Pokémon with the Harvest Ability are able to reuse the same berry infinitely within the span of a single battle, with a 50% chance of restoring a consumed berry. However, during periods of strong sunshine, the chances of this effect taking place increases to 100%.
Harvest doesn't have an effect in the Overworld.
Pokémon With Harvest
Related ThreadsDoes Harvest work at the end of a battle? - last post by @ Apr 13, 2015
Epic Heart Scale Harvest - last post by @ Jul 4, 2010
How the hell do you harvest 15 berries >_> - last post by @ Sep 21, 2008