Grassy Terrain is learned by 20 species by level, 111 by machine and 14 by egg

Effect, in PokéAPI's words

“For five turns, heals all Pokémon on the ground for 1/16 max HP each turn and strengthens their Grass moves to 1.5× their power.”

Who learns it

112 species in all, 7 games

Scarlet and Violet: 100 species

Species	Method	Level
Vileplume	Level up, Machine	1
Bellossom	Level up, Machine	1
Florges	Level up, Machine	1
Rillaboom	Level up, Machine	1
Smoliv	Level up, Machine	23
Dolliv	Level up, Machine	23
Arboliva	Level up, Machine	23
Flabébé	Level up, Machine	24
Floette	Level up, Machine	27
Oddish	Level up, Machine	32
Gloom	Level up, Machine	38
Calyrex	Level up, Machine	40
Comfey	Level up, Machine	48
Meowscarada	Level up, Machine	58
Shaymin	Level up, Machine	64
Wo-Chien	Level up, Machine	65
Bulbasaur	Machine
Ivysaur	Machine
Venusaur	Machine
Bellsprout	Machine
Weepinbell	Machine
Victreebel	Machine
Exeggcute	Machine
Exeggutor	Machine
Mew	Machine
Chikorita	Machine
Bayleef	Machine
Meganium	Machine
Hoppip	Machine, Egg
Skiploom	Machine
Jumpluff	Machine
Sunkern	Machine, Egg
Sunflora	Machine
Treecko	Machine
Grovyle	Machine
Sceptile	Machine
Lotad	Machine
Lombre	Machine
Ludicolo	Machine
Seedot	Machine
Nuzleaf	Machine
Shiftry	Machine
Shroomish	Machine
Breloom	Machine
Cacnea	Machine
Cacturne	Machine
Tropius	Machine
Turtwig	Machine
Grotle	Machine
Torterra	Machine
Arceus	Machine
Snivy	Machine
Servine	Machine
Serperior	Machine
Sewaddle	Machine
Swadloon	Machine
Leavanny	Machine
Cottonee	Machine
Whimsicott	Machine
Lilligant	Machine
Deerling	Machine
Sawsbuck	Machine
Foongus	Machine
Amoonguss	Machine
Chespin	Machine
Quilladin	Machine
Chesnaught	Machine
Skiddo	Machine, Egg
Gogoat	Machine
Phantump	Machine
Trevenant	Machine
Rowlet	Machine
Dartrix	Machine
Decidueye	Machine
Fomantis	Machine
Lurantis	Machine
Bounsweet	Machine
Steenee	Machine
Tsareena	Machine
Grookey	Machine
Thwackey	Machine
Flapple	Machine
Appletun	Machine
Zarude	Machine
Enamorus	Machine
Sprigatito	Machine
Floragato	Machine
Bramblin	Machine
Brambleghast	Machine
Toedscool	Machine
Toedscruel	Machine
Capsakid	Machine
Scovillain	Machine
Brute Bonnet	Machine
Iron Leaves	Machine
Dipplin	Machine
Poltchageist	Machine
Sinistcha	Machine
Ogerpon	Machine
Hydrapple	Machine

Brilliant Diamond and Shining Pearl: 17 species

Species	Method	Level
Vileplume	Level up	1
Bellossom	Level up	1
Roserade	Level up	1
Oddish	Level up	32
Gloom	Level up	38
Tangela	Level up	56
Tangrowth	Level up	56
Bulbasaur	Egg
Paras	Egg
Exeggcute	Egg
Chikorita	Egg
Hoppip	Egg
Sunkern	Egg
Treecko	Egg
Seedot	Egg
Turtwig	Egg
Cherubi	Egg

Sword and Shield: 36 species

Species	Method	Level
Vileplume	Level up, Machine	1
Bellossom	Level up, Machine	1
Roserade	Level up, Machine	1
Rillaboom	Level up, Machine	1
Oddish	Level up, Machine	32
Gloom	Level up, Machine	38
Comfey	Level up, Machine	48
Tangela	Level up, Machine	56
Tangrowth	Level up, Machine	56
Tapu Bulu	Level up, Machine	75
Bulbasaur	Machine
Ivysaur	Machine
Venusaur	Machine
Exeggcute	Machine
Exeggutor	Machine
Mew	Machine
Celebi	Machine
Treecko	Machine
Grovyle	Machine
Sceptile	Machine
Seedot	Machine
Nuzleaf	Machine
Shiftry	Machine
Cradily	Machine
Cherubi	Machine
Cherrim	Machine
Cottonee	Machine
Whimsicott	Machine
Lilligant	Machine
Maractus	Machine
Amoonguss	Machine
Lurantis	Machine
Gossifleur	Machine
Eldegoss	Machine
Zarude	Machine
Calyrex	Machine

Ultra Sun and Ultra Moon: 24 species

Species	Method	Level
Roserade	Level up	1
Florges	Level up	1
Tapu Bulu	Level up	1
Flabébé	Level up	24
Floette	Level up	27
Comfey	Level up	46
Oddish	Level up	47
Tangela	Level up	48
Tangrowth	Level up	50
Gloom	Level up	54
Bulbasaur	Egg
Paras	Egg
Exeggcute	Egg
Chikorita	Egg
Hoppip	Egg
Sunkern	Egg
Treecko	Egg
Seedot	Egg
Turtwig	Egg
Cherubi	Egg
Snivy	Egg
Sewaddle	Egg
Maractus	Egg
Skiddo	Egg

Sun and Moon: 20 species

Species	Method	Level
Roserade	Level up	1
Florges	Level up	1
Tapu Bulu	Level up	1
Flabébé	Level up	24
Floette	Level up	27
Comfey	Level up	46
Oddish	Level up	47
Tangela	Level up	48
Tangrowth	Level up	50
Gloom	Level up	54
Bulbasaur	Egg
Exeggcute	Egg
Chikorita	Egg
Hoppip	Egg
Sunkern	Egg
Treecko	Egg
Seedot	Egg
Turtwig	Egg
Snivy	Egg
Maractus	Egg

Omega Ruby and Alpha Sapphire: 18 species

Species	Method	Level
Roserade	Level up	1
Florges	Level up	1
Flabébé	Level up	24
Floette	Level up	27
Oddish	Level up	47
Tangela	Level up	48
Tangrowth	Level up	50
Gloom	Level up	54
Bulbasaur	Egg
Exeggcute	Egg
Chikorita	Egg
Hoppip	Egg
Sunkern	Egg
Treecko	Egg
Seedot	Egg
Turtwig	Egg
Snivy	Egg
Maractus	Egg

X and Y: 18 species

Species	Method	Level
Roserade	Level up	1
Florges	Level up	1
Flabébé	Level up	24
Floette	Level up	27
Oddish	Level up	45
Tangela	Level up	48
Tangrowth	Level up	50
Gloom	Level up	56
Bulbasaur	Egg
Exeggcute	Egg
Chikorita	Egg
Hoppip	Egg
Sunkern	Egg
Treecko	Egg
Seedot	Egg
Turtwig	Egg
Snivy	Egg
Maractus	Egg