For me, wearing the hair up means that the hair is bunned or bunched in some way (in contrast to plaits or pony tails), so it doesn't sound like the contrast that NHHL is looking for. (I encountered 'half up, half down' for the first time in discussions about Kate Middleton's hairstyle for her wedding. )
I like Copyright's suggestion of leaving the hair loose. (I'd also not heard of hairtie before - the usual term here is hairband.)
But then again, what do I know about ladies' hairstyles?
When hairs are not tied, they stay the nature way. How do you say when hair stays the nature way. As when talking to the hair dresser, I say I normally don't tie my hair/normally my hair (here is the word I need), could you give me a hair style that looks pretty when my hair (here is the word I need)?"