Starf

Starf is a type of Berry that can be planted in the Farm. It can be obtained by having at least 1 empty plot while encountering a shiny wandering Pokémon. At least one Gen 4 or  5 berry must be unlocked to unlock this berry. It emits an aura that increases Shiny Chance.

Possible Planting Formations
You can fill your farm in the following way to have the best chance to obtain Starf berries. Note that the setup omits a 24th Roseli, as that would allow Custap berries to mutate, which would occupy the empty plots. Purple squares indicate where mutations can occur.