The mystery seedlings can be bought ONLY during the special event, from either the billboard, or in the supply store, for 5 BBs each. Once bought you'll find them in your Tree inventory.
You then 'plant' them on an empty square. After 5 hours, it will be ready to transform into a Tree,
and will give you 200 EP when harvested. Then after a few seconds it will change into a Tree.This can be either a XL upgrade, a XXL upgrade (please note these include the base, so if returned to inventory will split into base and upgrade),
