Princess Alexandra met fellow enthusiasts when she unveiled a plaque to mark the official opening of the newly refurbished Thrive Gardener Workshop.
The building is based at the charity's herb garden in Battersea Park, south London. The princess is patron of Thrive, which helps people with a wide range of disabilities and uses gardening as therapy to help them rebuild their lives.
On a tour of the garden, Princess Alexandra exchanged ideas on growing sweet peas and using dibbers and extolled the virtues of people gardening together in a group.
The garden includes a glasshouse and plots for fruit, vegetables and flowers tended by people with autism, mental health problems or those recovering from strokes.
Kevin Smith, a visually-impaired Thrive client, visits the garden once a week and is working on an NVQ qualification in horticulture.
He said: "Working at Thrive is the best thing I've ever done and has opened a new window on the world. It's given me the chance of a job and I've started a gardening club."