How Far Away Do I Keep My Grow Lights?
Table of Contents
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
|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!
Growing Seedlings Under Indoor Grow Lights
Anyone who’s tried growing plants from seed knows that proper lighting is critical to producing an abundance of stocky, green seedlings. For those of us lucky enough to have a south-facing window with 12+ hours of full sun, lighting isn’t an issue. But for the rest of us, an indoor lighting system of some kind is a necessity.
This primer on indoor lighting for seed starting will help you choose the options that work best for you.
Light color is also referred to as color temperature, with cool light describing the blue end of the spectrum and warm light being the red end. Sunlight contains the complete spectrum of light, including all colors of the rainbow.
Although plants use the full spectrum for photosynthesis, red and blue light seem to be most critical. Red light stimulates vegetative growth and flowering (but if a plant gets too much, it will become tall and spindly). Blue light regulates plant growth, which makes it ideal for growing foliage plants and short, stocky seedlings (but too much will result in stunted plants).
You can tell which color a grow light produces by looking at its Kelvin rating. Lamps with a rating of 5000 Kelvins will appear bluish, while those with a 2500 Kelvin rating will be reddish.
The intensity of light that a plant receives is determined by the wattage of the bulb and the distance between the plant and the light source. So, for example, a brighter bulb that’s farther away from the plant could provide the same light intensity as a dimmer bulb that’s closer to the plant.
Different plants have different light intensity needs, but most seedlings grown for the garden will need higher intensity light to flourish. In general, the leaves should be about 2 – 4 inches away from the light source (assuming use of a fluorescent bulb – see below).
Duration of Light Exposure
There’s still debate about how many hours of supplemental light is ideal when starting seeds and growing plants indoors.
Most vegetables and garden plants require at least 16 to 18 hours of light each day; without enough light, they get pale and leggy. The conventional advice was to turn lights on for 16 hours each day. However, some growers maintain that 24 hours of consistent light every day provides a better outcome when growing seedling (i.e., there’s no need to give seedlings a nightly rest but this advice doesn’t necessarily apply to full-grown plants).
It’s certainly easier to leave your grow lights on all the time and that’s what I do. If you choose to go with 16 hours on, 8 hours off, put the lamp(s) on a timer so you won’t forget to turn the lights on or off.
Types of Bulbs
You can choose between incandescent, fluorescent, LED, and high-intensity discharge (HID) bulbs, each of which has its own pros and cons. Choose the grow light that works best for the type of plants you want to grow and where you plan to grow your seeds.
These include halogen bulbs and are the type of light bulbs still used in most homes (although they’re getting harder to find now that stores are carrying only more efficient bulbs, such as CFLs and LEDs).
Incandescent bulbs are a good source of red light, but a poor source of blue, meaning that plants will likely become spindly when grown under incandescent light.
Incandescent bulbs, and especially halogen bulbs, also produce a lot of heat in relation to the amount of light they give off; plants growing too close to the bulb can be easily burned.
Generally speaking, these are not the best type of lamps for growing seedlings.
These types of bulbs produce two to three times more light than incandescent bulbs for the same amount of energy and are the most inexpensive lights for indoor gardening. However, they usually require bulky external ballasts (like, for example, overhead shop lights) so aren’t as easy to work with as incandescent and LED bulbs.
Cool white bulbs are a good source of blue and yellow-green light, but are a poor source of red light. Plants grown under cool white bulbs will be stocky or even slightly stunted. Warm white bulbs emit plenty of orange and red light, but less light in the blue and green spectrum. These bulbs, when used alone, result in tall, spindly plants. If you are growing seedlings under two-bulb fluorescent fixtures, you can usually achieve a good color balance by combining one cool white and one warm white bulb.
A set of stacked shelves with fluorescent T5 lamps (or LED lamps) makes it easier to grow a large number of seedlings.
Full-spectrum fluorescent bulbs produce a balance of cool and warm light that replicates the natural solar spectrum, although these are less energy efficient than other fluorescent bulbs and tend to produce more heat. But, given the wider range of light frequencies emitted by these bulbs, they are a good choice for growing seedlings.
T5 lamps are fluorescent lamps that are 5/8″ of an inch in diameter, making them much less bulky than typical fluorescent bulbs. These are the lamps you’re most likely to find in grow light kits.
When using fluorescent lamps, be sure that all plants get ample light. For a typical seedling tray, that means using 2 bulbs, ideally with a reflector hood over them to focus all the light on the seedlings below.
LED Grow Lamps
Unlike other bulbs which produce light across a broad spectrum, light-emitting diodes (LEDs) give off light within a narrow band. LEDs designed for growing plants emit light in the two bands that plants need – red and blue. The result is a purple glow that not everyone likes.
LEDs are mercury-free and won’t shatter like glass. These bulbs are long-lived (up to 5x longer than fluorescent lamps) and very energy efficient, but they cost considerably more than fluorescent bulbs.
Research is still ongoing to determine which combination of light frequencies are best for plant growth and how LED grow lights compare to fluorescent bulbs in producing healthy seedlings. I haven’t seen a definitive answer on this one yet but more and more companies are producing LED grow lights and seed-starting kits, and even commercial growers are slowly moving in that direction.
High-Intensity Discharge Lamps
These lamps are used by commercial growers and serious horticulturists. These energy-efficient bulbs generally emit twice the amount of light (lumens) as a fluorescent bulb. However, the bulbs and special fixtures are considerably more expensive than those needed for incandescent or fluorescent lights. They also tend to be high-wattage bulbs, so you need to be sure your electrical system can handle the load. Some of these lights burn so brightly that they must be located in a special room and you’ll need to wear eye protection when working around them.
Metal halide lights emit an intense, bluish-white light that is excellent for growing plants. The foliage stays green and vigorous, and plants are usually stocky and strong. Metal-halide lights are currently the number one choice for serious indoor gardeners. Mercury vapor lamps emit a bluish, relatively well-balanced, high-intensity light. High-pressure sodium bulbs are usually used to promote flowering and fruiting but, when used exclusively, they produce leggy, weak-stemmed plants.
What’s the best option?
For the average home gardener starting seedlings indoors, a fluorescent or LED lamp will usually be the best choice to ensure that your plants get the quality, intensity, and duration of light they need to stay in peak condition.
A description and comparison of different grow lights for starting seeds and growing plants indoors. Covers incandescent, fluorescent, LED and high intensity discharge lamps, as well as issues around light color, intensity and duration.