Most often I hear the "dirty hair" thing from people who are thinking about starting locs, curious about locs or just not familiar with natural hair at all and are listening to the old wives tales about caring for our hairtype.
The bottom line is NO. Dirty hair does not grow faster than clean hair. And let me explain why some may think that it does.
The problem we have with washing our hair is not as much the frequency of washing as the type of products being used to wash it. And using inappropriate products combined with the amount of manipulation we do when we wash and style afterwards can cause hair breakage. Hair breakage can be mistaken for a lack of hair growth.
Shampoos containing SLS (Sodium Laurel Sulfate) and SLeS (Sodium Laureth Sulfate) and ALS (Ammonium Laurel Sulfate) can be downright dangerous to afro hair and to your scalp. More on Sulfates here.
These chemicals strip the hair of all oils and protective natural moisturizers. They also strip the scalp, causing dryness and dandruff. This is why your hair becomes dry after using them, no matter how much condish you slather on afterwards. The reason conditioners became necessary after shampooing in the first place is because of the stripping properties of the sulfates.
We use sulfates as driveway degreasers. That says it all.
If you are a coal miner, work in a dirty environment, sweat a lot have a tendency to get grime into your hair, or have overly oily hair then you will need something to cut through the grease and grime that builds up on your hair. But if you are like most of us, who work in clean environments, offices, supermarkets, libraries, schools then you do not need to use a sulphate based shampoo every time you wash your hair. You can use something sulfate-free and get a better result. And you can wash as often as you feel is necessary without damaging your hair.
Overmanipulation will cause hair to break off. Excessive combing, detangling after shampooing, without the necessary degree of care and using the wrong tools will cause breakage. If you detangle incorrectly or roughly, you wil experience breakage and this gives the illusion that your hair is not growing.
Look into some natural shampoos, especially shampoo bars. Read the ingredients on your product bottles. Stay away from sulfates and be gentle with your styling. Your hair will grow better, healthier and longer if you omit sulfates from your routine.
In summary, dirty hair does NOT make your hair grow. Harsh shampoo can dry your hair out and cause breakage, which gives the illusion of your hair not growing, when it is in fact just breaking off. However if you use the right tools and products, you can have fast growing, clean hair.