Want to be happy and productive at work? Keep a plant next to your computer. "Green" offices with plants make staff happier and more productive than "lean" designs stripped of greenery, new research shows.

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It found that enriching a "lean" office with plants could increase productivity by 15%.

"Investing in landscaping the office with plants will pay off through an increase in office workers' quality of life and productivity," said Marlon Nieuwenhuis from Cardiff University's school of psychology.

Researchers examined the impact of "lean" and "green" offices on staff's perceptions of air quality, concentration and workplace satisfaction.

They monitored productivity levels over the next months in two large commercial offices in Britain and The Netherlands.

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The findings showed plants in the office significantly increased workplace satisfaction, self-reported levels of concentration and perceived air quality.

A green office increases employees' work engagement by making them more physically, cognitively and emotionally involved in their work, added co-author Dr Craig Knight from the University of Exeter.