Gardeners have been knowing for centuries that their pastime gives them joy and peace. Many people will say that gardening is stress therapy. There is even a group called the American Horticultural Therapy Association “committed to promoting and developing the practice of horticultural therapy as a unique and dynamic human service modality.”

As with so many things, science introduces us to the physical wonders behind what we already know on a subliminal level. There are two interesting pieces of research that give credence to the feeling that our bodies and souls are better off from gardening.

Researchers reported in the journal Neuroscience that contact with a soil bacteria called Mycobacterium vaccae triggers the release of serotonin in the brain. This type of serotonin acts on several different pathways including mood and learning. Lack of serotonin in the brains is related to depression.

So basically, the things we do as gardeners—working the soil, planting, mulching, and so forth—can really contribute to happiness. We ingest the bacteria by breathing or through broken skin. The simple act of children playing outside in the grass and dirt can be a natural way for them to reduce anxiety.

In addition to increasing happiness and reducing anxiety, serotonin has positive effects on memory and learning. Research presented at the American Society for Microbiology shows that feeding live M. vaccae bacteria to mice significantly improved their ability to navigate mazes, due to the fact that the bacteria triggers the release of brain serotonin. It appears that this bacterium plays a role in learning in mammals.

Have you noticed that you feel really happy when picking those ripe vegetables, especially that first tomato of the season? Well, it turns out that harvesting fruits and vegetables triggers the release of dopamine in the brain. It is speculated that this evolved over 200,000 years of humans harvesting food as hunter-gatherers. Dopamine is strongly correlated with reward-motivated behavior.