Lamp Spacing

Marijuana Horticulture

by Jorge Cervantes

Light intensity virtually doubles for every six inches closer an HID is to the canopy of a garden. When light intensity is low, plants stretch for it. w light intensity is often caused by the lamp being too far away from plants. Dim light causes sparse foliage and spindly branches that are further apart on the stem.

Increase yield by giving growing area uniform light distribution. Uneven light distribution causes strong branch tips to grow toward the intense light. Foliage in dimly lit areas is shaded when light distribution is uneven.

Reflective hoods ultimately dictate lamp placement – distance between lamps and above the plants. Nearly all stationary lamps have bright (hot) spots that plants grow toward.

Growers prefer high wattage lamps – 400, 600, 1000, or 1100 watts – because they have lumens per watt and their PAR rating is higher than smaller bulbs. Plants receive more light when the lamp is closer to the garden. Even though 400-watt lamps produce fewer lumens-per-watt than a 1000-watt bulb, when properly setup, they actually deliver more usable light to plants. The 600-watt bulb has the highest lumen-per-watt conversion and can be placed closer to the canopy of the garden than a 1000 or 1100-watt bulb. When the 600-watt bulb is closer to pants, they receive more light.

A 1000-watt HID emits a lot of light. It also radiates a lot of heat. The bulb must be farther away from the plants to avoid burning them. In many cases it is more effective to use smaller wattage bulbs. For example, two 400-watt bulbs can be placed closer to plants than one 1000-watt bulb, and the 400-watt bulbs emit light from two pints. The disadvantage is that two 400-watt systems cost more than one 1000-watt system.

Although 400-watt lamps have a lower lumen-per-watt conversion, when used properly they may be more efficient than higher wattage bulbs. One 1000-watt halide produces 115,000 initial lumens and 400-watt halide only 40,000. This means each 400-watt lamp must be located closer to the canopy of the garden to provide a similar amount of light. It also means that several different point sources sustain more even, intense light distribution.