You can place flowers, tall grass, fern, and mushrooms on every type of blocks using snow layer (except dead bushes):
NOTE: Not working with double plant.

1. Place a grass block, and flower at the top.
2. Place a snow layer at the flower to maximum layer (8/8 full block).
3. Place some snow layers at the top snow layer (full block).
4. Replace the grass block with every block you want.
5. Destroy all the snow layer.
6. Then, your flower will stay at the top every block, ex: iron block.