What is the Best Distance to Keep the Light From the Plants?

Indoor gardeners often ask how far away their plant grow lights should be from their plants but the answer depends on what stage of growth they’re in. It’s important to get the correct distance as the effectiveness of the lights are greatly reduced by even being just a little farther away from the top of the canopy.

Plant grow lights are generally fluorescent, metal halide (MH), or high pressure sodium (HPS) bulbs.

Fluorescent bulbs don’t give off much heat but they’re also not as intense so they need to be closer to the plants. With proper ventilation and grow room temperatures, it’s possible to get fluorescent lights inches away from the top of the canopy without burning any leaves.

Fluorescent lighting can be used for every stage of the plant’s life. However, low wattage bulbs (125 watts and under) are perfect for clones (cuttings). When using a 125 watt fluorescent for this purpose, it’s best to keep the light immediately above your humidity dome.

Metal halide and high pressure sodium bulbs fall into a class of lighting called high intensity discharge (HID). HID lamps require more electricity and they must be cooled as they get very hot and will easily burn your plants. They’re commonly air cooled and the heat is ducted out of the grow room.

Often times, indoor gardeners will use metal halide bulbs for the vegetative stage of growth as the spectrum of light given off by these bulbs contains more blue and is better for this stage. The blueish light is exactly what plants need for photosynthesis. Not only that, using a metal halide bulb for the vegetative stage results in more compact plants with tighter inter node spacing. That being said, you can get great results using a metal halide lamp for your plant’s entire life cycle.

The granddaddy of all plant grow lights is the high pressure sodium bulb. Like fluorescent and metal halide lamps, HPS bulbs can be used for both the vegetative and flowering stages of your grow. However, they emit the yellow and red regions of the light spectrum so are best suited for the flowering stage. Another great things about HPS bulbs is that they don’t have to be changed as often as their metal halide counterparts. If you were to only go with one bulb for your grow room, your best option is an HPS lamp.

An even better choice is to combine both your MH and HPS bulbs so your plants get closer to a full spectrum of light during every stage of their lives.

Earlier I wrote that the effectiveness of your lights are greatly reduced by just being a little farther away from the top of your plants’ canopy. According to the inverse square law, “the intensity of light radiating from a point source (in this case a bulb) that reaches a surface is inversely proportional to the square of the surface’s distance from the source. So if an object is twice as far away, it receives only 1/4 the light.”

As a result, your goal is to get your lights as close to your plants as possible without burning them. To make your life easier, invest a few dollars and get the light lifters you can just move up and down as opposed to the chains you have to add and remove links from.

Start with your lights as high as they go. Next, turn them on, let them get warmed up, and place your hand underneath the lights, palm down. As you move your hand from the top of your plants’ canopy up toward the light, you’ll feel a definite zone of cool space vs. hot space. Move your lights down to a point where you can keep your hand right above your plants and the back of it is cool. If you move your hand up just a bit, you’ll feel where it’s warm which is too hot for your plants.

It’s as simple as that! Yes, you should have a thermometer in your grow room and you should monitor your humidity levels. However, when it comes to the height of your plant grow lights, it’s as easy as using the back of your hand. If it’s too hot for you, it’s too hot for your plants.


Source by Zach Waldman

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