LED Grow Light For Cannabis: The Best Way of Finding the Right Lamp
LED Grow Light for Cannabis: The Best Way of Finding the Right Lamp (with PPFD maps)
For beginner growers, underestimating how much light is needed to successfully grow plants is all too common. “If the lamp appears bright to me, then it’s bright enough for my plants”, is something we read every now and then. Even experienced growers can find it challenging to determine if a certain LED grow light is enough for their grow area.
In a previous article we discussed that the watt/sq ft principle gives a rough idea of how strong and powerful grow light is needed for a particular grow area. We discussed that for efficient grow lights with good hardware, somewhere around 30 watt/sq ft is a good rule of thumb. In other words a 3x3' space, or 9 sq ft, would need an LED grow light consuming around 270 watts. While this is a good indication, there are better and more accurate ways of determining what grow light is needed for any type of grow area.
Light intensity (PPFD)
To keep this article brief and on point we’ll stick to the basics. PPFD (Photosynthetic Photon Flux Density), sometimes misrepresented as “PAR”, is a measurement for light intensity. It’s possible to measure how strong light a light source emits. PPFD is measured with a so-called quantum sensor and the value will be presented in numerical terms, let’s say 500, in unit µmol/m2/s (micromoles per square meter per second).
In a dark room the PPFD will be around 0 µmol/m2/s while direct sunlight on a summer day will measure around 1500 µmol/m2/s. This is roughly the range you need to be aware of and keep in mind. Cannabis and hemp plants have grown in the wild for centuries and they have gotten accustomed to sunlight and sunlight intensity. This is what we try to replicate as modern, indoor growers.
Through trial and error the indoor growing community has learned what light intensity, or rather PPFD levels, marijuana plants thrive under. These are the generally accepted levels:
PPFD levels for cannabis plants
Clones: 50-100 µmol/m2/s
Seedlings: 100-200 µmol/m2/s
Early veg: 200-300 µmol/m2/s
Late veg: 300-400 µmol/m2/s
Early flowering: 400-600 µmol/m2/s
Late flowering: 600-900 µmol/m2/s (sometimes even a bit higher)
For smaller, non-fruiting, plants like basil, micro-greens, lettuce, much lower PPFD levels are enough to fully stimulate the plant. Around 200-300 µmol/m2/s are typically enough to see good growth.
This tells us, logically enough, that small, frail, young plants want less intense light than mature plants. Just like with nutrients, young plants need care and moderation as they develop but once they have grown large, they can take more and more. Both in terms of nutrients but also in terms of light intensity.
Light is strongly related to growth. Naturally, other factors play into the mix, too. Nutrients, temperature, humidity, CO2, pot size, etc. Giving plants less than recommended will hinder growth potential and bud development. In other words, to give your plants the best possible environment to grow and flourish, they will need an LED grow light that can deliver PPFD levels of 600-900 µmol/m2/s over as much as possible of your grow space.
Keep in mind that the light emitted from grow lights is typically very intense in the center spot right underneath the lamp but the intensity diminishes quickly away from the center. A lamp which has a PPFD measurement of 900 µmol/m2/s right underneath the lamp may only have 400, or 300, a foot away. In other words, the spread and distribution of light needs to be taken into account - not just the highest center spot value. How light is emitted and distributed is not something that a lamp’s wattage (draw power) tells us.
While a plant doesn’t need to receive PPFD levels of 900 µmol/m2/s across its entire canopy or surface area to grow well, the closer the average PPFD received by the plant is to 900 µmol/m2/s the better. High-light plants, like cannabis, will have positive photosynthetic reactions when given above 200 µmol/m2/s, but the higher the better. Below PPFD levels of 200 µmol/m2/s, the photosynthetic activity is marginal at best, i.e. the plant would grow with a poor structure.
How do I know what PPFD levels a grow light emits over a certain area?
This is where PPFD maps, or light footprint maps, come in handy. These maps are 2D images showing an area, let’s say 2x2’ (60x60cm), with PPFD spot measurements across said area. Grow light brands should, and most of them do, disclose this information alongside their lamp.
Let’s take a look at three PPFD maps of our own LEDTonic LED grow lights and how the light is distributed and also how the lamps’ performance differs from one another.
Based on the PPFD maps below, can you draw your own conclusions as to which areas and plants these lamps are suitable for?
The models we’ll be looking at are as follows:
LEDTonic Z2 LED Grow Light - Housing-type, medium efficiency (1.6 µmol/J) lamp drawing 50 watts and fitted with 90-degree focusing lenses (narrow light footprint). PPFD values measured in a 2x2’ (60x60 cm) tent.
LEDTonic Q7 LED Grow Light - Dual quantum panel, high efficiency (2.55 µmol/J) lamp drawing 320 watts. PPFD values measured in 3x3' (90x90 cm) and 4x4' (120x120 cm) tents.
Comparing these four PPFD maps (or light footprint maps) it’s immediately clear that the first two lamps in their 2x2’ spaces do not emit high enough light intensities over the 2x2’ area to perfectly stimulate mature, flowering, cannabis plants. While the PPFD levels are plenty high in the center spot under both the Z2 and Q2 lamp, the intensity quickly diminishes away from the center. This is not to say that the lamps are useless. They would work well for clones, seedlings or vegging, as supplemental lamps, or for a smaller area. We do see that approximately 1.5x1.5’ of the 2x2’ area does have PPFD levels between 350-800 µmol/m2/s, which means a decent average PPFD (of maybe around 400-500 µmol/m2/s). The PPFD levels within the 1.5x1.5’ would definitely be acceptable to grow cannabis, although they are slightly lower than ideal levels.
