Whenever someone uses the word "spiritual" someone else is bound to be offended, so I'm going to try my best to be as sensitive as I can possibly be while writing this. I'd appreciate it if anyone reading this keeps in mind that I am not promoting any of these ideas, nor am I against them, I just ran across them and thought they were interesting. In all honesty I'm the type of person that believes in science and finds any type of spiritual information simply an interesting way to look at things, nothing more.
Cutting hair by the moon
The thing I see most of the time is cutting hair by the moon cycles. The idea behind it is that the moon has a gravitational and magnetic effect on the Earth and on us. Some examples of these effects (backed up by science or not) are: the tides of the ocean, the full moon brings out the crazies, lunar cycle is associated to a woman's period, etc. In relation to hair it is said that cutting hair during a waxing moon will help your hair grow faster, cutting it during a waning moon will help it thicken but slow down length gain, cutting during a full moon is said to have different effects depending on where the moon is in reference to astrological signs and whether it is waxing or waning, the same goes for the new moon.
In my opinion cutting hair doesn't affect the roots much. After reading the Dr. George Michael book I came to the realization that hair cutting can affect the roots based on the weight they put on it (how long it is) and the scalp trying to balance itself out weight wise, but I don't really see a connection between this and the moon unless the gravitational pull really does affect the roots. However, if one is going to cut/trim their hair anyway I see nothing wrong with cutting it on a day that could give them the results they want according to the moon calendar. In fact it could have a placebo effect and actually do the thing it is said to do. Also it can't hurt, so if you're into that kind of stuff my advice is to go ahead and cut by the moon.
Here are some links with more information and actual calendars that tell you when and what:
Lunar Hair Chart & Hair Care (scroll down for the calendar)
hair growth and moon phases, Cutting Your Hair By The Phases Of The Moon, Does Moon Really Influences Hair Growth? Truth Behind the Superstition, Farmers' Almanac's Best Days to Cut Hair to Increase Growth.
Hail length and how it relates to spirituality
This is a subject that confuses me, the more I read about it the more confused I get, there is a bit of contradiction on the subject.
Some religions claim that long hair (especially on women) is a sign of pride and leads to all types of sin and temptations for everyone. This view claims that hair should be kept short to remove temptation, or should be covered at all times (by a hijab for example). Scientifically speaking hair is a huge part of attraction and how we view people, so there might be something to that claim. Then again nerfing the world against attraction is not the way to prevent "sin" or attraction. Temptation is all around us no matter what, even if we were all blind we would still feel attraction based on smell/pheromones, etc. So really this is not a solution, it is just a glorification of lack of self control. In my opinion self control is important and it is like a muscle, the more it is exercised the stronger it is, psychological studies back this claim up.
The other side of the coin is that cutting hair is like trying to improve on god's creation. In other words hair has the ability to grow to certain lengths and should not be interfered with because that is how it was intended to be. This view supports the idea of not cutting hair and letting it grow as long as it can. This idea really confuses me. On one hand trimming hair helps it grow to longer lengths by preventing breakage and spit ends from wreaking havoc, but a lot of that damage can come from hair products which are not always natural. On the other hand maybe it is suppose to break so it doesn't get too long to interfere with survival, this is more of an evolutionary point of view and would apply to prehistoric times when there were no such things as combs, hair ties and so on. So does that still apply today with all the tools we have at out disposal? *shurg* No idea. I've read some things about terminal length hair (hair that is as long as it can grow) being a type of blanket that keeps the body warm and helps diffuse sunlight. There are also some claims that more length can help by gathering more information from the environment and possibly give a better warning against predators/dangers.
I've seen articles on native Americans being excellent trackers but losing their abilities when their hair was cut. A lot of people say this is all false and there is no scientific proof of this. But it does make me wonder. In my opinion it is possible, but not definite. I'm not sure about it because of the lack of studies on this but I can see how that would be possible. If long hair is down it moves with any small vibration because hair strands are very light, and that movement can send a message to the root which would signal any small change in the environment. At the same time our sense easily get used to constant signals from the environment so that we can only sense changes in it, as opposed to having our senses constantly flooded by signals that are not important. This is called habituation and it is the reason we are not always aware of things like our clothes, air around us, etc. but we do feel a change if it happens whether it is in temperature (from AC to outside for example) or tactile (if there is liquid on our clothes that penetrates to the skin) etc. All of this information still just makes me think it is possible but I can't be sure.
And now for the more extreme claims. I'm just going to mention these without comments because they are extreme, basically unproven, not researched and some people feel very strongly about them one way or another.
Power: The hair of Samson was the source of his power, according to the bible and when it was cut he lost his power.
Luck: In Yao, China it is believed that long hair brings luck.
Communicating with aliens: The Vril society believed that very long hair helps pick up signals from space, messages from aliens.
Magic: Like in the new movie Tangled where Rapunzel can heal people with her hair, there are many magical claims about hair. I've already mentioned power, luck and communicating with extraterrestrials, but there are also: being as in touch with your natural self as possible (not altering your body so that it may be in tune with nature), putting time and energy into something (caring for it) gives it power, and picking up more energy from the environment (on Earth, not from aliens).
Being in touch with nature and caring about the environment in hair care
Part of the magic and hair being as it ought to be is also using all natural products on hair. No I'm not talking about the stuff in plastic bottles that say "natural" on the label but are made from ingredients that can't even be pronounced by most people. I'm talking about using herbs, water only, mud or baking soda, or nothing at all to wash hair, conditioning it with oil and humectants, etc. I've written many entries on the natural alternatives for hair care. There is also the other side of the coin where natural products do not harm the environment and thus do not contribute to our current environmental problems. It can also be argued that using natural products on your hair puts you more in touch with nature and thus it can be a magical/spiritual experience.
This was a very fun entry for me to write. I love learning about different cultures and religions. In closing I'd like to say that whatever you do with your hair there is probably some spiritual belief somewhere supporting your choice and one that is opposing it, so just do what feels right for you or what your belief system tells you to (if you want to follow it). At the end of the day it is just hair and whatever reason we have to keep it long or short is our own, and that's how it should be. It is part of our body after all, and decisions concerning it should be made individually.