I live in Colorado and it is that time of year that I spend a lot of time putting on and taking off blankets. You see, Denver's weather is what I call Bi-Polar...bitter cold one day, shirt sleeves the next. This makes it hard for my poor horses's winter coats to know if they are really needed or not.
Horse people are always discussing that eternally significant question: Should I blanket my horse or not? I saw this funny post on facebook from Auburn University:
I thought it was pretty funny! I especially love the question, "Is your horse a wussy?" --Yes!--Your horse probably needs a blanket.
I have read all sorts of articles arguing one way and the other about whether or not to blanket a horse. Those opposed claim that the blanket will compress the natural thick winter coat and thus destroy the natural insulation it provides.
On the other hand, most articles said blankets would probably be best if the temperature is below 10 degrees Fahrenheit or the horse is not able to find shelter, especially from wind. Wind seems to be more of a factor than cold.
Of course, if your horse is old or sick, he would need a blanket.
If he is accustomed to being in a stable, he would need a blanket if suddenly put outdoors.
If he is clipped, obviously he needs a blanket in cold weather.
One result of blanketing is that it prevents the winter coat from getting as long and thick as it might otherwise become. This is good if a horse is being worked a lot in the winter so that he doesn't get over-heated and it becomes hard to cool him off. This is my situation. I take my horse from an unheated stable to a heated arena for my dressage lessons. Even though I blanket him, he gets quite sweaty during our lessons.
Another reason to blanket is to help keep the horse clean. Dust helps insulate the unblanketed horse but that means a lot more grooming to get ready to ride.
After my reading I concluded that when all is said and done, it is up to you! (With the noted exceptions listed above.)