A root vegetable, where the food portion of the plant develops underground (carrot, beet, potato, etc.), could represent:
An activity where most of the action is unseen or unknown (as the part of the plant that we tend to see as food is developing unseen, underground)
The act of methodically stashing something away (money, ideas, etc.), as a root vegetable "stashes away" nutrients and moisture in its root
Something invested or planted (money in stocks, ideas in the subconscious mind, a problem situation, etc.) that grows or becomes bigger somehow, possibly without your awareness (since the root grows unseen underground)