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I'm not an expert on henna by any stretch, but I have applied it to my hair enough times to have come up with a pretty good system. I dabble in mehndi, but that's a post for another day. There is a lot of great information available on henna and how to get the most out of it on your hair, most if it at Henna for Hair. But, I keep my hair short, Jean Seberg short, and there isn't a lot of information (at least, not that I could find) on applying henna to short hair. So, through a bit of trial and error, this is the method I've come up with.
For starters, I've found it's best to henna hair just before cutting it. There are too reasons for this. First, the shorter the hair, the more apparent the roots; so if you don't feel like a day of hair maintenance, dye first and you won't have to walk around with obvious roots. Second, longer hair holds the henna in place. Henna is the consistency of mud, you don't want a mudslide down your face and neck. Just like trees, grass, and other plants prevent mudslides down a hill, hair prevents a mudslide on your head. This is also the reason you will want to mix your henna a bit thicker than the stirred up yogurt consistency that is normally recommended. Something a bit thinner than mashed potatoes works well when applied to my dry hair.
Keeping the mix simple works best, just henna, water, and lemon juice (lime juice and chamomile tea work in a pinch). Henna for Hair has an entire page devoted to the different things people mix into their henna, but adding terps is a bad idea, especially if you have short hair. Terps, or terpenes, are what you add to henna to get a better stain on the skin, but they have no effect on staining the hair.
When dying short hair, it is more important to get to the very base of the roots, especially around the face. At first, I was afraid of staining my face, so I erred on the side of not covering the hair, and keeping the henna off of my skin. This left me with bits of brown sideburn thingies (technical term) and auburn hair. It looked odd. I decided it was better to have a slight orange halo around my hairline for a day or so, than to have stray bits of different colored hair. This is why it's important not to terp the henna. You need to get the hanna on and covered fairly quickly, as it is only effective while it's wet, so it isn't likely you will be able to be incredibly precise.
Because the henna needs to be prevented from drying out, you need to wrap your hair in something like plastic wrap or a shower cap. I use plastic wrap. To hold it on around the edges, to prevent the weird sideburn thingies, I use petroleum jelly or Aquaphor. It is best to apply it after the henna. If it is applied before, it may coat some of the hair, which will keep it from accepting the henna. If it's applied after, and gets onto the hair, it will help keep the henna damp. When I apply the plastic wrap, I tear holes where my ears are, and wrap the ends around my ears, pressing it into the petroleum jelly. Then, I put on a cap I've designated as my henna hat, and wait four hours.
It isn't difficult to wash henna from short hair. It is usually recommended to wash the henna out with conditioner, but I don't find it necessary, and am allergic to every commercial brand I've tried. I simply use castile soap.
Henna will oxidize over a couple of days, so if the initial color seems overly orange, just give it some time, or hit it with a blow drier. You may want to henna your hair more often than people with long hair. The more times hair is hennaed, the richer the color becomes. Short hair doesn't stay on your head very long, so it doesn't have much opportunity to take in the henna, unless you decrease the time between applications. I prefer the color of my hair after two treatments, so I tend to henna twice in fairly quick succession. I usually henna when my scalp starts to itch, since it helps with my eczema/psoriasis.
Any extra henna mix can be frozen in a freezer bag. This makes the next application incredibly simple. Thaw out the henna, snip the corner off of the bag, and apply the henna to your hair like icing a cake with a pastry bag.
Henna on Short Hair Quick Tips
*Visit Henna for Hair
*Henna when your hair is at its longest, just before a cut/trim.
*Mix henna to a thicker consistency than normally suggested, something in the runny mashed potato range.
*Don't add terps to the mix.
*Be sure to cover all of the hair, even if it means getting a bit of the henna mix on your skin.
*Don't henna just before a big event, in fact, it's best if you don't have to be in public for a day or so.
*Apply petroleum jelly or Aquaphor (or a non-petroleum based alternative) around your hairline, after you've applied the henna.
*Wrap your hennaed hair in plastic wrap, pressing the edges into the petroleum jelly. Remember to make holes for your ears, you don't want to listen to everything through crinkling plastic for four hours.
*Put on a cap, it's far less cumbersome than a wrap, and your short hair will fit inside nicely.
*Use conditioner to rinse out the henna if you like, but shampoo or castile soap will also do the trick. Just make sure the henna doesn't dry out before you wash it out.
*Allow the henna to oxidize before judging the color.
*Henna again if you want a richer color.
*Scoop any extra henna into a freezer bag, and stash it in the freezer. When you are ready to henna again, thaw it out, nip off the corner of the bag, and use it like a pastry bag to apply the henna to your head. If you don't use all of that, put the bag in another bag before freezing it.