You can basiclly evolve it whenever you want, but you still need the stones. I personally never thought into training oddishes or glooms, but I do prefer Vileplume, but I usually just catch two glooms and get one of each.
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he evolves into gloom starting at level 21
There are two ways to evolve a Gloom, you can either evolve it into Vileplume or you can evolve it into Bellossom. To get Vileplume all you need to do is use a leaf stone on it. Likewise to get Bellossom you have to use a stone on Gloom, but this time it is a Sun Stone.
Raise an oddish to evolve to gloom, or catch a gloom in the safari zone, then evolve it with a sun stone. You should get bellossom.
no offence but its a bit of a silly question you use a leaf stone Yes, but it can also evolve with a sun stone into a bellosom
Evolve an Oddish to Gloom then use a Sun Stone to further evolve it to Bellossom. You must have the National Pokedex upgrade in order to evolve it otherwise nothing will happen.