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A Definitive Grow Light Study

Updated: Feb 8

Buckle up growers! It’s time to break down the mystery of grow lights and reveal the scientific concepts that we need in order to answer the questions we have about providing the right light for our plants.

It's important for us to get things right for our precious vegetable seedlings, so I spent a lot of time researching grow lights and even invested money in the right equipment to get solid answers to our questions when the answers could not be found. Now it's time for you to reap the benefits of following in our footsteps! In this post, I'll walk you through the big questions we had about providing sufficient light for plants and the answers we found that have led us to success. Here are the 5 big questions we'll cover:

  1. What is light?

  2. How should light intensity be measured?

  3. What intensity of light can plants use?

  4. What quantity of light do plants need per day?

  5. What length of time should plants be exposed to light each day?

These questions have been especially relevant for us during the seed starting phase of our season. Our winter months are cold and dark, so we are challenged with the task of starting plants indoors when there is little sunlight outside and still a thick layer of snow over the ground. The use of proper indoor lighting is necessary in our case. If you're in the same situation, take your time here and make sure to understand the quality of light that your plants really need. If you get your lighting wrong, you'll have no hope of growing great seedlings. Now, let's dive in...

Tomatoes seedlings under grow lights


1. What is light?

We understand light as energy that beams through space. With the right equipment, we can actually detect very tiny but quantifiable packages of electromagnetic energy. We call these tiny packages of light photons. Photons behave a bit like particles in that they generally travel in straight lines, and a bit like waves in that they travel at different wavelengths. Since most of these wavelengths are not visible to us or useable by plants, we will limit the rest of our discussion here to the visible part of the spectrum, which is just a small part of the larger electromagnetic spectrum shown in Figure 1 below.

The Electromagnetic Spectrum
Image Credit:

Plants, like humans, are most sensitive to light within this visible spectrum. Therefore, light that falls into this usable range of wavelengths between 400 nanometers and 700 nanometers is called Photosynthetically Active Radiation (PAR). That's just a fancy way of saying "light that works for photosynthesis". The range of PAR is shown in Figure 2 below. The blue and red peaks on this graph show an interesting difference between plants and human, which is that plants are better at using light in the blue and red part of the spectrum. Plants aren't as good at using green light, so most of the light they reflect is green and... drum roll please... that's why most plants appear green to us.

Graph of Photosynthetic Efficiency of Plants
Image Credit:


2. How should light intensity be measured?

In short, we can measure light intensity by counting the number of photons landing on a defined surface area. It's very important here, that we actually measure the same range of light that the plants are going to use, that is PAR. There are other forms of light measurement scales used in the lighting market such as foot candles and lumens, but these other measurements don't help us determine the light quality for plants. Lumen ratings, for example, focus on the amount of light in the yellow part of the spectrum, almost ignoring the blues and reds entirely. Lumen ratings are often found on common light bulbs and for common use these ratings are good enough, because human eyes are most sensitive to that yellow part of the spectrum too.

Photosynthetic Efficiency of Plants and Lumens
Image Credit:

WARNING: This is a good time to crush the myth that you can judge the intensity of your grow lights with your own eyes. This will never work for two reasons. The first, as we just discussed, is that our human eyes are just not sensitive to the full range of PAR so a light with a lot of extra blue and red light would not look much brighter to you. The second is that our pupils are constantly opening and closing to let an appropriate amount of light into our eyes. The moment you look away from one light source to another, your pupils change and you immediately lose any reference point that you think you have in your mind.

So, back to this light measurement business. Thankfully, there is a piece of equipment on the market that can measure what we need. It's called a quantum flux sensor, and it is designed to count the number of photons landing on a surface each second. It is also calibrated to measure the full range of PAR so it is ideal for our purpose. This sensor really makes our light measurement task easy. Unfortunately, it's is not the kind of thing you can pick up at your local garden centre, so this is the point where most growers fall back on using their best judgement. I'm not a typical grower though, and I wanted actual answers to my light questions once and for all, so I splurged for a quantum flux sensor of my own.

