhood with more public green space does not seem to improve your health. This is consistent with other research showing that in the modern world, ­people’s everyday exposure to nature and wildlife is likely to take place – if it happens at all – in their own gardens.

Finally, and perhaps most fascinatingly, there was an additional effect of income: poorer people reported worse health. No surprises there, it’s no secret that poverty is bad for you. But this negative effect of low income on health got bigger as average garden size declined; the worst health was reported by those who suffered the double whammy of low income and small gardens.

We still don’t know exactly why gardens, and especially large ones, are good for you. People with larger gardens might spend more time gardening and get more fresh air and exercise, or benefit more from the psychological effects of garden views, greater tranquillity and reduced noise.

Even those who don’t have large gardens themselves may benefit from living in neighbourhoods with larger gardens, for example from less extreme temperatures caused by the “urban heat island” effect. There’s also evidence that exposure to a wide range of bacteria and other bugs (from spending more time outdoors) is essential for the proper development of the immune system.

Of course, the beneficial effect of gardens could be a combination of all of the above. But one thing’s for sure, if we go on building denser housing with smaller and smaller gardens, the potential health effects could be serious.

Ken Thompson is a plant biologist. His most recent book is Notes From a Sceptical Gardener, a collection of his Telegraph columns. Visit books.telegraph.co.uk

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