Epic has made available over 50 high-quality 3D scans of plants and rocks for Unreal Engine free as part of their Meadow-Environment set. This set is part of their monthly free assets for July. The company, NatureManufacturer, also has free sets for desert and mountain terrains.
The list of assets included in the Meadow-Environment set has a wide variety of landscape types including read-made dirt rounds, low poly landscapes and much more.
You can follow this link to download (note: you will need to register at Unreal)