Can I grow...

[RiversideGuy]

thavnairian onions in potting soil? As I'm planting all 8 spots to yield the onions I have no crossbreeding in mind and they only yield one per plant... seems Thanalan or Shroud soil may not be warranted.

[Frizze]

So if im understanding you right, you bought* 8 Thavnairian Onion seeds, and you just want to grow them into onions so your chocobo can finish leveling up(growing them to sell would be the same i suppose too)? In other words, you only care about the onions themselves as an end result. If that is the case, then it shouldnt matter what kind of soil you use. Potting will grow them just fine, as would any of the other variants.

*I acknowledge that you also could have crossed them yourself, but most people who go to the effort of crossing TO seeds would see this as a bit of a waste and try to get something useful back in the process.

[Arazehl]

If you are just growing the seeds for the onions, then yes you can plant them in potting soil. You can even grow them in indoor flower pots.