Create only green smoke plants on the field then put them as close to the road as possible. Upgrade their speed and power first, then their production.
Ground remains blue
Unlock the standing dome factory. Answer "Yes" on level 1 when asked if it is your first time playing. Start by purchasing a standing dome factory instead of a green smoke plant. Then, purchase a green smoke plant. You will be told that you do not have enough money. However, you will still be prompted to place your green smoke plant in the blue blinking area. Skip the rest and the ground will remain blue.