Vallisaari is well known for its natural attractions. In the springtime, the island’s leafy woodlands are carpeted with white wood anemones, lilies of the valley, and wood stitchwort flowers, while the melodious songs of nightingales and icterine warblers fill the air. In late summer meadowland flowers like maiden pinks and moonworts thrive in grassy areas around the ramparts in the western part of the island.
The meadows around the old artillery battery and along the rocky shore are home to as many as 1,000 kinds of butterflies and moths. More than 100 of these species are rare or threatened. Vallisaari serves as a first foothold in Finland for southerly butterfly species, since its sunlit meadows and leafy woodlands are similar to the habitats favoured by such species further south. The sandy soils of the island’s sunlit meadows are also inhabited by rare beetles and weevils more usually found on sandy shores.