Understanding DLI & Choosing Optimal Supplementation Levels

Understanding DLI

By Eric J. Harrington, Dir Comm. Sales and Research, California Lightworks

When I discuss supplemental lighting with old school non-cannabis growers considering switching, I often hear the question, “Why would I need lights? It’s a greenhouse?  This is mostly because the ROI on supplemental lighting for traditional crops has never been very attractive. While fruiting crops can justify moderate levels of supplementation, it’s still not widely adopted.

But cannabis is by far the most light-hungry (and correspondingly fastest growing) plant cultivated commercially. In fact, it is becoming increasingly difficult to compete in the US market growing cannabis in a greenhouse without supplemental lighting.

Understanding DLI

DLI stands for daily light integral” and is a measure of light in mols(6×1023) of photons. Daily Light Integral is important in greenhouse horticulture because it measures the amount of photosynthetically active radiation (PAR) plants receive each day as a result of both light intensity and duration.  This has a major impact on plant growth and health.

With indoor lighting, light is traditionally measured in instantaneous photosynthetic flux density (PPFD). That is, the amount of PAR photons that land on a given surface each second. PPFD essentially measures the amount of PAR light that reaches your crop, and each plant species has different PPFD requirements for the best yield.

Because greenhouses have varying light levels, it’s not productive to try to measure the light levels in instantaneous PPFD. Instead, we have to measure the total photons that are transmitted into the greenhouse over a 24-hour period. This is Daily Light Integral.

Outdoor Vs. Indoor DLI

Establishing Your Baseline

How do you know what DLI your plants need? First, there is excellent DLI data collected by the national weather service and other agencies available through this link. These resources give the DLI averaged for each month of the year for locations throughout the US and they will help establish a Daily Light Integral baseline for your location.

For this example, we will use Salinas, a major hub for cannabis production in California.  When we enter Salinas in the search box on the website, we get the following:

Factoring In Greenhouse Transmission

The preceding DLI averages are outdoors, but we need to adjust those numbers for what we will get inside the greenhouse. 

Light transmission through typical greenhouse glazing ranges from 95% for clear glass to as low as 75% for highly diffused glass or polycarbonate. Most common glazing is in the range of 80-85%, but this number can be misleading as it represents the optimum transmission.

Optimum transmission refers to when the sun is at precisely 90 degrees, or perpendicular to the plane of the glazing surface. But as the sun gets lower in the sky both in morning, evening, and winter months, the transmission decreases with the angle of incidence.

In other words, the steeper the angle of the sun with respect to the roof surface, the less light is transmitted, and the more is reflected.

Calculating transmission over a day entails very complicated calculus, but the average for new glazing materials ranges from 75% for clear glass to as low as 60% for highly diffused materials. For the purpose of this analysis, we will use a common material, new single-glaze polycarbonate with an average daily transmission of 70%.

What 70% transmission means is that if we have 50 DLI in July, we are only getting 35 DLI inside the greenhouse. For most crops, even the light-hungry tomato, 35 DLI is a very productive DLI level.  But in December, 70% of the 17.1 outdoor DLI is only 12 DLI inside. 12 DLI is generally not considered productive for tomatoes, or even lettuce.

You are probably beginning to see why supplemental lighting is important even for greenhouse growth.   

What Is The DLI Upper Limit?

Numerous studies on various crops (except cannabis) have proven that yield increases proportionally with DLI up to a certain upper limit. Above that limit, the yield may continue to go up, but the rate of increase drops off rapidly and plant stress can ensue. Our experience has shown that cannabis closely follows this rule.

This upper limit ranges broadly for different crops. Lettuce is 15-20, Tomatoes is 35-40, and pretty much all other crops are in between.

That is, except for cannabis. Cannabis is unique for commercial cultivars in that it will have a uniform increase in yield proportional to DLI right up to 45 DLI, with many strains going as high as 50 DLI or more. This is easy to understand since most original cannabis strains evolved in equatorial mountains with very high natural light levels. Below 40 DLI, cannabis will start to see subtle changes in flower structure and resonation as well as the proportional decrease in yield.

So, 40-50 DLI is the sweet spot for flowering cannabis, and this is supported by the fact that a 1000w HPS over a 4×4 tray typically delivers around 40-45 DLI over 12 hours. Optimum Veg DLI levels should be a minimum of 75% of flower DLI to avoid shock when moving from veg to flower.

Supplementation Results

So, if we analyze the supplemental DLI required to get the indoor mixed-light to a min 45 DLI ( w/ lights on12 hours a day or less) in a new polycarbonate, light-dep greenhouse in Salinas, CA, we get the following:                  

The  “Supp PPFD req:”  PPFD levels in the above chart are the PPFD levels necessary when operating continuously for 12 hours a day to produce 45 DLI inside the greenhouse.

