How high they should hang their grow lights above the plants

One of the most common questions for indoor growers is how high they should hang their grow lights above the plants. The short answer is “it depends,” but the following explanation may provide more clarity.

Whether growing industrial cannabis or any other fruit or vegetable plants, plants require a certain amount of light, which means the distance between the plant grow light and the plants will vary.

That’s because the difference in height between the grow lights positioned 18 inches and 36 inches above the plants can result in approximately a 75% difference in light intensity.

Recommended distances between HPS/CMH/HID grow lights at 100% power and the plants:
150w: 3ft (seedling); 2ft (veg/flower)
250w: 3-3.5ft (seedling); 2.75ft – 3ft (veg/flower)
400w: 4ft (seedling); 3ft – 3.5ft (veg/flower)
600w: 4.5ft (seedling); 3 – 3.5ft (veg/flower)
1000w: 5ft (seedling); 4-4.5ft (veg/flower)

Distances between LED grow lights and the top of the plants:

In general, LED grow lights can be kept at 24-26 inches during the seedling stage (first 3 weeks) and 18-22 inches during the vegetative and flowering stages. If the light is too intense, raise the grow lights a few inches higher, and if the light is not strong enough, lower them.

The challenge for users is that almost every LED grow light is different because they use different brands of LEDs, different lenses (or no lenses at all), and have different wattages.

Most LED grow light manufacturers provide recommended hanging heights, but these are just estimates and may not apply to all situations.

When the distance between the plants and the grow lights is too far, the plant stems will stretch. The plant intentionally stretches its stems to get closer to the light source and receive more intense light. Genetics can also play a role in plant stretching, but if you notice this happening, it indicates insufficient light intensity, and the grow lights should be hung closer to the plants.

Using the inverse square law to determine the hanging distance of the grow lights above the plants:

The inverse square law is a physical law that can help determine the intensity of the grow lights at a specified distance from the plants. This will help determine whether the grow lights need to be hung closer or farther away from the plants.

The inverse square law of light applies to plant lights, where when the distance between the plants and the grow lights doubles, the area covered by light increases, but the intensity of the light decreases to 25% of the original distance.

Please note that the intensity at 2 meters in the diagram is 25% of the intensity at 1 meter. The intensity at 4 meters is 25% of the intensity at 2 meters.

Although the units in the diagram are in meters, this law applies to any distance. If your initial test distance is 4 feet, then the intensity at 2 feet would be 25%.

Suppose your LED grow light manufacturer claims that their grow light emits 1000 micromoles of light at a distance of 18 inches above the plants, and you determine that the optimal light intensity for your seedlings is 250 micromoles. If you increase the hanging height from 18 inches to 36 inches, according to the inverse square law, the light intensity will decrease to 25% of 1000 micromoles (250 micromoles).

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