Apogee MQ-500 Quantum Meter

When placed under any light source, the quantum sensor starts measuring the rate of photons that hit the sensor surface. It gives us a numerical reading in micro moles of photons per square meter per second, expressed as μmol/m2/s. We call these measurements the Photosynthetic Photon Flux Density (PPFD). Typical readings of daylight intensity are in the range of 200 to 2000 μmol/m2/s.

Here's where a few of you might need a refresher course on Moles:

Scientists use the concept of a “mole” like lay people use the concept of a “dozen”. When you say that you’ve got a dozen eggs, it means that you’ve got 12 of them. A mole is just much much larger than a dozen. Instead of 12, a mole is 602,214,150,000,000,000,000,000! So if you said you had a mole of eggs, it would mean that you had 602,214,150,000,000,000,000,000 eggs. It’s pretty annoying to write all those zeros or use scientific notation all the time, which is part of the reason scientists use the idea of a "mole" to help communicate very large numbers…like the very large number of photons that come from the Sun or grow lights.

Alright. Now that we've got a legitimate method of measuring the usable light that reaches our plants, we are ready to proceed to question 3.


3. What intensity of light do plants need?

There's no point in measuring light if we don't know how much our plants need so here's another important question. To understand this one, we need to start with a look at photosynthesis. Plants use light to drive the process of photosynthesis which converts carbon dioxide and water into usable forms of energy for the plant. The core reaction of photosynthesis is described below:

Diagram showing the process of photosynthesis
(Image Credit:

Plant growth is limited by the rate of photosynthesis, so if we want to help plants grow better, we need to increase the rate of photosynthesis. As we can see from the equation above, the rate of photosynthesis will be limited by the supply of the carbon dioxide and water as well as the energy input in the form of light, but it turns out that light is usually the limiting factor affecting plant growth assuming basic levels of moisture and soil fertility are met.

Most plants can handle quite a lot of light to drive photosynthesis, but every plant will have a light saturation point where the photosynthetic process slows because of another factor such as a shortage of carbon dioxide. At this point, there is no reason to add additional light in an attempt to accelerate plant growth. The graph below shows this relationship visually. As light intensity increases on the horizontal axis, the line on the graph increases very quickly at first, showing a corresponding increase in the rate of photosynthesis. However, notice how the rate of photosynthesis increases dramatically ONLY with the early increases in light intensity. After those initial gains, there is little improvement to be had by adding even more light. Because there are significant gains in growth from these early increases in light, we want to make sure that we are supplying enough light to at least get to the bump in the curve.

Graph showing influence of light intensity on the rate of photosynthesis
Image Credit:

So, what are the minimum and maximum PPFD numbers that a plant can use? Well, according to a cultivation guide by Fluence, a good level for plants just germinating from seed is 150 - 300 μmol/m2/s, and a good level for vegetative growth of young seedlings is 300-600 μmol/m2/s. Larger plants will need more than 600 μmol/m2/s for flowering and fruiting later on, but they'll have no trouble getting that outdoors. After several more hours of searching I found a few slightly different recommendations, but they were generally fell into the same range so I came to the following conclusion. Since we are using our grow lights primarily for raising young seedlings, we will need to aim for a minimum PPFD 100 μmol/m2/s and based on our research and first hand experience there is no need to be concerned with providing any more than 300 μmol/m2/s.


4. What quantity of light do plants need per day?

A measure of light intensity only tells us the amount of light landing on a surface in a single instant, but we need a way to keep track of the total amount of light that a plant is exposed to in a whole day. So, to answer this question, we first need to introduce one more scientific term, the Daily Light Integral (DLI). This is the total number of photons landing on a surface in one day and it is expressed in the units of micro moles per square meter per day or mol/m2/d.

Thankfully, we don’t need to take any more measurements to figure out this number, we just need to do a little math. As long as we know the rate that photons are hitting a surface, the PPFD as previously defined, we can calculate the total number of photons that hit a surface in one day.

For example, if a PPFD reading of 300μmol/m2/s was measured under a set of grow lights and the lights were turned on for 12 hours (or 3600 seconds) each day we could determine that

300μmol/m2/s x 3600s/h x 12h/d x 0.000001 mol/μmol = 12.96 mol/m2/d

We could graph a day of this artificial light at 300μmol/m2/s and it would look like an ordinary rectangle as shown on the graph below. The area under the rectangle represents the total amount of light falling on a surface for the entire day, and in this example, that's 12.96 mol/m2/d.

