Grow Light Distance

Light placement is an important aspect of a successful grow. Too close, and you may burn your plants. Too far, and you can cause the plants to stretch and become leggy. The distance you hang your light above the canopy will vary depending on your light source and wattage. 

This article details our hanging height suggestions for popular types of lighting. You will also learn the “why” behind our recommendations, which really is the important part.

How high should I hang my grow lights from my cannabis plants?

To be clear, light hanging height refers to the distance from the bottom of the light to the top of the canopy. Having a level canopy is beneficial for increasing yield because bud sites are at an equal distance from the light source. An easy way to achieve a level canopy for new growers is by using the low-stress training (LST) techniques of bending and tying down branches. Learn more about low-stress training for cannabis on our blog.

Although different types of lights and many variables go into selecting the perfect hanging height (which we will discuss), there are a few universal truths. The appropriate hanging height is directly related to the type of light, wattage, grow space dimensions, and life stage of the plant. 

In general, lights should be kept 24”-36” away from seedlings and 12”-24” above the canopy of mature, flowering plants. Keep reading to find out which hanging height is best for your style of light. You should also download Robert’s Grow Bible for a complete guide to growing great cannabis at home.

Heat is a serious concern when hanging grow lights

Too much light can burn and bleach buds, but heat generated by the fixtures can also wreak havoc on the health of your plants. Each of the three lighting systems mentioned in this article has a different heat signature. This difference in heat signatures factors into the main question of “What is the best height to hang my grow light”?

One watt of energy has the same heat output (BTUs) regardless of what fixture it comes from, so you may think they would all be equal. However, high-pressure sodium (HPS) systems produce the most heat. A combination of factors contributes to this.

First is the increased infrared radiation (IR) wavelengths emitted by HPS bulbs. Second, most HPS systems use reflector hoods that direct light that would normally shine up directly down on the canopy. Lastly, HPS bulbs are less efficient in converting energy into light and lose that energy by creating heat.

Don’t be fooled; you can burn or heat stress your plants with CFL or LED grow lights if they are too close to the canopy. The more wattage being used, the more this risk increases. Know the signs of heat stress and always inspect your plants daily. 

How high to hang LED lamps from your cannabis plants 

Many LED lights have dimmers allowing growers to control the intensity without adjusting the height. This article’s recommendations are based on light distance and dimmer settings.

LED light distance for cannabis seedlings

Seedlings are sensitive to intense light regardless of the source. LED lighting will be placed furthest from the canopy during the seedling stage.

LED light distance for cannabis in the vegetative stage

As the plant develops more roots and foliage, the amount of light it can tolerate increases. Grow lights can be positioned closer to the canopy, increasing the light intensity delivered. Vegetative plants require a higher intensity than seedlings but do not require as much as flowering plants.

LED light distance for cannabis during the flowering stage

Cannabis plants require the highest intensity during the flowering phase. This is when you will position your light closest to the canopy. Due to the reduced heat output towards the canopy, LED lighting can be run closer to the canopy than HPS systems.

LED wattage lamp height for each stage.

How high to hang HPS lamps from your cannabis plants

Many of the digital ballasts (a device that regulates the power current) that come with HPS systems have dimmers. Unfortunately, when you run them at anything less than 100%, the gases do not heat properly, affecting the desired color spectrum. Our tests were conducted at 100% of the intended power draw.

HPS light distance for cannabis seedlings

HPS lighting systems produce not only intense light but also intense heat, which dissipates in the direction of the canopy (thanks to increased infrared radiation [IR]). Growers should be cautious of placing lights too close to seedlings.

HPS light distance for cannabis in the vegetative stage

During this stage, lower the light to increase the usable light the canopy receives. Vigorously growing plants will enjoy the higher light intensity.

HPS light distance for cannabis during the flowering stage

Some HPS lights use an open “winged” reflector, while others use an air-cooled hood. Air-cooled hoods can be placed slightly closer to the canopy due to the reduced heat signature at the canopy and because the glass diminishes light intensity by 10% or more if it is dirty.

HPS wattage lamp height for each stage.

How high to hang CFL lamps from your cannabis plants

Compact fluorescent lighting (CFL) is a common household lighting option. Due to the frequency of daily use, CFL bulbs are also widely available worldwide. This is the lighting style most growers start with. Compact fluorescent grow lights come in many shapes, sizes, color spectrums, and wattages.

Two popular CFL color spectrums for indoor growing are 2700K and 5000K. The “K” stands for Kelvin, which measures color temperature on a scale of 1,000-10,000K. 

Gardeners use the 5000K for vegetative growth. The increased blue spectrum helps keep plants compact. Growers will then switch to the 2700K for flowering. It is a “warmer” color, with more orange, yellow, and red, similar to the spectrum from HPS bulbs. 

Also read: CFL vs LED grow lights – The Comparison Guide

CFL light distance for cannabis seedlings

The most forgiving light on seedlings are CFL bulbs. They aren’t extremely intense and have a low heat signature compared to the other styles of lights. Depending on wattage, bulbs can be placed less than a foot from seedlings.

CFL light distance for cannabis in the vegetative stage

At this point of the life cycle, the lower-wattage CFL bulbs will not sustain the required photosynthesis in a vigorously growing plant. At a minimum, growers should be using 100W per plant. Occasionally growers use multiple lower-watt bulbs to achieve this benchmark.

