Grass-type Pokémon

Pure Grass-types

Strong against Electric, Grass, Ground and Water attacks; weak against Bug, Fire, Flying, Ice and Poison attacks
Tangela
Chikorita
Bayleef
Meganium
Bellossom
Sunkern
Sunflora

Bug/Grass-types

Strong against Electric, Fighting, Grass, Ground and Water attacks; weak against Bug, Fire, Flying, Ice, Poison and Rock attacks
Paras
Parasect

Grass/Flying-types

Immune to Ground attacks; strong against Fighting, Grass and Water attacks; weak against Fire, Flying, Ice, Poison and Rock attacks
Hoppip
Skiploom
Jumpluff

Grass/Poison-types

Strong against Electric, Fighting, Grass and Water attacks; weak against Fire, Flying, Ice and Psychic attacks
Also weak against Bug attacks in Red, Yellow and Blue Versions
Bulbasaur
Ivysaur
Venusaur
Oddish
Gloom
Vileplume
Bellsprout
Weepinbell
Victreebel

Grass/Psychic-types

Strong against Electric, Fighting, Grass, Ground, Psychic and Water attacks; weak against Bug, Dark, Fire, Flying, Ghost, Ice and Poison attacks
Exeggcute
Exeggutor
Celebi