Graph of Grow Light Intensity

A graph of outdoor light is not nearly as uniform. For example, here is a graph of the light intensity outdoors in our garden the week of our spring equinox. There is a lot of variation throughout the day due to the changing angle of the sun, clouds, and nearby trees. However, as you'll probably notice right away, there is a LOT more light available. Even though the graph is much less uniform, we can still figure out the area under the curve by counting the blue squares and doing a bit of math. This process resulted in a DLI of 35.1 mol/m2/d for this particular graph.

Graph of Outdoor Sunlight Intensity

I was curious how these DLI calculations stacked up against other more professional research and a little searching led me to this collection of outdoor daily light integral maps of the United States. Our data was collected near the end of March on several particularly sunny days so our measured DLI was likely a little higher than our seasonal average. Our location in Saskatoon, Canada, places us just north of the centre of this map so it looks like our DLI of 35.1 mol/m2/d was quite reasonable for this time of year.

Image Credit: Korczynski, Pamela C.; Logan, Joanne; Faust, James E. (2002-01-01). “Mapping Monthly Distribution of Daily Light Integrals across the Contiguous United States”. HortTechnology. 12 (1): 12–16.

Ok, I realize that I haven't answered the question yet about the total quantity of light that plants need per day. We had a bit of background content to cover there, but now we are ready to get back to plants.

Based on the DLI maps above, it looks like vegetables grown outdoors in the summer have between 30 and 50 mol/m2/d of light to grow, but is that amount of light really necessary? Perhaps our artificial light example with a DLI of 12.96 mol/m2/d would be enough?

Another round of research was in order, and I found that the answers start to become less clear at this point because the amount of light a plant needs per day varies between crops and also depends on the stage of growth of a crop. Here are some of the specific recommendations that we found and their sources:

  • Roberto Lopez, Ph.D., researcher at Purdue University, developed a thorough set of guidelines to recommend the average daily light integral (DLI) for most common plants. His research showed that in order to produce crops at a high quality, most plants require a minimum DLI of 12-20 mols/m2/d.

  • "Many shade-loving indoor plants and ornamentals require a relatively low DLI. African violets and phalaenopsis orchids prefer an average DLI of 4-6 mol/m-2/day. Many ferns perform best at a DLI of 4-6, cyclamen at 6-8, fuchsias at 10-12, chrysanthemums at 10-14, petunias at 16-18 and cut-flower rose plants at 18-22 mol/m-2/day." Vanq LED 2019, How to Measure Lighting Level in Horticulture: PAR, PPF, PPFD, and DLI, Vanq, accessed 8 April 2020, <>

  • "For butterhead lettuce production, plants need a DLI of approximately 14-16 mol/m-2/day for high-quality head formation, while iceberg lettuce requires even higher levels. Larger, warm-season plants such as tomatoes, cucumbers, capsicums and eggplants require DLIs of 20-30 mol/m2/d for maximum production. The actual optimal light levels depend on density. Higher-density crops produce more inter-plant shading and require a higher DLI to completely penetrate the thick canopy." - Morgan, Dr. L 2016, Hydroponic Illumination and the Daily Light Integral, Maximum Yield, accessed 8 April 2020, <>

  • A DLI of 6-10 mol/m2/d is recommended for early stage seedlings and 10-15 mol/m2/d for late stage seedlings, and fruiting vegetables will require a DLI of more than 15 mol/m2/d. - Runkle, E 2019, DLI 'Requirements', Greenhouse Production News, accessed 8 April 2020, <>

Based on these recommendations and a few others, we aim to provide a DLI of 10 to 15 mol/m2/d for our vegetable seedlings indoors. Early in the seed starting season, we start with at least 10 and phase up to 15 as plants increase in size and we phase into sun loving crops like peppers, tomatoes, and cucumbers.

With those numbers in mind, we are ready to set up some grow lights to collect some data and see what it takes to generate at least DLI of 10 to15 mol/m2/d.

