My hair is long on top and short on the sides, can I dread? - Dreadlocks FAQ's
Sure. It is ideal to have it all one length but as long as all your hair is at least 3" long you should be fine. If the hair on the sides is really short and you can't dread it you can always start the dreads on the top and dread the sides later, keep the sides shaved short, or just wait until the sides are long enough. I've seen all three look cool but that doesn't mean a whole lot....just that it's possible to pull off nearly anything with dreads, it's hard to go wrong. If you are in it for the long term though, and want to keep your dreads for like 6 years, I would recommend growing it out to one length, making the sections perfect, and doing them as well as you can, you'll be glad you did.