One reason why I became a gardener was because I have a tendency towards depression, having suffered badly from it as a young adult. The idea of gardening for a living grew on me, if you'll excuse the pun, from helping on an organic farm, to studying Permaculture, to working for my friend's gardening business to finally getting a qualification in horticulture. I noticed that being outside improved my mood and physical exercise helped me to feel more capable and improved my general physical health, leaving me feeling more energy than I would usually have. I also liked the idea of having specific tasks and the feeling that there was always plenty to do, and being able to take pride in the difference I made to people's gardens. Later on, I relished the idea of working for myself, with all the flexibility and freedom that that entails; not to have a manager breathing down my neck, but a customer who respected my skill and knowledge and treated me as a co-conspirator in creating their perfect garden. Generally, this has been the case.
I have come across such programmes as 'Green Gyms' for people who have mental health needs and they sound great, but actually, in a sense, we all have mental health needs! Our culture is currently very sedentary and many people don't work outside, or do practical things with their hands, other than operate computers or phones in offices. Often people can feel very caught up in their heads, perhaps obsessed with social media, and doing something that involves using the body can break them out of it. Being in fresh air and braving the weather can give one a feeling of resilience, as often the grey winter's day looks far more unpleasant from inside the house, than outside, with the right clothing.
Completing a physical task can give one a sense of satisfaction, of having used one's muscles for a purpose, which is arguably more acute than the satisfaction of running around the block or going for a random walk. When one is in the grip of depression, it is difficult to motivate yourself to do the simplest of things, but when you literally see the fruits of your labours, by growing edible plants, it can help connect you to nature and to the reasons you can find for living, particularly when those reasons are hard to come by.
Sometimes, caring for plants can be a classic displacement activity, just like when people who have wanted but haven't had children, or whose children have left the nest, can become obsessive about pets. Plants can be just as demanding and some may argue, just as rewarding! Just think of the trouble some people go to with Auricula*, just as an example. Look at it one way, and it seems ridiculous. Look at it another and it is a wonderful form of art. I straddle both views at times. The same is true of life. Which glasses will you wear today?
Lots of people pursue gardening as a second career, perhaps after a break caring for children, but also many people who manage depression can find it a good choice as a career for all of the reasons I mention. Never assume that if you are employing a gardener, that they are doing that job because they don't have other skills from other lives, or that they are uneducated, which sadly, has been assumed about me. (I've got a degree in Philosophy.)
I admit I don't have much time for the highly competitive kind of gardener who professes to be an expert on everything, spewing Latin terms in a kind of machine gun effect to keep non-nerdy types at bay. It's not my style. I give my job away by getting my customers so enthusiastic about working in their own garden they start doing it themselves and don't need me! I prefer it that way, than by encouraging a false dependence by giving the impression I know everything and the customer knows nothing. This is rarely true. Why would I choose dis-empowerment as a career when I've striven to find something that gives me a feeling of well-being?
*Correct name Primula auricula; here are some images of 'auricula theatres': http://bit.ly/2e9yNHL
Here is a leaflet produced by Thrive, the gardening therapy charity, which outlines the positive benefits of gardening for people with mental health needs: http://bit.ly/2esa6tg. You can also learn more on their web site: http://www.thrive.org.uk.