Oh wait! We need to talk about one more question first, because it’s not clear yet if this amount of light needs to be delivered to our plants over 10 hours, 12 hours..or hey, maybe even 24 hours. If we can leave our grow lights on 24 hours a day, maybe we can get by with less light bulbs? Does the length of the lighting period even matter?


5. How many hours should plants be exposed to light each day?

Does it even matter? The short answer is yes. Plants are sensitive to the length of light exposure they receive each day. However, their responses vary with the crop. I found one particularly valuable piece of research that looked at all the studies published that experimented with exposing plants to 24hours of light per day. These researchers spent far more time on this subject than I ever will and still their study was inconclusive. In their words:

  • “The continuous lighting was found to give benefits to some tolerant crops, which do not develop leaf injuries and can take advantage of the extra light energy provided by continuous lighting.” - Sysoeva M, Markovskaya E, Shibaeva T, (2010) Plants Under Continuous Light: A Review, Plant Stress 4 (1), 5-17

  • “Despite quite a lot of research having been conducted on the influence of long photoperiods on plant growth and development, there is no universal agreement among authors as to the mechanisms involved in the plant response to continuous light and the exact cause of negative effects of continuous light (foliar chlorosis, limited or reduced plant growth and productivity) still remains to be elucidated. “ -Sysoeva M, Markovskaya E, Shibaeva T, (2010) Plants Under Continuous Light: A Review, Plant Stress 4 (1), 5-17

I wish there was a clear answer, but the results truly varied. In some cases 24 hour lighting accelerated flowering, seed production, and other stress responses. There were however cases where a lower intensity of 24 hour lighting was used with good results. The different responses of different crops could relate to photoperiodism, which is the defined as the physiological response of a plant in relation to the length of daylight and darkness it experiences.

Growers classify plants into three categories of photoperiodism:

  • Long day plants that require greater than 12 hours of sunlight or less than 12 hours of darkness to initiate flowering. Spinach and lettuce fall into this category which is why they are so prone to bolting in early summer when the days are longest.

  • Short day plants that require less than 12 hours of sunlight or greater than 12 hours of darkness to initiate flowering. There are few vegetables that fall into this category.

  • Day neutral plants only flower after reaching a certain developmental stage. Cucumbers and tomatoes are examples of day neutral plants that will flower and produce fruit only after they reach a certain developmental stage and continue to do so as long as the temperature and light levels are in an acceptable range.

What about onions you say? Isn't their bulb growth triggered by increases in day length. Yes, it is. However, a study I found showed that bulb growth doesn't begin until the onion plant has at least 6-8 leaves. Therefore, we are safe to expose our onion seedlings to more than 12 hours of light early on without worrying about them forming bulbs too soon.

High Mowing Seeds has published a post with a little more information about photoperiodism and you can find that here if you're interested in more reading:

After all this, our recommendation on lighting timing is this. You can try leaving your lights on for 24 hours because your artificial light is so much less intense than sunlight. You will not cause immediate damage to your plants. However, be on alert for signs of stress if you choose to experiment with constant lighting.

If you're lighting lacks intensity and you want to play it safe, I suggest using a maximum of 20 hours. This was the highest number of hours I found suggested for supplemental greenhouse lighting in a Michigan State presentation.

What length of photoperiod do we use? Well after all this research, the short answer is that we stick to a maximum photoperiod of 16 hours. Our longest summer days here are around 16 hours and even though science shows I could keep my lights on a little longer, I feel that modelling our indoor growing environment after our outdoor growing environment as much as possible can help our plants make a smooth transition to their outdoor conditions.

Does this mean that you can just plug in your lights for 16 hours a day and get the same results as us? No. Remember that this last question was just about picking the right photoperiod for your plants. The DLI you can achieve at home will also depend on the light intensity your provide your plants throughout that photoperiod, and that light intensity totally depends your unique lighting configuration. These two variables must combine to give us the necessary DLI.


It's Time to Review

If you've made it this far, congratulations! You clearly care enough to master your grow lighting and you are now "light" years ahead of the competition. Before we proceed to setting up our grow lights, let's wrap up with a review of the key points we've learned:

1. What is light?

Visible light is electromagnetic radiation travelling through space in the form of photons with a wavelength range of 400-700 nanometers.

