Abstract: At least one high intensity discharge lamp is utilized in the growing of plants within a growth chamber. The lamp is connected to a power supply that provides pulses of alternating polarity to the lamp through a ballast which first provides a relatively high inductance and which after lamp current has reached a certain level provides a matched T-configured impedance network. Initially, a microprocessor, there being one for each lamp ballast (or one for a group of lamps), closes a switch to connect the lamp to the power supply through a step-up transformer, the secondary winding of which during start-up contributes to the high impedance condition during the early stage of lamp operation and which is thereafter part of the T-network. When the lamp is ionized, both the lamp current and the lamp voltage are sensed.