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How Far Away Do I Keep My Grow Lights?

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Signs of Light Stress

How Far Away Should I Keep My Grow Lights?

Although this detail is often overlooked, in many cases cannabis growers can increase their yields and potency simply by keeping their grow lights just the right distance from their plants!

How far away should you keep your grow lights for the best cannabis yields and potency?

Cannabis plants capture energy from light and use it to grow bigger buds. In order to maximize your cannabis yields you’ll want to give your plants as much light as possible, but without causing light burn. In other words, you want to increase light levels but without going overboard!

While standard fluorescent grow lights like T5s and CFLs really don’t produce enough light to overload your plants, powerful lights like HPS, LECs and LEDs can do so with ease. Although LED, LEC and HPS grow lights can produce the biggest yields and most potent buds of all grow lights, they need to be used correctly to get those results!

You get bigger yields and more potent buds by using powerful grow lights and keeping them just the right distance away from your plants

By positioning grow lights the right distance from your plants, you’ll create the highest levels of light your cannabis plants can use. Put your buds in this “sweet spot” light level zone, and they’ll reward you!

Notice with these plants how all the top buds are big, while the lower buds are smaller. You want to keep grow lights close, because buds closest to the grow light get the biggest. (Do I need side lighting?)

More light is better when it comes to yields, but unfortunately that’s only up to a point. Although it may be tempting to keep trying to move grow lights closer and closer, your plants simply can’t use more light past a certain point!

In fact, if you keep your lights too close to your cannabis plants, you’ll see bleaching, yellow leaves and nutrient deficiencies, so it’s important to find a balance!

Each type of light needs to be kept a different distance away, so if you feel confused, this tutorial will show you how far to keep your grow light so you get incredible results every time, no matter what your setup!

Metal Halide & HPS Grow Lights

HID grow lights like Metal Halide and HPS (High Pressure Sodium) are the most widely-used and debatably the highest-yielding grow lights available for growing cannabis.

Metal Halide lights are well-suited to the vegetative stage and tend to encourage plants to grow short and squat though they are less electrically efficient than HPS. HPS lights are suitable for both the vegetative and flowering stage of cannabis growth and the light they give off encourages plants to quickly grow tall and produce big buds. Despite these differences, MH and HPS bulbs need to be kept about the same distance away from your plants. The proper distance actually depends on the size/wattage of your lamp.

Here’s a quick reference guide to use as a starting point:

150W – covers 2′ x 2′ (0.6m x 0.6m) area

250W – covers 2′ x 2′ (0.6m x 0.6m) area up to 2.5′ x 2.5′ (0.8m x 0.8m)

400W – covers 3′ x 3′ (0.9m x 0.9m) area up to 3.5′ x 3.5′ (1m x 1m)

600W – covers 3.5′ x 3.5′ (1m x 1m) area up to 4′ x 4′ (1.2m x 1.2m)

1000W – covers 4′ x 4′ (1.2m x 1.2m) area up to 5′ x 5′ (1.5m x 1.5m)

With all grow lights, always do the “hand test” after you’ve positioned your light. If you put your hand where your plants are for 30 seconds, does it feel too hot for you? If so it may be too hot for the plants and the light should be moved a bit further away even if it’s already within the acceptable range.

A 600W HPS light should be kept about 16″ (40 cm) away from the tops of your plants

This grower didn’t keep moving the 600W HPS light up as plants got taller, and some colas on the left plant got within 12″ (30 cm) of the bulb. As a result, the top buds and leaves on that plant got burnt and/or bleached.

In addition to the wattage, it’s important to realize that older bulbs stop giving off as much light over time, so new MH/HPS bulbs generally need to be kept further away and older bulbs should be kept closer than the normal recommendations.

If you’re using new bulbs, never start closer than the “

Sunlight” distance on the chart and only move lights closer if your plants are stretching towards the light (getting tall and lanky).

