Minimum required (by almost all info you can find on the internet) is 2000 lumens per square foot. Just to barely keep it alive. 5000 as a medium and 7500 as primo. So 1100 is a long way from that.

And actually they aren't just 18watts of light. They use 18watts which is why people use them i.e. for the energy savings.

But I agree with CS. You need more juice.

Maybe you should look into LST (low stress training) instead of going 12/12 when it's just barely past the seedling stage (6-7"). Relieves some of the concerns with height constraints.