How long does my hair have to be to dread? - Dreadlocks FAQ's
If you don't mind a little extra maintenance you can start them at about 3". You will spend more time dreadballing and putting in loose hair if it's started this short. Rubber bands will help keep them together until they start to lock up. Rubber bands never look "cool" but they really help when the dreads are new....and if they are really short like 3", they are more and more necessary. If you can wait until your hair is at least 4" we guarantee it will lock using the Supa Dupa Dread Kit.
If you don't mind waiting, some extra length will reduce maintenance and make things go a lil easier. Six to eight inches is pretty ideal. It won't take long to put them in at this length but they will be long enough that they won't be quite so delicate.
Locking up very long hair has it's advantages too. Namely you get to start out with longer dreads and longer hair is more forgiving over the first few months. The downside is that it can take quite a while to backcomb longer hair, especially if you are making smaller sections and doing lots of dreads. If this is you, I'd highly recommend putting together a dread party to help you put them in. One or two people are going to be working on it for well over 8 hours!