There are many workflows for creating such objects, but we suggest using Chaos scattering system.
- How to use Corona Scatter? - for Corona 7 and older;
- How to use Chaos Scatter? - for Corona 8 and newer.
For realistic plant materials, see: How to create realistic leaf or grass material?
For rendering 3ds Max's native Hair and Fur, see: How to render Hair and Fur in Corona?