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OOver the years, many theories have arisen to try to explain why spending time in green spaces can reduce feelings of stress and anxiety, improve self-image and, in some studies, even significantly reduce measurable need for analgesics. From their alleged ability to purify the air, to the alleged mood-elevating compounds emitted by soil bacteria, to a mysterious effect that just seeing the color green has on our brains, let’s look at how the evidence stacks up.

Probably the most common claim I see is that plants, especially houseplants, purify the air. While they do do this on a planetary scale, the latest research now suggests that in indoor spaces the volume you would need to achieve this makes it implausible. How much ? Well, several hundred per square meter of living space to get the same benefit as just opening a window. Even I can’t stand this.

Another of the most persistent memes on social media is that soil bacteria, inhaled into our bloodstream, have been proven to improve our mood. Intrigued, I went to get the scientific article. This was a single study where the bacteria were injected directly into the brains of live lab mice. Unless you’re a caged mouse, this experiment doesn’t confirm much, and even then, the delivery method doesn’t sound like much fun.

What we do know, however, is that there is something unusual about the color green that seems to affect how our brains work. Researchers studying the effect of exercise on mental health have found that seeing views of the natural world projected onto screens has additional benefits beyond just the workout. However, when they manipulated the color of these screens to show the same views in black and white or in shades of red, the effect was markedly reduced compared to the original green. This may mean that using evergreens or, potentially, even something as simple as painting your fence green could help with its effect.

Likewise, designing your garden to require more maintenance so that it isn’t just a place to sunbathe in the summer can enhance the beneficial effect by encouraging gentle workouts. We know that prolonged low-impact exercise, like gardening, can burn more calories than an intense gym session, even if it sounds easier. It’s a way of being active that may appeal to a lot more people – if it’s for no other reason than being outside in the fresh air and it’s not as boring as watching the treadmill flashing lights, which brings me to my final point…

Gardening is also a classic form of mindfulness practice, which can help focus our thoughts on the here and now, distracting us from any worry or stress. Not my thing, but mindfulness exercises are well documented to have a positive impact on mental health, and are even recommended by the NHS. So ditch boring, low-maintenance projects like herbaceous plants for a more absorbing design with a spine of evergreens, and you could turn your garden into a welcoming therapeutic space.

Follow Jack on Twitter @Botanygeek

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