Let's see...
Kanto based games
Bulbasaur line, Tangela, Bellsprout line, Exeggcute line, Paras line, Oddish line (Bellossom is only available in Gen III games, additionally National Dex is needed),
Johto based games
Bulbasaur line, Bellsprout line, Exeggcute line, Oddish line , Hoppip line, Chikorita line, Paras line, Sunkern line, Tangela,
Tangrowth ,
Leafeon (needs help from a Sinnoh based game)
Bold names indicate they are only available in Gen IV games.
Italic names indicate they can be only caught by trading from Gen I games.
Hoenn based games
Tropius, Treecko line, Seedot line, Lotad line, Shroomish line, Cacnea line, Oddish line, Lileep line, Roselia line (Budew and Roserade are only available in Gen VI games)
Sinnoh based games
Budew line, Turtwig line, Cherubi line, Snover line, Wormadam,
Tropius,
Leafeon,
Tangela line, Carnivine
Bold names indicate they are only available in Platinum normally, they can be found and caught during pre-National Dex.
Unova based games
Snivy line, Sewaddle line, Cottonee line, Deerling line, Foongus line, Maractus line, Pansage line, Petilil line, Ferroseed line, Carnivine,
Budew line,
Tangela line,
Tropius,
Sunkern line,
Leafeon (Post-game)
Bold names indicate they are only available in Black 2 and White 2 versions normally.
Any hacks are welcomed as long as they provide a wide variety of Grass-type Pokémon. In case of non-Grass types edited as Grass-types, they can be used. While Grass-types that are no longer part of the said type
cannot be used.
So I can evolve Servine right away? Sweet! I'd like to do that!
Done, please pick a new partner.
This is me as well. It seems like there are so many Grass-types I view as feminine for some reason, likely because most of them have cute appearances or something. In fact every time I used Roselia's evolution line in a playthrough I always intentionally caught a female.
But as for that list, it is impressive that Hoenn has a lot of Grass-types in its Dex alone. I guess it's likely so that you have something that counters a lot of Water-types (even Ground to a lesser extent) that you'll be running into around the watery areas.
Even the name 'Roselia' sounds feminine. :)