If it were long, I would use 'haar' as well. So you are right about the singular form being the 'normal' one. Sometimes, the plural is used (preferably), but I cannot explain when (because when that happens, hte singular is also acceptable). In this sentence, you can switch to 'korte haren', it is not wrong. ;)
Thank you. I guess it's just one of those peculiarities I'll have to live with. ;) I did understand it either way, but despite the length of time I've been regilding my tree, I don't think I'd ever come across the plural version before. It's handy to know it can sometimes be that way, even if I wouldn't use it myself.
In English, when I started going grey, I suppose you could say I had "some grey hairs" (if it's still distinct individual ones, we'd use the plural). I think now it's more than a few, we'd just say I have "some grey hair" (not "hairs")
This seems to be the case
"You say "het haar" when talking about hair in general (such as body hair) or your haircut/style/do."
"You say "de haar" when talking about a single hair, or "de haren" in case you mean a collection of single hairs as opposed to a hairdo."