That is normal. Those roots will wind out of any drainage holes you may have. You have something under the pots to catch the water, right? If so, the roots that come out will drink that. Try to make sure there is not much standing water in the dishes underneath which could be a breading ground for algae which would then effect the roots. Just because the plant has reached the bottom, does not mean it is root-bound. In fact, they reach down first, then progress laterally. The topping will control their size. When your tallest is 18 inches, I'd flower so It's not to dense for that light to penetrate. That light will do a 3 by 3 area pretty well. Keep the light as close as you can without heat burn. Keep a breeze on them. You're looking good.
-Dude