CFL light distance for cannabis during the flowering stage

Growers should keep lights as close to the canopy as possible without causing heat stress to the upper buds. If using multiple CFL bulbs for an individual plant, place them around the top of the plant instead of concentrating in the center. This action will help the lower auxiliary buds receive direct light too.

CFL wattage lamp height for each stage.

*Used as a singular bulb, these lights perform below the minimum requirements for vigorous plant growth. 

Light distance for autoflowers: Does it make a difference?

Yes, the height at which you hang your light will be important. This variable will directly relate to the number of light hours your autos need to thrive in your home garden.

To help guide you toward a bountiful harvest, read our article outlining the best light schedule for autoflowers.

Grow light distance calculators

There are dozens of grow light hanging height charts available online, but calculators are hard to find. A reputable lighting company commonly has hanging height recommendations on its website. Be sure to check with your lighting manufacturer for guidance as well. 

The important things to know about grow light distance

An important factor in determining ideal height is the light’s wattage. Different wattages produce different photosynthetic photon flux density (PPFD), which, in turn, affects the daily light integral (DLI). Lower-wattage lights need to be run longer and placed closer to the canopy to equal the same DLI as a high-powered light.

In short, you can find the appropriate height for your grow lights by knowing the PPFD of the light, the duration the light will be on for, and the required DLI for the life stage of the plant. Because most growers lack the tools to calculate this, we created the charts above. 

Also read: 5 Tips when buying LED lights

Grow light brands with good products and information

A handful of commonly used LED lighting companies serve the home grow community. These manufacturers have detailed PPFD maps on their websites. These maps are meant to be an estimate instead of a hard number. Truthfully, these companies use a few tactics to inflate the numbers slightly, but the charts still serve their purpose.

Spider Farmer

• Consumers will find two available PPFD maps for each of their listed lights. The provided maps give a PPFD reading for every 6”x6” square of the tent. A few lights have PPFD numbers for two different tent sizes, be sure you are looking at the map that matches your dimensions.

Mars Hydro

• Each light has two PPFD maps provided on the description page. Although they are different hanging heights and numbers, the important thing to note here is one map is for using CO2, and the other is without CO2. The majority of hobby growers do not supplement CO2 in their grows. Make sure you are looking at the applicable map here.

Horticultural Lighting Group (HLG)

• This website offered the least information in regard to PPFD maps. Only a handful of units had them that I could find. The PPFD map wasn’t listed as part of the product description page; it was a separate link to an independent lab analysis. It wasn’t until I started googling that I found some PPFD maps for the HLG lights.

Viparspectra

• I found varying information here depending on which lighting series you are looking at. Most of the lights on the website provided two PPFD maps on the description page. Each map represented a different hanging height for the light. The exception to that was the KS Series of lights. The PPFD maps included for those lights do not mention a hanging height and are almost useless with no point of reference.

On all of these PPFD maps, you will notice the numbers are highest in the middle and lower in the corners. If the tent’s footprint is larger than the footprint of the light, as you lower the light, the PPFD numbers in the center will increase, but the numbers in the corners will fall. This is something to consider when searching for the best balance of intensity and even light distribution when determining the proper hanging height. 

To understand why light is important to a plant and how it fuels growth, read our article detailing how a marijuana plant converts light energy. If you have not yet bought an LED light but have been thinking about it, read our article highlighting our top five tips when buying LED lights.

Important terms to know

Daily Light Integral (DLI)

• The total amount of usable light (PPFD) a plant receives in 24 hours. Cannabis has a minimum DLI to maintain healthy growth and a maximum DLI. The maximum is when more light becomes detrimental. 

Photosynthetically active radiation (PAR)

• This describes light wavelengths (color spectrum) measured in nanometers (nm) within the visible spectrum. PAR ranges from 400nm-700nm. It is also the range of light used by plants for photosynthesis.

Photosynthetic photon flux density (PPFD)

• This refers to usable light received by the plant. It measures the biologically-active light radiation as micromoles per square meter per second (umol / m2 / s).

Lumen

• Lumens are for humans. It is a unit of luminous flux and measures the amount of visible light (brightness) to the human eyes. It is not an accurate assessment of light received by the plant.

Lux

• This is a calculation of luminous flux per unit area. It is equal to one lumen per square meter. Again, this measures the intensity as seen by the human eye, not a photosynthetic plant.

Inverse square law

• The amount of light received decreases in increments that are compounded by distance. If you double the height of your light, the canopy will receive only a quarter of the light that it was. If you move your light three times higher, your canopy will only receive a ninth of the same light.

PPFD and the inverse square law 

This equation demonstrates how finding the right hanging height is important to the success and vigor of your home garden. It also points out that PPFD numbers on lighting websites may be misleading if you are using a different hanging height than they are for their test. With some quick math, you should be able to get a ballpark for the PPFD numbers received by your canopy at the height you chose to hang the grow light.

Final thoughts on grow light distance

We all want the best for our plants because that will mean larger yields at harvest. Finding the right hanging height to meet the DLI for the current life stage of your plant is an important part of the puzzle. In this video, Dr. Bruce Bugbee of Apogee Instruments highlights how much wattage affects the best hanging height for your cannabis lights. He also demonstrates the need for different recommendations for the different wattages, just like we included above.

Setting up your lights correctly is critical to growing the best weed. Check out our free plant care guide for more tips on growing healthy plants.

Now that you know how to calculate the proper hanging height for each light source, will you be making any changes in the grow room? We’d love to hear what light source you are using and how high you hang them. Let us know below in the comment section. Until the next time, happy growing!