Just one type of plant everywhere, would give the impression of "dull unvaried green", but a mixture of plant colors and textures would be "a sea of foliage"
The ocean isn't just one color....being a mixture of shades and colors, and I think that's the analogy that this is trying to make.
In her poem 'A Sea of Foliage' Toru Dutt compares the greenery surrounding her family's garden with the sea. She goes on to say that while the sea has an unchanging green colour, her garden is filled with different and exciting shades of green: the light green of the tamarind trees, the deep green of the mango grove, the grey green of the stately palms. A contrast is found in the brilliant red of the flowers of the seemal tree which takes one by surprise like the sudden shrill sound of the trumpet.
She says that the loveliest however are the lines of bamboo trees, growing towards the eastern side of the garden. When the moon shines through the bamboo trees and when the white lotus looks like a silver cup, the scene is so enchanting that one might almost faint, intoxicated by its beauty or gaze in wonder at what looks like the Garden of Eden- the first garden created by God for Adam and Eve.