Overgrow

In-Game Text: Powers up Grass-type moves in a pinch.

In-Depth Description: Grass-type moves have their power increased by 50% when the Pokémon’s HP falls below ⅓ of its maximum.

This is the standard ability for all of the Grass-type starter Pokémon and their evolutions. Unfortunately, it’s not that useful on most of them, as they tend to be on the more defensive side of things, or they simply have better Hidden Abilities. One example of this is Venusaur, which gains Chlorophyll, and even Thick Fat upon Mega Evolution, making Overgrow an afterthought. Meganium is far too bulky and slow to do any decent damage in the first place. Torterra might be able to make some use out of it with a Rock Polish/Seed Bomb set, if you’re lucky. Serperior has Contrary, which is far superior (haha, get it?), and Chesnaught has Bulletproof, which makes it almost a perfect Gengar counter. That leaves Sceptile as the only decent user of Overgrow.

One common way I’ve seen this set utilized is through Substitute and a “pinch” berry, which activates when your Pokémon’s HP drops below a set amount. Petaya Berry Sceptile makes use of this to hit the foe with nuclear-powered Leaf Storms, but it can only do so much damage before switching out or being taken down itself. This is the only acceptable use of Overgrow that I can think of, and even in this case, Unburden is a better Ability choice, for the Speed boost it provides for Sceptile is far more beneficial in the long run.

Now that Mega Sceptile has been revealed, Overgrow is looking far less useful, as Lightning Rod gives Sceptile an immunity to Thunder Wave, making its sweeps much more difficult to stop.

Pokémon With This Ability Available:
Bulbasaur
Ivysaur
Venusaur
Chikorita
Bayleef
Meganium
Treecko
Grovyle
Sceptile
Turtwig
Grotle
Torterra
Snivy
Servine
Serperior
Chespin
Quilladin
Chesnaught
Pansage (Hidden Ability)
Simisage (Hidden Ability)