1. Create a Platformer Area
Set Perspective to Platformer for a game viewed from the side. Use it for plants on floors, ledges and walls. Resize the Area rectangle to cover the surfaces you want to paint.
2. Author surface-friendly types
Use sensible pivots and width/height ranges. For grass placed along surfaces, keep the visual root aligned with the sprite pivot so bending and shadows remain predictable.
3. Try a short strip of grass
Paint a small section of floor in Manual mode and check its size and angle. If you use surface placement, add a Collider2D to the ground and select its layer in the placement settings. A plant is skipped when the surface check does not hit that layer and the selected side of the surface.
4. Use masks for geometry exclusions
Use painted Placement Mask when walls/hazards should only hide existing vegetation. Use Automatic Scene Mask when the generator must actually reject those positions.