We see in the chart that the maximum indoor DLI we get without supplementation (labeled ”@70% Trans:”) for June is 37.5 DLI. So even in jJune, by adding 174 PPFD to achieve 45 DLI, it will result in an increase vs un-supplemented summer yield of ~17%.  And that same +17% yield can be delivered year-round with roughly 750 uMols of supplementation.

Many cultivators may not have the budget to buy 750 uMols of supplementation, just to get exactly 45 DLI in December, but for the three most popular levels of supplementation we sell to our customers namely 400, 500, 600 PPFD, as well as 750 PPFD, we get the following total indoor mixed-light DLI:

Interpreting The Chart

In the previous table, the months with a DLI of 45> represent that the mixed-light DLI is equal to or higher than 45. But in real-life practice, that threshold can be whatever the user chooses.

To limit the DLI to 45 (or 50 or whatever), the simple way is to adjust the light schedule to turn the lights off as much as necessary on either side of the noon hour to maintain 45 DLI.

This is a crude approach and will not maintain a narrow DLI target without calculations and continuous adjustments. The easier and far more accurate approach is to use a photosensor with the appropriate software to sense the varying daylight and weather conditions and automatically adjust the supplemental light output to maintain a user-selected PPFD threshold that directly correlates to your DLI target.

This photosensor control is available in our California Lightworks MegaDrive Greenhouse line, and with our system you can accurately and reliably get the results shown in the preceding chart. (Please Note: unusual weather conditions can drive the mixed DLI down from the averages, but such occurrences are rare.)

Different strains have different DLI stress points, and while the range is wide virtually all strains can handle somewhere between 40-50 DLI.  This assumes all other plant systems can support the metabolism demanded by the light.  If not, you will get light stress because if an absorbed photon doesn’t create plant, it creates heat in the plant. And that heat can cause stress.  So, while a plant can only grow as fast as the weakest link, the primary driver of plant metabolism is light.

In the table, while we limited the DLI to 45 for clarity, the actual maximum indoor mixed-light DLI available in March with 600 uMols of supplementation is 49.8 DLI. So, for strains that can utilize up to 50 DLI or more without stress, you would want to set the Photo-sensor threshold to whatever the plant can tolerate to take best advantage of the higher DLI months. Careful testing and logging will allow you to optimize your DLI targets.

As for the months with negative percentages (in red,) those percentages will generally correlate to that percentage decrease in yield versus what would be achieved at 45 DLI. This again assumes that the grower has the necessary root health, nutrient uptake, etc., to support that level of metabolism. If light-stress is seen below 45 DLI, the grower should investigate ways to improve their nutrient throughput etc.. to be able match the metabolism demanded, or dial the DLI target back.

Also in the table we see that with 400 PPFD supplementation in January, February, and March, yield would drop roughly 32%, 26% and 10% respectively, vs the 45 DLI yield.   With 600 PPFD it dropped 23% and 7% for January and February respectively. But with 600 PPFD, the January and February DLI are still very close to 40, so the flower structure and density should be consistent year-round.

That’s why 600 PPFD is our most popular design as requested by large commercial customers. They accept small drops in yield in December and January, but see uniform quality12 months a year. Uniform yield and quality help keep manpower requirements and cash flow consistent, as well as being able to take advantage of months with lower competition and respectively higher prices.

As for the energy consumption with these higher levels of supplementation, in general the value of cannabis yield exceeds the cost of the electricity required to produce it by 20+ times.  So, electrical consumption is just not an important consideration compared with the yield results. Only if the necessary power infrastructure is unavailable, or if the budget is not feasible should lower supplemental levels be chosen.  At current prices, cannabis always justifies the electricity necessary to produce it up to the DLI limit.

And cannabis justifies the cost of the lights very quickly as well.  The payback based on increased yield alone for any level of supplementation from 400 to 700 uMols is typically 2-3 harvests at a pound price of only $600. That’s a 400 -500% ROI! You can borrow the money at 20% interest over 2 years and still make over 300% return on their money with properly designed cannabis greenhouse supplementation.

But most importantly, once the lights pay for themselves, as your yield per square-foot goes up using lighting supplementation, the biggest cost centers in cannabis, namely real-estate, capital equipment, and Labor(except trimming) all stay the same. So, as your average annual indoor DLI goes up with supplementation, your profit goes up proportionally. 

There is a compelling argument to be made that supplemental lighting is the single best investment you can make in a cannabis greenhouse operation.

For questions, catalogs, or to discuss your project, or request a light plan,  please contact Eric@CaliforniaLightworks.  And don’t forget to ask for a complimentary Yield / ROI Calculator which will spell out every aspect of the Daily Light Integral and yield performance for MegaDrive system investment with your actual situation. 

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