MH stands for metal halide. This bulb technology emits a blue spectrum of light making it best for use during the vegetative stages of plant development. Although MH lamps are slightly less efficient than HPS lamps, the blue spectrum they emit is best for vegetative plant growth. This causes your plants to grow upwards at a slower rate, focussing their growth on leaf and root development instead. This makes them stronger and more suited to the height limitations of indoor growing. Just be sure to swap over to one of our HPS lamps for the flowering stage, or choose one of our dual spectrum bulbs instead!