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How Close Should LED Grow Lights be to Plants?

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Some growers are looking for answers to what the best grow light height is above plants or how close should LED grow lights be to plants.

It is important to understand that this proper height is to hang the light as low as possible on the plant so that as much light as possible is shining on the canopy, rather than having a hot spot in the center of the canopyA hot spot concentrated in the center of the canopy can cause some damage to the top of the plant. Not only that, but the plant will not get even light around it, which will cause slow growth.

Here I will give you a guide to show you where this trade-off occurs and how to best evaluate and weigh your own cannabis seedling lighting against the plants for your own particular setup as best as possible.

LED grow lights

To give cannabis grows an average of about 700 to 900 micromoles, so you want to try to get the central measurement and not exceed too much. But at 1000 micromolar, this high light intensity might lead to stress at some stage of the plant.

When LED lights for cannabis are left on all day, too much light intensity can cause stress on plants. You will see the leaves curling and a slight bleaching on the tips of the leaves. We are trying to avoid this at the same time. But sometimes it’s good to see a little bit because it’s giving you indications.

HIDs emit a lot of far-red and also radiate a lot of heat, and the distance of grow light from plants is farther than LED grow lights. So HIDs are not included in this paper, only LED grow lights.

If you’re interested in this aspect of cannabis light distance, let’s take a look at the Viper Spectra in a 2×2, the Auxgrow in a 2×4, the Spider Farmer in a 4×4 and the HLG 600 Rspec in a 5×5.

LED Grow Lights Distance from Plants​

In fact, there is no universal rule for how far grow light height above plants at each growth stage.

Different manufacturers construct their cannabis LED grow lights in different ways. There are multiple details to consider, the power and angle of the fixture, and the use of a lens to reflect light downward. The best way to find the best grow light height is to consult the plant growth lights manufacturer.

The second thing to remember is that the correct placement of the LEDs will change with each growth stage. If your LEDs are dimmable, you can use dimming to adjust the light intensity on the plant surface. If your LED lights for growing weed are not dimmable, your only option is to change the hanging height.

Viper Spectra XS 1500 hanging height

The first one is the Viper Spectra XS 1500, which is a sort of quantum board style light panel board. They recommend 12 to 18 inches hanging height over the growing area. 

And now looking at LED Gardeners tests, he runs them all the way from 6 inches and way up past 24 inches. In terms of hanging height,12 inches is showing a center measurement of so the highest reading of 1150 micromoles. It’s an efficiency of 2.02 micromoles per watt so that’s the total amount of power reaching the plant canopy divided by the power consumed.

So that’s 2.02 which is a uniformity of pretty much 50%.  The uniformity is the lowest reading over the highest reading, so you know ultimately you want it to be 100% that all the readings are exactly the same but it’s not really achievable. Anyway, that’s the benchmark for this light is 50%.

However, at 18 inches as they recommend, the efficiency drops to 1.76. Thus, this 13 drop efficiency goes up to plus or so uniformly goes up to 60 to 79 which is good. Viper’s recommendations, I would agree. Basically,12 inches to get the most light down it’ll be. But you know, not as uniform as it might otherwise be eight inches 18 inches to get a really uniform light distribution. I’ll be a little bit less light.

That’s good well done to Viper Spectra. They’re improving the grow light distance chart Autoflower from all manufacturers has much better.


The next light is our own Auxgrow VG22A02D. This is a 240-watt light for 2×4. We recommend a minimum suspension height of 12 inches. 

You can see in the chart that at an altitude of 12 inches it has an efficiency of 2.9 and shows a center measurement of 1351μmol/s. It has a uniformity of 60% and a minimum reading of 810. It is right at the limit as far as the hot spot in the center is concerned, increasing it to 18 inches. So that’s a 9% reduction, but the uniformity jumps up to 82%. 

By adjusting the led suspension height, you get a truly uniform distribution over the entire planting area.

You see, this quantum board has a lamp bead design that is sparse in the middle and dense all around, and it improves uniformity very significantly.

The manufacturer’s recommended LED grow light height above plants is now accurate and realistic, and they will have the appropriate power meter. This is really very helpful for grower setups.


Spider Farmer SE7000 hanging height

Spider Farmer SE7000 hanging height recommends 8 inches to 16 inches hanging height.

They say with co2, so generally, it’s recommended if you’re going over a thousand micromoles on average that you would supplement with co2 in a closed system. And bring that co2 and ppm in parts per million up to around the same as the micromoles. If it was 1500 micromoles, you’d have 1500 ppm co2.

In this case, the SE7000 is in a 4×4 at 8 inches maximum center, maximum measurement, maximum power reading of 1430 micromoles, and efficiency 2.5 uniformity of 57. The uniform is a bit low and the center reading’s a little bit high.

But as I said, this is with co2 12 inches the center or the highest reading drops to 1200 system efficiency drops a little bit was 4% down to 2.41 and the uniformity is 72%. 

I would probably recommend in this instance 16 inches,45 centimeters which gives a 2.31 efficiency. It’s a very slight drop but you get that 82% uniformity across the growing area. This is my own recommendation.

But in this case, spider farmer is pushing it because they want to really maximize that efficiency and the potential yield. They’re recommending sort of aggressive hanging heights. The bar light shows really good uniformity across the growing area so impressive.

HLG 600 Rspec distance from canopy

The fourth one is the HLG 600 Rspec, a decentralized design of quantum board-style plant light. HLG 600 Rspec distance from canopy recommends a 28-inch hanging height.

In this case, that’s perfect. And you have a maximum value of 927 micromoles and an efficiency of 2.21 at the highest reading. Unfortunately, the uniformity is pretty low 37.

You’re getting a big drop off there between the highest and the lowest and you should adjust a little bit higher. Look there, at 32 inches high, the efficiency drops to 2.13, but you get a uniformity of 48. This type of lamp has a lower level of uniformity because it does not physically spread the light out as much. 

It’s interesting to see that the range of you’re talking about increasing hanging height by about 50 over the minimum and you’re going to get an increase in uniformity. But of about 30 to 50% but you’re going to get a reduction in efficiency by you to know anywhere 10 to 20% that sort of range.

It is that trade-off really good to see however that the recommended hanging heights by manufacturers are now accurate and realistic. they’re coming with the power charts to suit. It’s really helpful for growers to set up.

If you want to reduce potential hot spots and increase uniformity, you can go to about 40-50% of the suspension height. You’ll see a drop in efficiency from 10 to 15″, but you’ll get really good uniformity.