Again, not enough light and too much heat. Incadescent lamps (and halogen lamps fall under this category) are not very efficient and only convert about 20% of the electricity consumed into light. The rest is dissipated as waste heat. Additionally, as mentioned above, the color temperature is wrong for good growing. I'd use a compact fluorescent (CF) bulb, like a GE EnviroLite, which are not much bigger than standard incandescent bulbs. CF lamps are available in a wide range of wattages and spectra, and produce considerably less waste heat than incandescents.