My parens are strict and want me to get a haircut :/???!?!?

I need to know how to let them know I want to let my hair grow out... They think it looks dirty and clean haircuts are always better, but I really love long hair, how can I convince them to let me grow it out ove the summer ?!, I'm almost 13 and it's really bothering me. I got my phone tooken up for a week just for that, I just got it back just because I said I would get a haircut this Saturday :( please help on how to convince such monsters :)