How to Test Under Canopy Lighting in Cannabis

A light meter held above a cannabis crop can tell you what reaches the top leaves. Move the same sensor into the canopy and the reading can drop sharply. That difference is the case for adding light lower in the crop. The grower still has a practical question to answer at harvest. How much additional dry flower met the intended grade, and how much electricity did it take to produce it?

What the studies tested

Hawley and colleagues compared overhead lighting alone with overhead lighting plus two different subcanopy spectra in one cannabis cultivar. Both supplemental treatments increased flower yield, including in the lower canopy. Garrido and colleagues later compared overhead lighting with light added from below and light distributed at two depths. Both supplemental treatments increased light in deeper layers. In that setup, the treatment that lit the canopy at two depths produced the largest dry flower gain, about 30% over the overhead control. Both experiments added photons to the crop. An indoor cannabis study by Rodriguez-Morrison and colleagues also found a dry flower yield response to increasing overhead intensity. Together, the results make light quantity a plausible part of the explanation when a grower adds fixtures below. [1] [2] [3]

Use the zones to test one question

If the question is whether to keep the supplemental lights in the program, compare the current ramp with overhead lighting alone in the same crop cycle. In a room with four independently controlled zones, make two pairs with similar overhead light and air movement. Randomly assign one zone in each pair to the current ramp and the other to overhead only. Use plants of the same genotype and similar starting size. Keep density, irrigation, pruning and harvest criteria consistent. Check the boundaries with a meter and leave border plants out of the harvest comparison where neighboring zones spill light into each other.

Treat each zone as the experimental unit. Weighing twenty plants in a zone improves the estimate for that zone. The plants still share the same light treatment, so the comparison has two zones per treatment. Keep the paired results visible, including a pair that moves in the opposite direction. Repeat the comparison in another cycle and swap which physical zone receives each treatment within each pair. If you want to test an earlier start, compare start dates in a separate run and record total light dose. Starting earlier also changes how many photons the crop receives unless the programs are designed to match that dose.

Measure below the tops

Mark repeatable points across each zone. Record photosynthetic photon flux density (PPFD) at the top, middle and lower flower positions. With light coming from below, take separate readings with the sensor facing up and down at the lower and middle positions. Record its orientation and the distance from the fixtures. These are measurements at specified points and orientations. Leaf angle and shading still determine how much light the crop intercepts. Repeat the map after stretch and once the canopy has filled in. At the same points, log canopy air temperature and humidity. If the supplemented zones are consistently warmer or drier, carry that observation into the interpretation. The trial evaluates the lighting program as it operates in that room. Matching the environment more closely in a follow-up trial helps isolate the effect of the light itself.

Daily light integral (DLI) expresses photon exposure over a day. At a given measurement point, a steady additional PPFD of 100 µmol/m²/s for 12 hours amounts to 4.32 mol/m² for that day. For dimming steps within a day, multiply the PPFD measured for each step by its duration in seconds, add the results and divide by one million. Record when ramp steps change across the crop cycle. If you eventually want to compare placement at the same light input, design a separate comparison with similar total fixture photon output over the lighting period and map where those photons land. Equal electricity use can give different photon output when fixtures differ in efficacy. [4]

Follow the dry flower and the meter

Set a drying endpoint and trim standard before harvest. Keep the upper, middle and lower flower separate through weighing. Residual water can skew the weight after drying, so use the same method to assess the endpoint for both groups. Apply the same grading criteria across zones. Record total dry flower and dry flower that meets the intended grade per square metre of measured crop area. A lower canopy can gain flower without improving the grade mix. Conversely, the grade mix can improve with little change in total weight. Tracking both outcomes gives you a way to see either result. [2]

Read actual kilowatt hours for the lighting circuits over the same crop period. Report dry grams per total lighting kWh and additional dry grams per additional lighting kWh. Suppose the overhead control yields 4,000 dry grams on 1,000 kWh and the supplemented zone yields 4,400 grams on 1,150 kWh over the same area. Yield rose 10%, while overall lighting efficiency fell from 4.00 to 3.83 g/kWh. The added lights returned 2.67 dry grams per extra kWh. If you are comparing whole room energy, account for measured cooling and dehumidification separately and document how you allocated shared equipment.

I would keep the zone results alongside the canopy maps and the grade weights. The useful result is a gain that shows up again under the same conditions, with enough detail to tell where the extra flower came from and whether it made the grade.

References

[1] Hawley D, Graham T, Stasiak M, Dixon M. Improving Cannabis Bud Quality and Yield with Subcanopy Lighting. HortScience. 2018. 53(11), 1593–1599. Read source

[2] Garrido J, Corral C, García-Valverde MT, Hidalgo-García J, Ferreiro-Vera C, Martínez-Quesada JJ. Subcanopy and Inter-Canopy Supplemental Light Enhances and Standardizes Yields in Medicinal Cannabis (Cannabis sativa L.). Plants. 2025. 14(10), 1469. Read source

[3] Rodriguez-Morrison V, Llewellyn D, Zheng Y. Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment. Frontiers in Plant Science. 2021. 12, 646020. Read source

[4] Virginia Cooperative Extension. Calculating and Using Daily Light Integral (DLI): An Introductory Guide. Publication SPES-720. Read source