If you’re using older MH/HPS bulbs, you should replace them if you can, but if that’s not possible you can keep your bulbs closer to help make sure your plants are still getting good light levels. This is why it’s beneficial to be able to test your current light levels with a lux meter.

In general, MH bulbs need to be replaced about once a year (every 2-3 grows), while HPS grow lights last a bit longer, up to 2 years (every 4-5 grows). Older HID bulbs give off less light and more heat, so in addition to keeping the right distance, make sure you’re aware of the age of your bulbs. To maximize the light to my plants in the flowering stage, I personally replace HPS bulbs every 3rd grow, but that’s a bit excessive.

If you see that all the leaves are turned up toward the light, it’s a sign your plants may be getting too much light. This puts them at risk of getting light burn unless the light gets moved up.

LEC/CMH Grow Lights

LEC stands for “Light Emitting Ceramic” while CMH stands for “Ceramic Metal Halide”. These are just two different ways to describe the same grow light technology. “LEC grow lights” are only sold by Sun System because the term “LEC” is trademarked by them. Sun System LECs only come in 315W and 630W sizes, as do most CMH lights. LEC lights usually have proprietary reflectors, whilesome CMH lights use more typical MH/HPS reflectors.

315W is the most popular size for LECs/CMH grow lights. Both the 315 LEC and 315 CMH grow lights should be kept at least 18″ from the top of your plants, with some models needing to be kept as far as 3 feet away.

“CMH” grow lights have been around for many years, and used to come in typical HID sizes like 400W, 600W and 1000W. Recently, they almost always come in 315W or 630W sizes so they can compete more directly with the branded LECs.

Regardless of the name, LEC grow lights are basically a hybrid between Metal Halide and HPS grow lights. It is a type of Metal Halide bulb that is constructed with ceramic like an HPS. As a result, LEC grow lights last longer and are 10-20% more electrically efficient than MH lights. They also produce relatively high levels of UV rays, which may contribute to increased trichome development on cannabis buds, and can potentially make buds more potent.

When it comes to distance, LEC grow lights produce an extremely powerful light and typically need to be kept further away from your plants than HPS grow lights of a similar wattage. The proper distance varies wildly between models.

Sun System LEC Grow Lights

LEC Grow Light Model Distance From Plant
315W 18-20″ (45-51cm) or more
630W 24-26″ (60-66cm) or more

Other CMH models may need to be kept up to 3 feet away! Always read the instructions or ask the seller. Here are some examples I’ve found the distances for:

  • Grower’s Choice 315W CMH – 2.5-3 feet (76-91 cm) from plants
  • Hortilux 315W CMH – 3 feet (91 cm) from plants
  • Vivosun 315 CMH – 24 inches (60 cm) from plants

*Keep LEC grow lights an extra 6-12″ (15-30cm) away for young plants, or if you see signs of leaf stress. If you’re noticing symptoms on just the tops of the plants closest to the light, as opposed to evenly on all the leaves, the problem may be that the light is too close, even if it looks like a nutrient deficiency!

Same as with MH/HPS grow lights, always conduct the 30-second “hand test” after you’ve positioned your light. If the light bothers your hand after 30 seconds, it may bother your plants!

LEC grow lights are surprisingly powerful and must be used with caution to prevent light burn, especially with seedlings and strains that tend to be more sensitive to light. This 315 LEC is being kept 2.5 feet (75 cm) away from the tops of these young plants until they get bigger. Some CMH models need to be kept 3 feet away from the tops of plants!

LEC bulbs last longer than either MH or HPS grow lights, and only need to be replaced every 6-7 grows.

LED Grow Lights

LED grow lights are powerful and can get great yields when used correctly. But when growing cannabis with LED grow lights it can be a little tricky to get the distance just right.

