Hi, I had a question how to realize such material?
What have you tried so far?
For the vegetation part I would recommend you to take a look at this thread: https://forums.unrealengine/showthread.php?3268-Nature-Environment-in-UE4 
i am a noob but for the rock i think the best its two textures, the rock texture and the moss, and then painting the rock.
Check some rocks in cavern or effect (not remember) they use something like that i think, and you can reuse the textures.
The rock can be done in different ways:
- use the UE4 vertex painting
- use vertex group which were made in your 3d program
- use masks/variation textures in your material
e.g This moss was added with some nodes in the material editor -> the same setup like in the grass material from the starter content was used for this (instead of just one color I used a texture) (of course it will look better when you invest some more time into it -> I just made it in some few seconds to show it :D)
In the material I would recommend you to take a look at the moss material from the starter content to get a feeling how to create moss (but it will need some adjustments + probably also a better texture) 