Although different plants may have different lighting requirements, this should provide you with an effective starting point for deciding how to set up your indoor garden's grow lighting. The most important thing to consider when designing your indoor garden is the amount of light coverage your plants will receive. Inadequate lighting will lead to small, weak plants that are unable to completely fruit. Cutting corners on your lighting system will waste all of the time and expense you have put into setting up your indoor hydroponic system.
The most popular sizes of garden grow lights are 100W, 250W, 400W, 600W, and 1000W. A good general rule is that each light will cover a certain square area based on their wattage output. You can use reflectors to shift some of this light to other areas, but this square area is where your light will produce the most light, so it is a good benchmark for planning your indoor garden's design. Here is a list of the expected coverage areas for the more popular sizes of HID grow lights:
Day and Night Cycles
Light requirements vary based on the type of plant and growth stage. For most plants, 18 hours of light followed by 6 hours of darkness is ideal for the vegetative growth phase.
Seedlings should be under continuous light until the first real leaves appear. At that point, they should be switched to a regular 18/6 light cycle. Fluorescent lights are best for seedlings because of their soft light and low heat output. Using a timer will automate your garden and ensure consistent light cycles. You can find inexpensive timers at any hardware or Home Depot/Lowe's type store.
Light and Photosynthesis
The duration, intensity, color of light that your plant receives will drastically affect the amount of fuel it creates through photosynthesis. Certain portions of the light spectrum increase the plant's ability to photosynthesize. Blue light (simulates summer sun) and orange light (simulates autumn sun) seem to stimulate photosynthesis best.
You may notice that this corresponds to the light spectrums emitted by metal halide bulbs (blue light) and high pressure sodium bulbs (red/orange light). This is a big reason why these types of high-intensity discharge (HID) lights are so popular among indoor gardeners. A combination of metal halide and high pressure sodium bulbs will provide your plants with the complete spectrum of the light produced by the sun.
Use Caution When Working With Lights
The growroom can be one of the most hazardous areas in your home because there is usually water, electricity, and chemicals in a closed-in space. Always keep your ballasts elevated above any water-containing areas of your hydroponic growing system. Make sure all wires are secured and kept away from water. Your light bulbs will be hot when burning and for a short while after shutting off. Be careful not to spray a hot bulb when misting your plants or it could crack.