2. How should light intensity be measured?

We can measure the Photosynthetically Active Radiation with a quantum flux sensor.

3. What intensity of light can plants use?

Since we are using our grow lights primarily for raising young seedlings, we will need to aim for a minimum PPFD 100 μmol/m2/s and based on our research and first hand experience there is no need to be concerned with providing any more than 300 μmol/m2/s.

4. What quantity of light do plants need per day?

We aim to provide a DLI of 10 to 15 mol/m2/d for our vegetable seedlings indoors. Early in the seed starting season, we start with at least 10 and phase up to 15 as plants increase in size and we phase into sun loving crops like peppers, tomatoes, and cucumbers.

5. What length of time should plants be exposed to light each day?

We use maximum of 16 hours. If you wish to experiment with photoperiods up to 24 hours in length, do so at your own risk.


How to Set Up Your Grow Lights

Now that we know everything about the light our plants need we can actually measure the output of our grow lights and figure out what lighting configuration is best to deliver the necessary light. Hooray! The two styles of lights that we recommend are T5HO lights and LED strip lights because they both have a low profile and offer a full spectrum of light. Therefore, I focussed my testing on these two types of lights.

I conducted numerous tests with our personal configuration first by making a test grid for our data collection and varying the heights of our grow lights to three different settings. The results quickly showed that the height of the grow lights above the test surface changed the light intensity significantly! But I didn't stop there. I knew that many of you would have different numbers of lights and set them up differently so I wanted to see how other configurations performed. The T5HO lights were tested in banks of 1, 2, 3, and 4 and the more intense LED strip lights were tested in banks of 1 and 2. In all cases, the lights were suspended over a 20 inch by 44 inch grid and a quantum sensor was used to collect a light reading at the centre of each square in the grid. For each light configuration the distance between of the light bulbs and the sensor was also varied to collect data from 4, 6, and 8 inches above. All other sources of light were blocked from the test area.

Light readings from each light configuration were recorded, and from this data we determined the average PPFD, the Standard Deviation, and the total DLI for 16 hour and 24 hour of lighting periods.

From this data, we then graphed the DLI for each height and each configuration.

Based on these numbers, we could determine which of the configurations are most effective for growers. The key number we are looking at in each situation is the DLI, but the standard deviation is also important to consider because this represents the degree of difference there is between the brightest and darkest spots in each test. Ideally, we want to supply our growing surface with an even light intensity avoiding darker zones.

The full study is available below. It presents results from each of the configurations listed below along with our corresponding recommendations about how to use each configuration. More than one of these configurations can be used successfully.

Wide Configuration Example: Trays are perpendicular to lights.

Wide Configurations: These configurations study the light exposure over a growing surface that is 44 inches long and 20 inches wide. In this case, seedling trays are typically oriented perpendicular to the grow lights as shown above.

  • Configuration A: 1 T5HO Light

  • Configuration B: 2 T5HO Lights spaced 8 inches apart

  • Configuration C: 3 T5HO Lights spaced 6 inches apart

  • Configuration D: 4 T5HO Lights spaced 4 inches apart

  • Configuration E: 1 LED Strip Light

  • Configuration F: 2 LED Strip Lights spaced 8 inches apart

Narrow Configuration Example: Trays are parallel to lights.

Narrow Configurations: These configurations study the light exposure over a smaller growing surface that is still 44 inches long but now only 12 inches wide. Seedling trays oriented parallel to the grow lights.

  • Configuration G: 1 T5HO Light

  • Configuration H: 2 T5HO Lights spaced 4 inches apart

  • Configuration I: 1 LED

This is where the lighting theory comes to an end and you take action...

Hopefully, you find this resource helpful in setting up your grow lights at home. It pains me deeply to see photos shared of tall spindly seedlings that have been deprived of light. Please don't let this be you! You have no excuse now. With your newfound knowledge, lighting should never be a problem for you again.

I believe all the answers you need to light your vegetable seedlings properly are right here, but if anything is unclear, just connect with me in the Classroom and I'll do my best to help.


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