I've had a Rastafarian coworker back in my Heineken period, and he told me that true Rastafari's rarely ever wash their hair. They take care of it in another fashion. Personally i think they're just lazy.
And to be honest, besides the penetrating ganja scent that followed him everywhere he went, he smelled a bit funky. His dreads were so long, they reached all the way down to his knees, so he mostly had them in a knot, or stuffed in one of those hats in the colours of the Jamaican flag. I guess the only way for him to get clean again is a big sharp razorblade, and get it all off. There isn't a shampoo in the world that can deal with something like that.