After defeating Roxanne and getting the HM Cut from the Cutter's House in Rustboro City,teach the HM Cut to one of your Pokemon.I think Treeko and its evolutions,and Torchic and its evolutions can learn Cut.I don't think Mudkip and its evolutions can though.If you don't have a Pokemon that can learn Cut,catch a Nincada on Route 116 near Rustboro City.Nincada can learn Cut.Then go to he Petalburg Woods and cut Down those trees near the Exit of Petalburg Woods.There will be a Girl in there and talk to her.She will give you the Miracle Seed. When held by a Pokemon,the Miracle Seed raises the power of its Grass Type Moves.
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You can find a Miracle Seed on Route 32.
Try catch a Tropius, some of them hold miracle seed.
It boosts the power of grass-type moves
South of Violet City. There is a man standing in between some trees and a rock. He will not battle you, but if you talk to him he will give you the Miracle Seed.
You can find it on Route 32 after beating Falkner.