You have to breed a female Venusaur with a male Pokemon that knows it out of the following:
Victreebel Exeggutor Bellossom Sunflora Grovyle Sceptile Shiftry Tropius Turtwig Grotle Torterra Snivy Servine Serperior
Source: http://pokemondb.net/pokedex/venusaur/egg#move-leaf-storm
