You should evolve sunkern when you are in need of a strong grass type, and when you have a spare sun stone on hand. In Pokemon Ruby, Sapphire, and Emerald, you can get a sun stone in the Rocket Institution in Mossdeep City.
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gloom can evolve into bellosom and sunkern can evolve into sunfloria.
It turns into a Sunflora. Hopes this helps
Sunkern evolves by giving it a Sun Stone, not by leveling up
A Sun Stone will evolve Oddish, Sunkern, Cottonee, Petilil and Helioptile.
You have to use the sun stone on sunkern at any level but it is hard to get a sun stone in soul silver.