What other plants are present ? ...and are you planting or floating it ?
I'm currently growing on some Anacharis in a 30g (nothing else in there,not even substrate) to add to other tanks.
Just running a simple T-12 single bulb and only supplementing with a tablespoon of Miracle-Gro (no fish) every few days.
I'm just floating it,so in close proximity to the light,and it's sprouting shoots (and throwing roots) and averaging an inch a day of dark green growth. If you are planting it,then the 1wpg probably isn't enough considering the depth of a 90g.
The supplements will help,it will grow,but imagine it might get "lanky" growth with that light.
Just think about it.