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When the wielder is at 1/3 HP or less, all of its Grass-type moves will have 50% more power.
Most Grass-type starters, thanks to the Dream World, now have far better abilities than Overgrow. Venusaur now has Chlorophyll, and while its usefulness is debatable, Sceptile has Unburden. Torterra really can pick and choose between Overgrow and Shell Armor so if you do see it, that will be the Pokemon you see it with. Serperior has Overgrow as its only ability, but once it gets Contrary from Dream World, the chances of you seeing it use Overgrow are next to nothing.
Found in Dream World