You cant get grass knot until you defeat the Pokemon league. Its in pinwheel forest. But you can get energy ball in aspertia city by surfing in the southeast corner. There is also solarbeam which is also found in pinwheel forest (only accesible after the Pokemon league).
-So you only choice is energy ball which is found by surfing in aspertia city.
-But if you need a grass type move, you could also go to the move tutors where you can teach seed bomb for 6 red shards in driftveil city or giga drain for 10 yellow shards in humilau city