I got  rootlings of 4 henna plants from Natasha Papousek at Sirus Rising, at the henna conference, and they have grown very nicely in the south window over my desk in the last 7 months. 

I water them with distilled water, when their soil has gotten very dry.   If they look "leggy" or whimpy, I pinch them back and give them a heavy drenching with water and indoor plant food or Miracle Gro.  They respond extremely well to pruning, and put out new growth promptly. 

Once they got an attack of spidermites and aphids ... but I dusted them with my cat's flea powder, and they haven't had any problem since. 

This picture is of the plants in February, when they have been looking out the window for 3 months at nothing but grey skies and snow .... I expect them to put on new growth rapidly as the sunshine hours increase with springtime.  In hot humid summertime, henna houseplants can shoot up very rapidly to 6' tall!
 
 

When you're planning to raise henna plants at home in a cool temperate climate, you should try to approximate the conditons that they most love where they grow naturally!  They grow along homes in the Sudan, in yards in Cairo, between rose bushes in India, along oasis and roadside damp spots in Saudi Arabia, in southern Iran, alongside vinyards in Israel, back yards in Singapore, in coastal areas in Spain and Greece, and the Mediterranian Islands.  If you can get lemons to grow outdoors, you can get henna to grow outdoors.  Otherwise, you'll HAVE to protect it against cold!  Henna LOVES HEAT, and regards  cold weather with fear and loathing!  If your henna plant gets chilled, expect it to drop ALL it's leaves and sulk!   Though henna likes an occasional drenching, and the best henna color is from leaves picked during the monsoon season, it needs to be dry most of the time for its health.  Henna is very sensitive to aphids, termites and other "dampness diseases", and you will have to protect your plant from those!
 
 

If PJ can grow henna at his apartment in Buffalo, N.Y., you can grow a little henna anywhere!  It will have to be greenhoused or grown inside anytime that the temperature is below 60F, and certainly kept away from all frost.  Since henna is a perennial, loves a HOT desert climate and aspires to be a shrubby tree, you will have to allow it plenty of heat, sunshine and room to grow.  In warm summer days (of which Buffalo has precious few) your henna plant will be happiest outside in the sunshine. 
 
 

You can add fresh, or freshly dried henna leaves, pulverized in your food processor, to your regular henna paste to create a more robust red color.  Until your front room is overrun with potted henna bushes, you won't really have enough to make a steady supply of henna paste. 

In India, henna bushes are a frequent household hedge plant.  A prudent Indian housewife, as early as 400 BCE, was urged to make certain that she cultivated henna plants as part of her househoold garden, in between the rose bushes.  On hot days, an Indian mother will send children out with a little bucket to collect fresh henna leaves.  When there are plenty of leaves, a child will take a large flat rock, put the henna leaves on that, and sprinke them with lemon or lime juice.  They grind the leaves and juice  to a pulp with a second smaller flat stone. When the henna paste is smoothly smooshed, they can dip their hands and soles in it, and it will make a brilliant cherry-red stain in only a few minutes!

In Sudan, women  know that some henna bushes give browner stains, and some give redder stains ... so there IS variety in the henna, due to different soils, microclimate, and genetic variation.