You have to beat the gym leader in Canalave City. you get the hm in the lost tower on the way to Soclan town.
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You have to beat Erika, who is the fourth Gym Leader, raises Grass-Type Pokemon, and resides in Celadon City.
I believe that is after you've beaten the 4th Gym in Volcanoridge Town.
ieryjioet
you use strength. you get strength on route 42. you can use it in the wild after you beat the goldenrod city gym.
By defeating the the fourth gym leader