by Nebula Haze
Table of Contents
Introduction: How Far to Keep Grow Lights?
- Metal Halide or HPS Grow Lights
- LEC or CMH Grow Lights
- LED Grow Lights
- Fluorescent Grow Lights (T5s & CFLs)
Measuring Optimal Light Levels
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!
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!
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:
|Grow Light||Closest||~ Sunlight||Furthest|
|150W||8″ (20cm)||10″ (25cm)||12″ (30cm)|
|250W||10″ (25cm)||12″ (30cm)||14″ (35cm)|
|400W||12″ (30cm)||14″ (35cm)||19″ (48cm)|
|600W||14″ (35cm)||16″ (41cm)||25″ (64cm)|
|1000W||16″ (41cm)||22″ (55cm)||31″ (79cm)|
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. Most LEC and CMH lights have proprietary reflectors that look a little different from typical MH or 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
“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.
Learn more about LEC grow lights
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.
|LEC Grow Light Wattage||Distance From Plant|
|315W||18″ (45cm) or more|
|630W||24″ (60cm) or more|
*Keep LEC grow lights an extra 6-12″ (15-30cm) away for seedlings until they’re a few weeks old
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.
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
Learn more about growing cannabis with LED grow lights
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, LEC, MH and HPS grow lights!
Quick Guide – Lux Levels for Optimal Cannabis Growth
|Vegetative||70,000 lux||40,000 lux||15,000 lux|
|Flowering||85,000 lux||60,000 lux||35,000 lux|
< 15,000 lux – sparse or “stretchy” growth – plant isn’t getting enough light
15,000 – 50,000 lux – good amount of light for healthy vegetative growth
45,000 – 70,000 lux – optimal amount of light for cannabis plants in the flowering (budding) stage
70,000 – 85,000 lux – a lot of light, some strains thrive at this light level, but some plants (especially auto-flowering and indica strains) lose their top leaves early under this light intensity and need lights to be kept further away for healthy growth
> 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!)
Learn more about using lux meters to increase yields
Note: Unfortunately, a lux meter may not be a good tool for estimating the brightness of LED grow lights. Lux meters have been calibrated to be accurate for incandescent light bulbs. Although this is close enough to fluorescent and HID lights that it doesn’t make a huge difference to your plants, the unique color spectrum of 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!
Compare Different Cannabis Grow Lights: Review
Beginner’s Guide to Growing Weed
Cannabis Problems & Leaf Symptoms
More about MH/HPS Grow Lights