"Comb your hair!"
--Uncle Vernon to Harry, frequently
The wild state of Harry’s hair was a constant irritant to Vernon Dursley:
About once a week, Uncle Vernon looked over the top of his newspaper and shouted that Harry needed a haircut. Harry must have had more haircuts than the rest of the boys in his class put together, but it made no difference, his hair simply grew that way — all over the place. (PS2)
He shot a nasty look sideways at Harry, whose untidy hair had always been a source of great annoyance to Uncle Vernon. (PA2)
Aunt Petunia sometimes took matters into her own hands, but it didn’t really help:
Once, Aunt Petunia, tired of Harry coming back from the barbers looking as though he hadn’t been at all, had taken a pair of kitchen scissors and cut his hair so short he was almost bald except for his bangs, which she left “to hide that horrible scar.” Dudley had laughed himself silly at Harry, who spent a sleepless night imagining school the next day, where he was already laughed at for his baggy clothes and taped glasses. Next morning, however, he had gotten up to find his hair exactly as it had been before Aunt Petunia had sheared it off He had been given a week in his cupboard for this, even though he had tried to explain that he couldn’t explain how it had grown back so quickly. (PS2)
All of this was evidence that he was a wizard, as Hagrid pointed out to him:
Harry looked into the fire. Now he came to think about it… every odd thing that had ever made his aunt and uncle furious with him had happened when he, Harry, had been upset or angry… dreading going to school with that ridiculous haircut, he’d managed to make it grow back… (PS4)
Harry inherited this untidy hair from his father, which he noticed when seeing his parents in the Mirror of Erised:
The tall, thin, black-haired man standing next to her put his arm around her. He wore glasses, and his hair was very untidy. It stuck up at the back, just as Harry’s did. (PS12).