What is the wattage on that halide?
With that set-up you need to try hydroponics. It is loads of fun and the growth is amazing. Get a bucket system and drop a pup in there under that halid and stand back.
Keith
An old page on Hydroponic bananas:
http://www.geocities.com/k2benson/Ba.../Hydroban.html
If I had the space I would use:
http://www.genhydro.com/genhydro_US/megafarm.html and replace the airpump life with a small aquarium pump.
You could put one of the super small varieties in:
http://www.genhydro.com/genhydro_US/waterfarm.html
But to me that defeats the purpose of seeing the tremendous growth. General Hydro used to make a system where a 3.5 gallon bucket nested ina 5 callon bucket. It was the precursor to the waterfarm. It was perfect for bananas as long as you didn't let the pups break through the sides . . .