Unlike with MH/HPS/LEC grow lights, there is no “standard” distance to keep each LED grow light and it’s hard to summarize with a quick chart like the others. The distance needed between an LED and the top of the plants varies quite a bit from model to model and from manufacturer to manufacturer. Many things can affect the optimal distance from plants, including the individual size of each LED bulb and the way the manufacturer used lenses to reflect light downwards at your plants.

Different models of LEDs have different optimal distances

Contact the Manufacturer If Possible – Ultimately when it comes to LED grow lights, the best thing to do is contact the manufacturer (or read the specifications) to see what is recommended as far as distance from the tops of plants.

If in doubt, keep LEDs further away – It’s very common for cannabis growers to believe that as long as the temperature is under control they can keep LED grow lights as close as they want. Unfortunately this is not true. Although LED grow lights run cool, the LEDs available today give off an intense amount of light which is great for yields, but can give your plants light burn if kept too close.

The reason this can be such a problem with LEDs is it can take a few weeks before your plant really starts to show symptoms. Usually the upper leaves are affected first (yellowing or turning brown or spots) by too-close LEDs, but symptoms can also affect other parts of the plant.

Keeping LEDs Too Close Can Give Your Plants “Sunburn” That Looks Like a Nutrient Deficiency!

Depending on the make and model, there’s a lot of difference in operating different LED grow lights, but if you weren’t given any instructions with the light, here’s a quick reference guide:

Type of LED Grow Light Distance From Plant
1W Bulbs at least 12″ (30cm)
3W or 5W Bulbs at least 18″ (45cm)
High Wattage (300W+) up to 30″ (75cm) or more

Powerful LED grow lights need to be kept relatively far away from your plants to prevent light burn

Fluorescent Grow Lights (like CFLs & T5s)

As long as it’s not too hot, you basically can’t give your cannabis too much light with these types of grow lights. Fluorescent lights like T5s and CFLs typically can’t produce enough light to cause “light burn” the way you get with the other, more powerful lights. So the idea is to keep your plants as close as possible as long as the distance passes the “hand test.” You know your lights are a good distance away as long as you can put your hand where your plants are for 30 seconds and it doesn’t feel too hot for you. Just make sure to give enough extra space for fast growing plants because the heat can hurt them if they grow into the lights!

Keep Fluorescent Grow Lights As Close As You Can to Cannabis Plants Without It Getting Too Hot!

With fluorescent grow lights, it becomes especially important to learn how to use plant training techniques. You will need to train your plants in order to achieve an even canopy and get good harvests. But with good training, you can grow cannabis under CFL grow lights that look like this!

Or T5 fluorescent lights to grow cannabis like this!

For the best results with T5s and CFLs, try to keep the lights as close as possible and adjust them every day to prevent the plants from growing into the light.

Measuring Optimal Cannabis Light Levels

If you have a Lux Meter to measure your brightness levels, you can get even more accurate with your grow light distance. A Lux meter isn’t a perfect measurement of light because “lux” isn’t exactly what matters to your plants (lux is a measure of light as humans see it).

However, a lux meter is accurate enough for indoor cannabis gardening when it comes to fluorescent, CFL, MH and HPS grow lights!

Quick Guide – Lux Levels for Optimal Cannabis Growth

Life Stage Maximum Good Minimum
Vegetative 70,000 lux 40,000 lux 15,000 lux
Flowering 85,000 lux 60,000 lux 35,000 lux

85,000 lux – at this light intensity, you’ve hit the plant’s “saturation point” which means your plant can’t use all the light (be careful of light bleaching!)

Note: Unfortunately, a lux meter may not be a good tool for estimating the brightness of LEC/CMH or LED grow lights. Lux meters have been calibrated to be accurate for incandescent light bulbs. Although this is close enough to fluorescent and some HID lights that it doesn’t make a huge difference to your plants, the unique color spectrum of LEC/CMH LED grow lights doesn’t always translate well when the lux meter is trying to measure light levels.

Increase Marijuana Yields Even More With Training!

