Supplying adequate light coverage for strong, healthy growth is critical to the indoor gardener. Using a light that is too small can result in uneven growth and weakened plants. Using too large a lamp can result in burn or other damage as well as wasted electricity. Deciding which High Intensity Discharge Light to use doesn’t have to be difficult. The size of your indoor garden will determine what wattage system you should purchase.
The chart below gives general guidelines for the area coverage a particular lamp will provide. Plants such as tomatoes or basil that need strong, direct light will do best in the primary areas shown in blue. Most salad greens and other leafy plants including kale and spinach will find all the light they require in supplementary areas shown in white.
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A number of variables including the type of reflector used, the reflective qualities of the grow room walls and distance of plants from the light source will affect the amount of light that reaches your plants. The intensity of the light is greatest near the bulb and diminishes relatively quickly as the distance between source and plant increases.
To avoid burning plants — HID lamps are also a source of heat as well as light — follow the chart to keep your lamp a safe distance from tender plant tops. You can generally determine if your plants are a safe distance from the lamp by putting the back of your hand level with the plants to test for a comfortable temperature. If your hand begins to feel uncomfortably warm (use caution) the lamp is too close. Lamps must be adjusted up as plants grow taller.
|Area Coverage Recommendations||Recommended Mounting Height Above Plants|
Growing areas can be expanded by adding additional lights and by using a light mover. Depending on the size of your grow space, it’s often more efficient — both in lighting and in electricity use — to use multiple, smaller wattage lamps than a single large lamp. Using three 400 watt lamps rather than one 1,000 watt lamp will increase a grow area by 30 to 40% and allow more plants to be closer to the point of light. Smaller lamps, which generate less heat, can also be hung closer to the tops of plants.