Since we've been getting these posts about every 3 days now that winter has hit for the Northern hemisphere, I figure I'd give the solution its own post, and stick it in the memories.
This process is COLOR SAFE and good for all hair types.
It's also a lot of information, so follow
First, make sure you actually have dry scalp!
Most people do, fungal dandruff is pretty uncommon, even though that is what dandruff shampoos are designed to treat. Dry scalp is white flakes, that tend to clear up immediately after a shower, but return. Fungal dandruff is linty or pilly, grey, yellow, or greenish, and does not go away with showering.
Fix the symptoms
Moisture is key:
1. Take a tablespoon or two of olive oil (extra virgin is my preferred, it's heavier and more nutritious) and massage it in to your scalp with your finger tips. Not nails, tips. Oiling your fingertips will help you get oil on the scalp, putting it in your palm tends to apply it to the hair on top.
2. Wrap in plastic and let sit for 30 minutes or longer. The longer the better, so more oil can absorb.
3. Wash out. Start with a rinse of diluted apple cider vinegar in water, about 1:4 ACV:Water. If you don't have ACV white vinegar is totally fine, if you only have red wine or rice wine vinegars they are fine too, but check the acidity. ACV and white vinegar tend to be 5%, others may be stronger or weaker, and might be better with different concentrations. Try to use mostly vinegar rinses to get the oil out, but you will have to shampoo some until the hair doesn't feel greasy any more. If it still feels greasy in the shower, it's going to be really gross out. Don't worry about smelling like vinegar, that will go away when hair dries. Condition as usual.
Make it fancy:
If you want, you can add sugar (white or brown, I would avoid sugar in the raw as it's too big/rough) or cornmeal to the oil for some extra exfoliation.
Honey is OK to add as well, it's moisturizing and quite nice. However, it causes big nasty dreads while it's in, which makes the massage part difficult. It will wash out easily though.
Essential Oils: If you must, use A DROP of something like lavender, rose, sweet orange, or jasmine. Notice that tea-tree is not in this list. Tea tree is astringent, and will dry things out. If you buy essential oils, be sure you are getting a true essential, not a synthetic or 'perfume oil' Synthetics and perfumes don't have the healing properties of true essentials. Rose and Jasmine are very expensive EO's, so most of the time you will find either diluted EO's for perfume use (companies like Simplers), or synthetics. Know what you are getting.
Keep doing the oil treatments, starting with every couple of days, and tapering off as symptoms improve. Once you have things under control, feel free to switch to a lighter oil, such as lighter olive oils, sweet almond, apricot, or jojoba. Do not use grapeseed, it is astringent.
Stop the cause
Change your products and routine!
Shampoo is a major culprit here. The detergents in shampoos are generally harsh and strip moisture from the scalp along with dirt. Many shampoos, even those claiming to be gentle use too many. Sulfate free is ultimately the way to go, but if you can't find that, try for only one sulfate. The best sulfate is Sodium laureth sulfate (SLES, although there are some sources that say it's more carcenogenic than others), followed by sodium lauryl sulfate (SLS), then the Ammonium laureth sulfate and ammonium lauryl sufates. Origin of the sulfate doesn't matter much, so when you read JASON (I believe) brands SLS from coconut! thing, don't bother, unless it's the best you can do. Check the ingredients on single sulfate shampoos as well, they may have sulfinates, sulfico-somethingorothers, etc. If it sounds like a sulfide, it's a detergent, which isn't bad, but sles +3 sulfinates isn't good either. ALSO CHECK CONDITIONERS! you'd be shocked how many contain a sulfate.
Check the memories at longhair for lists of sulfate free shampoos. There will be an adjustment period, be patient.
-cones. Ingredients like silicone or dimethecone that end in -cone aren't directly a culprit in dry scalp (hair yes, scalp, not so much). However, they can cause major buildup, and result in limp and greasy looking hair. They frequently need some heavy sulfates to get rid of, so tossing them isn't a bad idea. If you must use products with -cones, add a vinegar rinse to your routine to help cut them. if you drop -cones, don't be surprised if your hair suddenly seems really damaged. It's not more damaged, you just see it more now that the cones aren't gluing everything together. Trim and move on.
The less you wash, the less often oils are stripped from your scalp. Elementary, but culturally many of us are caught up in a cult of cleanliness that has far reaching consequences. It does take time for your body to adjust to this, so be patient. Look into between shampooing options like conditioner only (CO, check longhair), dry shampoo, or no-poo (again longhair or naturalliving). I personally use a little jojoba or aloe to condition after a rinse, my hair tends to be dry and it works nicely.
Alcohol, butyl, and butane are common in styling products. The alcohol in hair spray or gel is much different from the fatty alcohols in conditioner and shampoo, so don't worry about cetyl or stearyl alcohols in cleansing products. Unfortunately, tossing these three ingredients leaves you pretty much without options. Aloe gel works for a very light hold gel, there are a few volumizing products I've found that have none of these, and work as light hold hair spray. There are a few natural options, like flax seed hair gel, and lemon+sugar hair spray, but they can be complicated, and lemon juice is acidic enough it can contribute to dryness, plus change hair color.
Bad, bad, bad. Hot water, heat styling, blow drying. Cut back as much as possible on all these. Shower in water that is warm, but not hot. Save the heat styling and blow drying for really special occasions.
Stay hydrated! Especially during winter months, all your skin needs water. Drink a lot of it. Make sure you get some good oils, you can supplement with ONE of the following: vitamin E, Flax seed oil, evening primrose oil, fish/cod oil. Take as directed, with E watch for easy bruising and cut back if you experience it. If your diet includes a lot of oils like these, don't supplement. Eating fatty fish like salmon, or using olive oil frequently (preferably unheated) can give you enough of the 'good' fats, and too much of any fat is bad.
Heaters can be drying. If you can, a humidifier in your room may be a good idea. Alternately a bowl of water placed on a radiator and changed frequently, or just left in the room can help some. Use moisture like this with care, some environments need it, some don't, and don't go overboard as that can encourage mold.
Wind and severe cold can happen in winter. Wear a hat when you are out in these conditions to help protect your scalp.
Use products designed for dandruff. Medicated shampoos are not what you need, they contain anti-fungal agents, which can disrupt natural balances. Most of them also have really harsh ingredients-Head and shoulders for DRY SCALP has Sodium lauroyl sarcosinate, Ammonium lauryl sulfate, AND Sodium sulfate.
Tea tree oil is touted as the cure-all. It's fantastic, and I support people using it and other natural solutions to problems. However, it is an anti-fungal/bacterial, and unless that's the cause of your dandruff, it's unnecessary, potentially harmful, and drying.