Tip: In addition to knowing the right distance to keep your grow lights, many cannabis growers can increase their yields even more by using plant training to take advantage of that “sweet spot” where plants are getting the best amount of light. Training plants to grow with many colas of the same height (like the cannabis plants pictured below) allows dozens of buds to be the same distance from the grow light, so all the buds can be in the sweet spot at the same time!

Get many colas close to the light for the best yields!

Signs of Cannabis Light Stress

Here are some examples of light stress appearing on the parts of the plant closest to the light. Learn more about light stress.

If the leaves start pointing up like this, it’s a sign the plant is getting a LOT of light. Some growers like to see their leaves “pray” but if you see this keep a close eye to make sure leaves don’t start turning yellow early.

Keeping grow lights too close hurts your plants! These plants were too close to their LED grow lights for too long, and their top leaves started getting bleached from light burn!

Sometimes the tips of leaves will turn yellow in response to the grow light being too close. This is different from nutrient burn which produces brown tips, or nutrient deficiencies which typically appear all over the plant and are often caused by incorrect pH at the roots.

Edges of leaves may turn up from light stress. If it goes on too long, the leaves will eventually get crispy or brittle, and you can easily break them off with simple bending.

Closeup of yellow leaf from light stress

Light Stress on Leaves – Notice how the leaves closer to the light have turned yellow, while the leaves further from the light are still green. This is a sign the plant is experiencing light stress and the grow light should be moved up several inches.

Light Bleaching – Although it was a comfortable temperature in the grow space, the grow light was too close, which caused the yellow leaves and bleaching on the buds

Each type of grow light (CFLs, LEDs, HPS, etc) has a "sweet spot" as far as how far to keep from your plants. Learn what the sweet spot is for your light!

How to Give Seedlings Enough Light to Be Healthy

Why your seedlings aren’t getting enough light

Light is the number one underestimated factor of growing, even at the seedling stage. If seedlings don’t have enough light, they grow more slowly and can become leggy. Overall, growers who do not give seedlings enough light have longer crop timelines and higher rates of seedling mortality.

Why do growers mistake the level of seedling light?

  • They don’t have enough lighting.
  • They place lighting too far from seedlings (bad coverage).
  • They don’t leave lights on long enough.

Usually, these problems are easy to fix: add lights or adjust your seedling light schedules! Here are some general seedling light guidelines to speed up production and save you money on seeds.

How far away should I place seedling lights?

You’ve got to keep two goals in mind: the amount of light you’re giving seedlings, and the coverage of light that you’re giving seedlings.

1) Amount of light:

We can measure the useful light to a plant (PAR—photosynthetically active radiation, or the parts of light that the plant can use to photosynthesize) in micromoles per meter squared per second ( micromoles/sec/m² ) . The goal here is to deliver 120–150 micromoles/sec/m² of PAR to your seedlings .

Some people will use a PAR meter to figure out how much PAR they’re getting to their seedlings and will adjust the distance of their lights based on that measurement. Don’t worry—you don’t need one of those if you’re using LED bars or fluorescent bars. Just remember the following distances.

LED bars (like Phillips): 8–12 inches away from seedlings

T5 fluorescents: 5–6 inches away from seedlings

2) Coverage of light

The next question is coverage. You can usually just eyeball this to figure out how far apart your lights need to be to deliver the same intensity of light across your entire seedling tray.

Again, we’ve outlined two simple rules to follow if you’re using the most common seedling lighting types:

LED bars: 8–10 inches apart from each other

T5 fluorescents: 4–5 inches apart from each other

Stick to these rules, and your seedlings will be ready to transplant in no time.

How long should I keep lights on my seedlings?

One convenient thing about artificial lights is that you can set them to turn on and off by themselves using timers.

But how long should you keep light on your seedlings?

Balancing light and electricity costs

Growers are going to need 12–18 hours of light per day.

Your place in that range depends on the costs of two things: real estate and electricity. Which one is inherently higher for your facility?

