Below ground, just like a carrot.
They grow above ground. It is a stem plant.
Above the ground
above.
A Swede grows below ground.
Above ground. It is a flower.
Cabbage grows above ground.
Both
Below ground.
Above ground as the oat plant is like wheat.
No corn grows on a corn stalk above ground.
The potato tubers (potatoes) grow below the ground, but most of the potato plant (leaves etc.) grows above ground.
The roots of parsnip, the part that is eaten grows underground, but the foliage grows above ground.