When I first started dying my hair years ago I always used henna. Back then it was the cowpack powder stuff, I believe you can get henna now a days in pretty boxes like normal hairdyes.
Just wanted to give a couple of tips before you go ahead with the Henna.
Yes your roots will most definitely go a different colour. Thing with Henna is I never got an even colour, even colouring Henna on Henna.
You can make Henna go more dark by adding coffee, red by adding red wine.
Hairdressers are reluctant to touch your hair if you've used Henna on it as Henna puts a protective layer on the strand of hair. So subsequently if you want to dye your hair with a standard hair dye, it won't stick very well and fade real quick. It's almost impossible to perm too (that is if you were thinking of that). Most hairdressers say once Henna always Henna.
So just bear that in mind before the weekend.
There are other non amonia products out there if that's what you're concerned about