Is the garden just a place where people can talk about more important matters, or is the garden itself the inspiration for the discussion.
Do you want to bring out the connection between gardening and philosophy. In the Italian Renaissance, gardening was thought to be a branch of philosophy. So Italian gardens contain statues, grotesque images and mythological references. You could have a statue of Venus (claimed by some to be goddess of gardens).
Many fairytales and stories involve plants and gardens. You could have a collection of quotations or illustrations on placards, from such stories. Jack and his beanstalk, James and a giant peach, Eve and her apple, the mistletoe and the Golden Bough, etc, etc.
Also, the plants that flower in May will all have some mythology and folklore associated with each one of them, which you could illustrate and explain. May gives its name to the Hawthorn, which should be in flower then. That's an important tree in Celtic beliefs. This painting by John Collier is titled Queen Guinevere Maying (she's surrounded by boughs of flowering May)...