How High Should You Hang Grow Lights? LED and HID Chart

How high should you hang your grow lights above cannabis plants? The answer is deceptively complicated if you want to break down the nuts and bolts. Grow light distance seeks to wash the canopy with as much light, at the highest intensity possible, without burning or otherwise damaging the plants. Getting the combination of factors right promotes healthy growth, bountiful harvests, and minimizes damage. 

The first step is easy: read the manufacturer's recommended hanging height. But, beyond the grow light manual, many other factors play into how high you hang your lights above your plants. These include light intensity, type of light (HID or LED), lifecycle (vegetative or flower), growing environment, and the light coverage area. If these is already over your head, start with this grow light resource first.

Take all these factors together, the task of calculating grow light height seems a bit daunting. However, these are all general guidelines to find the ideal hang-height. Getting the height within the general ball-park will still produce great buds.

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First things first: read the owner's manual. If you have purchased a cannabis-specific grow light from a reputable dealer, it will come with suggested hanging heights. For example, the Growcraft X2 330W LED Grow Light recommends a hanging height between 12" to 24". Others may vary based on the following details.

What Type of Light Do You Have?

If you work with LED grow lights, these do not produce as much heat as their HID counterparts. You will likely be able to place these much closer to the canopy than a metal halide or High-Pressure Sodium set up.

Are the Lights Dimmable?

Many new fixtures come with a dimming feature. This allows you to dim the intensity, rather than adjust the light height. Non-dimming fixtures require you to move the entire grow light up and down depending on what the canopy requires.

Stage of Growth Cycle Matters

Depending on where you are in your plants' life cycle, the hanging height will need to change. The younger the plants, the higher the lights need to be. In vegetative, lights need to be as close as possible. In flower, the suggested height shifts up again.

Intensity and Angle of the Lights

Intensity matters. If you have a super intense HID light, then it needs to sit high above the canopy to avoid light burn. A less intense LED should sit closer to bathe the plants in as much light as possible. Intensity also has something to do with the angle, or canopy coverage, of the system. Some grow lights are quite dispersed, covering a larger canopy space.

With dispersed light landing at a wide-angle, that typically means the grow light can sit much closer to the canopy because it's intensity is distributed. HID lights and some LED grow lights have a small angle, which increases the intensity and therefore requires a higher fixture height.

HID grow lights, like Metal Halide and High-Pressure Sodium fixtures, produce significantly more heat than the LED variety. One fixture which delivers extreme intensity is the Gorilla Pro Series.

In terms of grow light height, that translates to a higher hanging distance above the canopy. Because HID grow lights get so hotter, they increase light burn and crop damage.

As with any grow lights, you'll first want to check out the owner's manual for recommended hanging height. If this does not exist, or your light was not designed specifically for cannabis, you can also perform a rough hand test.

Hold your hand under the fixture at the same distance your plants will sit for at 30 seconds. If your hand heats up to the point of discomfort, your plants will experience the same. Keep testing at different distances until the heat feels warm but comfortable.

If you don't find anything in the manual and want some numbers to start with, the following reference table should help get you started:

Keep in mind, LEDs are a bit more diverse when it comes to intensities, spectrum, and recommended heights. Furthermore, not all grow light manufacturers use the same measurements to talk about their products' intensity. You might find lumens, lux, PAR, PPFD, and wattage equivalency. It's much harder to estimate appropriate distances between plants and fixtures.

Unlike an HID light, you cannot perform a hand-heat test with an LED fixture because it doesn't create enough heat. Hanging height comes down to sheer-light intensity, not so much heat. 

The intensity of the light all comes down to the strength of the individual diodes and how much of that reaches the plants. Most lLED lands somewhere between 1W to 5W diode power. The lower the diode strength, the closer you can place the lights to the canopy. Other manufacturers may talk about PPFD or lux ratings, but the higher the rating, the farther away the lights must sit.

The key takeaway from this is to always follow the manufacturer's suggested hanging height, but don’t be afraid to tweak it based on the specifics of your fixture, grow, and grow tent. A few inches up or down can boost yields and save your plants from light stress.

Monitor your plants daily as you experiment with hanging height. If you notice stretching, you’ve hung the fixture too high. On the flip side, if you see light burn, the lights sit too low. Adjust a few inches at a time until you get it right.