Roots Techniques -with video!
To prevent your hair from darkening more than you wish, once your hair is the color you like, you should stop putting more henna on the length and just touch up your roots (in other words, only re-henna your length if you want more color, or if you have hennaed multiple times and reached “maximum henna saturation” where your hair doesn't dark any further when you re-henna). You'll use the same mix and the same timing as you do for the rest of your hair. Using leftover paste from last time that you popped into a zip lock bag and tossed in the freezer make this process very quick and convenient.
This works the same for cassia/henna/buxus mixes and cassia/henna/indigo mixes as well. If you have frozen paste that contains cassia (either cassia alone or mixed with other plant dyes), the cassia will still work, but if it contained buxus or indigo, those blue/green dyes don't survive freezing well and will be pretty well demised at that point. To make up for that, you'll add freshly prepared buxus or indigo to take its place. (Keep in mind your frozen henna/indigo paste will act like diluted henna because the indigo/buxus portion of it is now inert, so you will reduce the amount of fresh indigo/buxus to accommodate for that.)
There are a number of methods to apply just to roots. Here are a few popular ones that users have described on the forum:
Tint brush: Part hair in the middle, dab paste on roots with tinting brush, flip that section over to the other side. Section 1/4 to 1/2 inch away and repeat. Once you get to ear level, flip all of that hair back over and start again in the middle, sectioning and pasting row by row down the other side. Then do the back, which won't be as precise since you're doing it by feel (if you have another mirror behind you, you can get the sections more precise). This only takes about 15 minutes to apply this way. Instead of wrapping your head in saran wrap, you can use a disposable thin shower cap and cover with a tight-fitting knit hat to keep the air out.
Condiment bottle or tint bottle: Get one of those empty condiment bottles sold in the grocery store, or a tint bottle from a beauty supply store. It's good if it will hold at least a good 12 ounces or more of paste, but you can make do with a smaller one if necessary, refilling it if you run out. Cut the hole in the tip large enough to avoid getting clogs from any small lumps in your paste (a larger hole than you would need for a chemical dye). Apply to sections as above, or zip in and out randomly if you are not a stickler for even coverage. Apply plastic wrap, grocery bag or tint cap, mush gently to make sure all of the roots are covered, and put on a warm cap.
Root shooter: This is basically a big syringe sold by Mehandi/Ancient Sunrise. You can use it to part your hair and to apply to the roots. Make the mix a little thinner than usual and try to avoid getting air in the shooter; it can make it sputter a little when applying. If your hair is long enough, you can put lots of little braids all over your head, and put your beads of paste in between them.
Frozen baggie: Freeze a bag of prepared henna paste (your leftovers from last time work well). Thaw it, mash it up until it is smooth, the consistency of thick custard. Snip a corner off the bag and squirt a bead of henna paste down your part, then on the rest of your scalp as above. Using gloves, mash it around at root level to even out the coverage. Cover with plastic bag or shower/tint cap, and top with a ski hat.
Quick-n-dirty: Make your mix quite thin, add conditioner to it and hang your head upside down over the bath as if washing your hair, then rub the mix all over your scalp with fingertips (like when you shampoo your hair). When you feel you have all the roots covered, wrap your head as usual. This is the least precise method so not recommended if you are covering gray or need to avoid overlapping onto previously dyed hair to avoid going too dark.
If you add conditioner to your roots mix, it will be runnier. To keep runs and drips to a minimum, you can add a spoonful or two of cornstarch or instant pudding mix, stirring very well.
Check out this link to see a video of the lovely Deborah Brommer demonstrating how she does her roots. www.youtube.com/watch