Potato Mine (if you have nine seed slots and you are replaying)
Gloom-shroom (if you have ten seed slots and you are replaying, or replace Split Pea with this)
Avoid planting Sun-shrooms in the first column, unless you want extra defenses. When a Digger Zombie reaches close to your Lawn Mowers, it will eat your Sun-shrooms. Instead, plant them in the second column.
If you prefer not to use the Split Pea, use a Squash or a Potato Mine when you see a tunneling Digger Zombie.
It is recommended to use Fume-shrooms in this level, because these mushrooms will deal damage to many zombies at once. A Gloom-shroom can also be used, but it is not recommended because the level only has one flag.
Whenever you see the Digger Zombie, put a Squash on the first square so that when it pops up, the Squash will crush it.