how cold does it get there? Cause really we shouldnt blanket horses unless there shivering constantly,sick,under weight... if you blanket them to soon their hair wont grow and also when there cold there hair stands up to keep the heat in and when the blanket is on they can't do that
This is all mostly true, but it's really not going to hurt the horse if he or she is appropriately blanketed (so not sweaty, ever, etc) and they perfectly healthy. There is nothing wrong with blanketing, as long as you do it properly, ie heavyweight blanket when it's REALLY cold, medium when it's sorta cold, light weight when it's not too bad and doubling up if it's extremely extremely cold.
Also, the blanket doesn't stop the hair from growing, it just slicks it down and makes it look less long. It's the amount of light the horse receives during the winter that determine how much winter coat grows in. So, a horse in natural light, even blanketed, will grow a full winter coat under the blanket, the hair will just be laid down and appear short. If that same horse is under lights all winter, he or she may not grow any extra coat at all if it gets a full 12 hours of light a day.
And, the thing with the hair laying down is why you want to blanket appropriately. You are making up for their hair laying down by blanketing them, so it's a terrible idea to put a lightweight, no fill, blanket on a horse in thirty degree (F) weather because you've just laid down the hair and not added any warmth. But a medium weight blanket would be just fine to put on in thirty degree weather since the blanket has enough fill to replace the lost hair mass.
Hopefully that all makes sense. :)
To the OP: When it's above 45 degrees (F) and sunny while being about 35 at night, I take my mare's blanket off. But, as soon as it drops below that, or it gets rainy/windy, her blanket goes right back on.
Once the real spring hits and she starts shedding, I generally take her blanket off and leave it off. However, I will still put it on if the temperature drops below 40 or if it starts raining cold rain.
Hopefully that helps answer your question. :)