Choose the 16 plants for the ground-cover sheet. Pick a cell, try candidates in it (36: your 27 plants and the 9 long grasses), switch between 300 / 600 / 900 triangles at the bottom, and Use the one you want. Copy picks when you're done and paste them in chat.
Each version: corners at the same spot merged, faces turned one way, flat faces merged, then shrunk toward 900, 600 or 300 triangles (Blender, labs/plants/process.py). Plants made of many separate little pieces stop shrinking at whatever their pieces need, so their 300 can read more.