Finnish bogs have formed in three different ways. When a lake or a pond overgrows, bog vegetation will slowly spread from the edges of the lake towards its center. In clear-water lakes, this process can begin from the bottom, where the increasing amount of lake vegetation slowly takes over the lake and before being taken over by bog vegetation. In dark-water lakes, there are fewer bottom-dwelling plants because of the low amount of sunlight available. However, these lakes can still overgrow from the top, when bog plants such as peat moss take over the lake by growing on top of it.


When a lake overgrows, bog vegetation slowly takes over the body of water, spreading from its edges towards the center.

Sometimes, forests can also turn into bogs. In wet depressions where groundwater is partially visible on the ground, haircap moss and other bog plants will slowly take over the forest floor. Over time, they produce peat that makes the growth of non-bog plants impossible.

The third way in which a bog can form is via post-glacial rebound. This can happen near the seashore when previously submerged areas of land rise above the water's surface. Bog plants often take over these new, wet habitats.