"Dr. Rev. Chuck, M.D. P.A." wrote:

Sabaa Mundia wrote:
...i want to know that if plant it will i get onions growin underground or
what will happen...

They'll grow into bigger onions.

Onions are heavy feeders. Give them 10-10-10 or diluted 20-20-20
every couple of weeks. Harvest the bulbs when they "trip," i.e. when
the tops die down. Harvest the tops for soup any time you like.



Since the onion started out fairly large (or it wouldn't have been in
the supermarket), it was probably already mature. Further growth will be
to set seed, a process which will detract from the quality of the bulb.
Some onions will divide and form two or more bulbs, but you are unlikely
to get a significant increase in the volume of onion you have to eat for
the amount of work required to grow (and weed) it.

The note on fertilizer is good. They also need regular water if the
weather becomes dry (keep soil moisture up, but not soggy). A reasonable
way to judge soil moisture is to take a handful of soil and squeeze it.
If it drips water, it's too wet. If it forms a fairly solid ball, it's
good. If it won't form a ball, it's dry.

You can:
(1) eat the onion
(2) plant the onion and enjoy the flowers (or use the tops for soup as
mentioned above)