This gives you four situations.

1) If the cost of real estate is higher, then you want to move things through your space quickly. This means that you will be paying for more hours of light (18 or so) to move crops along.

2) If the cost of electricity is higher but you have plenty of space, then you’ll want to reduce lighting a bit (12 or so hours). This means that your crops will grow slower.

3) If both costs are equally high like they are at our farm here in Laramie, then you’ll find a balance. We give our crops about 14 hours each day.

4) If both costs are equally low, then go ahead and run them fast, at about 18 hours a day. (Just remember to give crops everything else that they need as well, like airflow and CO2, or they won’t be able to use all that light.

Don’t forget to harden your seedlings before transplant!

Remember to harden off your seedlings before transplanting them! Hardening seedlings involves hardening them to things like temperature and humidity, to reduce plant shock. It also means hardening them to light.

To harden your seedlings, place them in the main growing area for a few hours a day in increasing periods over about a week.

Need light help with the rest of your farm?

Light is one of the most important factors for growing healthy crops. Being a modern farmer like you comes with both a blessing and a curse: on one hand, light technology has never had so many quality options. On the other hand, choosing a lighting type can be a very overwhelming task.

To help you cut through the noise, we wrote The Modern Farmer’s Guide to Artificial Lighting. This guide explains the factors of a good light, how LEDs work, and some special considerations that indoor farmers may have for lighting.

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Kindest regards
Rezaul Haq
[email protected]

How are the red/blue diode color combinations better than white light?

Hi Jack, plant pigments absorb lights at very specific wavelengths. Chlorophyll a and Chlorophyll b, the two main photosynthesis pigments, absorb red and blue lights.
You can learn more about how plant use lights and different types of artificial lighting in this free eBook: http://info.brightagrotech.com/the-modern-farmers-guide-to-artificial-lighting

Hi,
I can’t open the link you posted. Page not found 🙁

Hey Nicola, sorry about that. Could you specify which link? I’ll get the correct one in there asap!

Great info here! Would you mind elaborating on the importance of hardening (first time I hear about it…), how much shock is there generally, and how does that affect operation / any tips?
One other thing I’m having trouble figuring out is how do you crunch the numbers on cost of real estate vs electricity then derive the number of hours of light?

Thank you very much!

Hey Edmund, sure. If the environment where you start a seedling and the environment where it is moved to mature are very different, then the plant might not have the necessary structure and defenses to deal with differences is temperature, airflow, light, etc. For instance, if a seedling is grown in a system that is sheltered and the air if very still, then it will not have the cues to grow a thick stem. If it is moved into a system with a lot of airflow (or outside where there is wind), its thin stem won’t be able to handle the change. Hardening a seedling is just breaking the transition from one environment to another into small steps so that the plant has the time to get ready for its new environment.

I am in the start up phase of setting up a vertical indoor micro green production facility and I am confused about the lighting systems that would work best for micro greens:

1. LED grow lights from Phillips (expensive)
2. T5 Florescents
3. T5 LEDs

From what I have read all 3 of them will work for micro greens, but which one is more cost-effective, t5 florescents or t5 LEDS. Any help with regards to this would be most welcome

Hey Atif – great question. At the seedling stage, plants typically only need 100-150 um/m2/sec. Fluorescents would provide plenty of light in this case. Just make sure that you have enough airflow to help with the heat!

I’m new at this. Starting veggies and flowers in my basement. I gave grow mats and 2 led light boxes. From reading I see the seeds need light for 12 to 18 hours a day. How about the heat mats. How long do they need to be on? Water daily? The one system I bought has self watering system.

Heat mats generally only need to be on until seeds have germinated. Daily watering is a good place to start. The media needs to stay damp during germination, but not waterlogged or soggy.

To give seedlings enough light, you'll need to keep LED bars 8–12 inches away from seedlings and T5s 5-6 inches away from seedlings. Learn more!