Henna is fine to use on waist length hair... heck let us check out some of the Indian/Pakistan women O.K.?
I have always henna'ed since the early 2000's. It is only messy if you hadn't streamlined the process. The only time it will go everywhere
almost is in the shower while rinsing..that's it. I actually love the process.
But I am going to switch to Cassia because I am letting my gray hair grow out.
Henna is not really a growth aid per se as it is actually something that improves the strands of the hair giving way for healthier looking hair that will grow. The growth is simply windfall from the healthy hair/scalp.
For those who haven't tried it, give it a try if you don't mind the translucent color. But try Cassia if you don't want to alter color.
Henna/Cassia actually will tame the hair especially at longer lengths.