Huge asset collection (6.5GB) for creating forests and landscapes. It’s been around since 2015 and it’s still free (and awesome). EPIC recommend using it to create procedural landscapes and grassy plains:
- https://docs.unrealengine.com/5.0/en-US/procedural-foliage-tool-in-unreal-engine/
- https://docs.unrealengine.com/5.0/en-US/grass-quick-start-in-unreal-engine/