The 'Garden Grotto' seems a quaint oddity from the past and it may surprise readers to find us making a case for its continued existence. Let us consider first elementally and then historically.
Gardens are made with natural materials and represent ideas about nature. Scientists break down the world into innumerable components. Gardeners learn from scientists but see gardens as composed with earth, water, plants, structures and sky. Every garden has these elements - and they should be composed.
The best way of concentrating upon EARTH is in cave - or grotto. It yields depth, darkness, silence and mystery. With a water fountain in the cave, there is also life. Mosses and ferns flourish. When the water flows into the sunlight, one blinks and looks to the sky. This gives verticality to counterbalance horizontally, especially if there is a fountain jet. The garden gains. [The idea of bringing elements together is also found in feng shui]
The word 'grotto comes from the Greek word kruptos, meaning hidden. Grottoes originated when it was believed that a union between the Sky God and the Earth Mother created the world. Contact with the Earth Mother was closest in caves with a fountain of spring water. Such places became shrines. When Roman Emperors wanted Greek shrines in their gardens, they made grottoes with fountains. When renaissance princes wanted Roman gardens, they made grottoes with fountains. When English lords wanted classical gardens, they made grottoes with fountains.
But when the grotto became just an ornament, its noble and symbolic past was forgotten.