For achieving photorealistic results general taste in design is far more important than just a technical knowledge of the software.
The key for photorealistic vegetation, besides correct tools, is understanding how it behaves, what it does, what it smells like.
My way to achieve photorealism in lights and materials is always having good references and working strictly based on them.
Analyze distance to vegetation on your render and real necessity for detailed materials - this way you shorten rendering time.
I always try to tell a story. Could be some elements or a light hitting one spot in the scene.
The main components of a great render are: interesting design, good composition, balanced lighting, realistic materials and right camera angle.
Remember: although optimizing textures size, lights and "general rendering setting" speeds the work, overdoing it, can ruin the end result.
To set lighting in a scene, you need to know some photography basics to understand camera settings and white balance.
Be self-critical with your work: try to find the balance between the time spent on a project and the quality.
I prefer to spend less time in 3d-software and more in postproduction. For me Photoshop is the best render engine.
Start with a small scene and shoot it like a photographer, remember the composition - it considerably changes the image perception.
Photoimages serve as the basement for fixing materials and lights. Results in 3d strongly depend on the choice of references.