Mary is compared to the enclosed or closed garden as mentioned in the Canticle of Canticles (4:12 - My sister, my spouse, is a garden enclosed, a garden enclosed, a fountain sealed up). This is one of the oldest symbols of Mary's sinlessness and immaculateness. It was included in the Litanies of Loreto. Since Mary's purity is the equivalent of great beauty, it was the custom in the middle ages and later on to design the enclosed garden as paradise garden filled with flowers and aromatic plants. Here, the garden reflects the Renaissance canon of beauty with its penchant for symmetry and geometric proportions. In its midst we discover a tree, probably in reference to the tree of life, meaning Christ Jesus himself.
Mary's love is exclusive. There is room in her heart for Christ alone. Such is the meaning of the enclosed garden. But Jesus is the source of life and love, and thus love springs forth from the heart of Mary and becomes visible and palpable to all who contemplate her enclosed garden, meaning her uniquely dedicated heart.