What is henna?

Henna is a small tree or large shrub, growing to six meters high. It has lateral branches with leaves that grow in pairs, two to four centimeters long. Henna leaves have a red-orange dye, lawsone, and the highest dye concentration is in the petiole (the central vein). 

Young leaves have the highest petiole dye content.  Older leaves have lower dye content.

Henna will only grow where minimum temperatures stay above 50 degrees fahrenheit or 11 celsius. It will tolerate extreme heat and long droughts.  It grows wild near desert oasis, and in semi-aid regions. It thrives in alluvial soils, where there is annual precipitation of 0.2 to 4.2 meters and a soil pH of 4.3 to 8.0 (4.1-31).

Henna contains a red-orange pigment, lawsone, the molecule of which is also known as hennotannic acid. When henna leaves are crushed in an acidic medium and applied to skin, the lawsone molecules migrate from the henna paste, traverse the outermost layer of the skin.

 Why doesn't henna last forever like a tattoo?

Every day your body a grows new layer of skin cells below the surface of the skin.  These skin cells cells are interlocked and rise upward in columns, being pushed up by the new cells underneath.  As they rise in this column, they gradually die, dry out ... and become "corneated".  Corneated comes from the Latin word for "horn", and that means the cells get hard and horny... that is to say, that instead of being moist, soft and flexible, they get stiff and dry.  When they emerge at the surface of your skin, they are very dead, very dry, and getting loose.  These cells then fall off, along with the henna that stains them. The new layer of skin has a lesser henna stain than the layer that has just fallen off.  If you have very dry, flaky skin, you are shedding lots of corneated cells from your Stratum Corneum. That's why the amount of time that henna will stain your skin varies so much; it depends largely on your skin type.

For more information about henna, including diagrams of how henna stains the skin, download this .pdf file.

Pictures of the henna plant, flower and powder.