How Far Should Grow Lights Be From Plants

grow lights hanging aboveIf you’ve spent any time searching for the best height to hang your grow lights, you’ve probably found a bunch of wildly different answers.

That’s because it’s impossible to give an exact distance.

What I can give you, is a good starting point.

From there, you’ll gradually move the lights closer to the canopy, while monitoring your plants closely. This way, you’ll end up finding the sweet spot for your particular garden.

That’s the best anyone can do for you. The perfect hanging height depends on so many factors that no one could ever provide an accurate answer.

But if you follow the recommendations below, you will have no trouble finding the perfect distance to hang your grow lights above your particular canopy.

We’ll begin with LED grow lights, then we’ll cover HID lights and finally, fluorescent lights.

 

How Far Should LED Grow Lights Be From Plants

I know most of you are using LED grow lights and unfortunately, those are the hardest lights for which to estimate an ideal hanging height. There is just so much variance in the output of different LED brands.

They are also deceptively strong, especially the red and blue mixed lights (i.e. not the all white LEDs). The light looks dim and it doesn’t feel very hot, but it can scorch the plants nonetheless. For this reason, you want to keep a close watch over your garden, whenever you start using a new light, until you get a feel for the ideal hanging height.

Most manufacturers provide the recommended hanging height for their lights. If that is the case for your light, start with your light hanging at the distance they recommend. Watch your plants closely and back the lights off if their are any signs of bleaching (i.e. the leaves begin turning white).

If the plants are fine, move it a bit closer until you reach the point where they begin to bleach the leaves. Then back them off again and you have your ideal height (for flowering; for vegging, back it off 6 to 18 inches more).

If you do not have a recommended height for your light, start by hanging it according to the chart below, depending on the strength of your light. The wattages are HID equivalent, not actual LED watts, meaning a 1000 watt light in the chart below will actually consume between 550 and 700 watts.

Note that these hanging heights are for the highest quality lights with extremely powerful LEDs. If you get a lower cost LED grow light, you’ll probably need to hang it closer (but you always want to start higher and slowly lower the lights).

HID Equiv WattageVegBloom
200 watts12 - 20 inches8 - 16 inches
400 watts20 - 27 inches13 - 21 inches
600 watts30 - 38 inches18 - 30 inches
800 watts32 - 42 inches19 - 34 inches
1000 watts36 - 46 inches21 - 36 inches

 

Metal Halide And HPS Distance From Canopy

For HID grow lights, you can use the “back of the hand” test as a general rule. Simply hold your hand just above your plants where they are closest to the light, with the back of your hand facing the light source. Hold it there for about 30 seconds.

If your hand starts to get too hot for comfort, then the light will be too hot for your plants as well and you want to hang it higher. If your hand could be closer and still not feel discomfort, go ahead and bring the light closer. You want your hand to get hot, but not unbearably so.

To get an idea of how high to hang you light to start with, consult the chart below. I’d always begin with the light at the upper end of the range and gradually lower it.

Bulb WattageMetal HalideHPS
150 watts4 - 10 inches4 - 12 inches
250 watts5 - 13 inches5 - 15 inches
400 watts7 - 18 inches7 - 21 inches
600 watts10 - 27 inches10 - 28 inches
1000 watts12 - 31 inches12 - 35 inches

 

Fluorescent Lights: Distance From Plants

Fluorescent lights don’t get nearly as hot as other types of lighting, so you can get them extremely close. In fact, as long as they are not touching the plants, you should be fine (with mature plants).

You can use the same hand test I described above in the MH and HPS section to see if the light is too close or not. To begin with, hang it about 4 to 6 inches above the tallest plant, for vegging or flowering.

If you are using fluorescent light with seedlings, give them a bit more space. I’d start by hanging the light 6 to 10 inches from seedlings. Use the same distances whether you are using a large T5 fixture or just a regular fluorescent light bulb.

December 10, 2017

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