The Woodland biome is the second Biome one will unlock in the game. There are three evolutionary paths that can help you unlock this biome.
You can evolve a yellow Daisy to Buttercup, which will spread Woodland as it grows.
If you've done the task Control a Sheep (which grants access to mushrooms), you can make a mud green Grass Tuft evolve to Oregano.
You can also make an Oak Tree Evolve to Sycamore Tree.