How do I dread my roots as they grow? - Dreadlocks FAQ's
Well Rastafari believe that Jah forms your locks and the shape of your locks is influenced by your personality. I'll go with that for the most part but I also believe that Jah helps those that help themselves. There are quite a few things you can do to ensure that new growth locks as it grows.
The roots are, by definition, the point where the hair grows out of the scalp. Since the hair follicle's continue to push the hair out straight and unknotted (the knots don't ever come pushing out of your scalp pre-formed) you'll always have some undreaded or straight hair at the roots. It doesn't seem possible that straight hair held at both ends could lock at all. The hairs that are actually doing the locking are the new baby hairs growing out of the scalp and the ends of the old mature hairs that just fell out but can't go anywhere cause they are being held in place by the rest of the dread.
Having some straight hair doesn't mean your roots will look bad, far from it, they can actually look supa nice. With that understood what you want to do is keep the roots looking clean by taking care of loose hair and keep the new growth knotting as is grows. As the dreads tighten your hair will dread closer to the roots with less help, especially with larger dreads and textured hair.
The best way to knot up new growth (the straight hair at the roots) is to use clockwise rubbing. You'll also find that washing residue free shampoo makes it much easier for new growth to knot - not only while you're clockwise rubbing, but also on it's own, which will save you a bunch of work!
While your dreadlocks are new it's helpful to have rubber bands on the roots of each dread. Please see the FAQ question about how to wear bands on the roots properly before you put the bands on.