Looking at the Q7 on the other hand, we see that the lamp fills a 3x3’ space incredibly well and the lowest measurement is relatively close to the highest measurement. In other words, no extreme hot spots.
We also see the Q7 in a 4x4’ space and it’s interesting to compare how the same light performs over two different areas; 9 sq ft vs 16 sq ft.
We generally recommend the Q7 for 4x4’ spaces as the lamp provides decent PPFD levels over this area and for an area of 16 sq ft, the lamp is very economical. Naturally, some of our experienced growers prioritize incredibly high PPFD levels across the board over purchase price, in which case the Q7 excels a 2x4’ and 3x3’ space.
Generally speaking, the larger area that needs to be filled with light and the higher average PPFD is targeted, the bigger and more powerful lamp is required. The bigger and more powerful the lamp is, typically, the more expensive it is. The dilemma growers usually face is how high average PPFD they require versus how much they are willing to spend on a lamp.
Regardless of your preference, what you should take away from this is that PPFD maps are the most accurate way of comparing the performance of different LED grow lights and also determine which lamp is right for your and your space.
I’d like to add that one should always be a bit hesitant to accept PPFD maps and measurements at face value. It only takes a keystroke to change or falsify a number. When possible, try to verify that a PPFD map is legitimate by finding a video recording of the test. Few brands are this transparent with their data though. Here’s how we measure PPFD:
Fake PPFD maps
Also, beware of downright fake PPFD maps. According to us, PPFD maps are the best way of comparing LED grow lights and I believe plenty of growers would agree. With this in mind, it makes total sense for brands to fake and lie about their lamps' performance to try to get a competitive edge against other brands. Here are two examples from major manufacturers. The brands will remain anonymous as our intentions are not to discredit a particular brand but rather show how to spot a fake maps, once knowing what to look for:
Gray/semi-transparent lines (18”): There are clear patterns where PPFD are exactly the same, which in the real world is not just unlikely, but impossible.
Green vs red box (18”): Compare these two boxes and you will find that the outer values (415) are exactly the same in both boxes but the next row are not (671 vs 631/633). Also, the green box has identical values on the second row from the top (671) but the row below differs by 5% (965 vs 919). The fourth row from the top (1031 vs 1027) only differs 0.3%.
White box vs pink box (18”): In the white box we read 303 (left ) and 414 (right) but in the pink box we read 314 (left) and 303 (right). How can the two inner numbers differ 100 while the outer numbers are exactly the same (as well as the numbers in the green box)? They can’t.
Red circles, bottom right (18”): Here we see the values differ in the three boxes which makes sense but comparing these diagonal boxes to any other corner will show that the corresponding boxes in all other corners have exactly the same values (303).
Red arrows, corners (18”): We see vastly different corner values (163, 150, 135, 106) while the boxes surrounding the corners (except the bottom right corner) are all identical (261, 261, 303).
White lines (24”): At first glance the PPFD map looks mirrored from left to right, which is not uncommon, but if looking a bit closer some (many) values are not mirrored at all which raises the question if any number at all can be truly trusted.
Green vs blue boxes: These two boxes are equally far apart from the center which means they should have very similar PPFD values, which is true in the 18” (right) map, but not the left (12”) map. Here, on the 12” map, the values in the blue box are inflated by around 20%. Comparing the green vs blue box and also the values surrounding the blue box obviously shows that the numbers in the blue box are fake. The red arrows help illustrate this point as well.
Furthermore, the shorter the distance (12” / 18”), the higher PPFD values should be recorded at the center and near the center. We see that the PPFD values in both green boxes are very, very similar when they should differ about 10-20%.
Black line: The numbers on the 18” map (right) are illustrated logically (but probably still inflated), however, on the 12” map (left), there’s a number (on the black line, to the left of the green box) that reads 1023. The measurement to the right of this reads 1020. Measurements closer to the center (closer to the light source) will always be higher than values further away from the light source. This raises the questions, which of the two numbers is fake? Or are both fake?
Pink boxes & red arrows (12”): We highlight this area but the following claim is true for pretty much the entire map. PPFD values on opposite sides of a map cannot be this close. All corner values are within 3 PPFD of one another, which is practically impossible. Same goes for the red arrows. Here the difference is only 2 PPFD (714, 715, 713, 714) between the lowest and the highest. Not even the most reputable and highest regarded brands in the industry has this close readings. When something seems too good to be true...
There are, of course, plenty of more irregularities or lies on both maps. Maybe you can find these on your own?
Instead of using the watt/sq ft principle to determine how strong grow light you need, look at lamps’ PPFD output and how the light spreads over the area your plants will occupy. The number of watts consumed by a grow light will never tell you how the light actually spreads and how good the coverage is. Instead, look at the PPFD values as they tell the truth (assuming they can be trusted).
In addition, also consider the reputation and online presence of a grow light brand. Maybe shoot them an email to see how they threat customers or ask about their product to see how knowledgeable they are. Look at what hardware they use in their products. Look at what warranty they offer and if they live up to it. Look at where the company is based and if they are growers themselves. This way you get the full picture of both how well you can expect your plants to grow under the lamp but also how you’ll be treated if you ever need help from the company.