When to Turn On Under Canopy Lights in Flower (and How to Set Them Up)
Under canopy light works by reaching bud sites that top lights can't reach once the canopy closes. That makes timing as important as the bars themselves: switch them on too early and you light a plant that is still stretching; too late and you've given away weeks of bulking on the sites you bought the bars to fix.
Here is a practical schedule, with the evidence behind it.
Short answer
Turn under canopy lights on once stretch has finished, which in most rooms falls between day 14 and day 21 of flower. Run them on the same 12/12 photoperiod as your top lights, start dimmed and ramp up, and consider dimming or switching them off in the last two to three weeks before harvest.
Why not from day one?
In the first two weeks after the flip, most cultivars are still stretching and building the branch structure they'll carry to harvest. Light from below at that stage spends power on growth that is still changing shape, and the lower canopy isn't yet shaded enough for extra light to make a big difference.
The 2025 peer-reviewed trial behind our post Does Under Canopy Lighting Increase Yield? switched its supplemental lights on in week 4 (day 22). By then the plants were about 80 cm tall, and the lower canopy was getting a fifth of the top-level light or less. Subcanopy light in that trial raised dry flower yield by 24.58% over top lighting alone. The authors note that the best moment depends on how your cultivar stretches after the flip, so use the plants, not the calendar, as your trigger.
A practical trigger: when the canopy has closed and a quantum sensor at your lowest bud sites reads a small fraction of what it reads at the top, it's time.
Ramp, don't flip
Lower leaves that have grown in shade aren't adapted to strong light from below. Grower guides commonly recommend starting at about 45–50% intensity and ramping to full output over roughly 18–21 days. With dimmable bars this costs nothing but a schedule:
Days 1–3 with the bars on: 45–50%.
Raise in steps every few days while watching the lower leaves for bleaching or curling.
Reach full output by the start of the bulking window.
AG-140 bars are dimmable. On a Master Controller, put the under canopy bars on their own group so they ramp independently of the top lights. One controller drives up to 100 bars directly (50 per side), and the AG-140's built-in signal booster lets a single group run up to 1,000 bars.
Late flower: dim or switch off?
The same 2025 study found that the photosynthetic capacity of fan leaves dropped in the last two to three weeks before harvest. The authors suggest that turning supplemental light off in that window could save electricity, but they also say this needs confirmation, because younger tissue around the flowers may still be photosynthesising. A sensible approach is to trial it on one bench: dim the bars by about 20%, or switch them off, in the final two weeks, and compare weight and quality with a bench that ran full time.
Placement and height
Point up from beneath the canopy, along both sides of each row. In the 2025 trial this layout gave the best yield per kWh: 1.61 g of dry flower per kWh versus 1.49 for top lighting alone.
Keep the bars below the foliage, not touching it. A common guideline is roughly 8–12 inches below the lowest leaves you want to light. Adjustable stands make it easy to change as plants fill in. The AG-140 Under Canopy LED Kit includes cords and stands.
Expect water. Bars under a canopy get splashed during irrigation and wiped down between runs. AG-140 bars are IP65 rated.
Size the run first. See How Many Under-Canopy Light Bars for a 4×4 or 4×8 Bed? for bars per bed and per circuit.
Change your defoliation
Heavy lollipopping, stripping the bottom third of the plant, was designed for rooms with only top light. If you add light below, you want to keep the lower lateral branches and bud sites you're now lighting. Most growers who switch keep a light cleanup at the base and remove the large fan leaves that block light from reaching the sites, rather than stripping whole branches.
Plan for heat and humidity
Under canopy bars add watts to the room, and fuller lower canopies transpire more. Before you switch on:
Check that cooling can carry the added load. Each AG-140 draws 140 W.
Check dehumidification for the extra transpiration.
Move air under the canopy so heat doesn't pool around the bars.
Monitor temperature and humidity at the bottom of the canopy, not just the top.
A sample schedule for a 9-week cultivar
Flower week | Under canopy bars |
Weeks 1–2 | Off (stretch) |
Week 3 | On at 45–50%, ramping |
Weeks 4–6 | Ramp to 100%, hold |
Week 7 | 100% |
Weeks 8–9 | Trial: dim about 20% or off on one bench |
Adjust it to your cultivar's stretch and finish times.
Want the bar count, layout and expected PPFD for your rooms? Ask for a free light plan. Or browse all under canopy lights.
Sources
Garrido J. et al. (2025). Peer-reviewed supplemental lighting study. Plants 14(10):1469. https://doi.org/10.3390/plants14101469
GrowersHouse, Under Canopy Lighting: Yield Trial Data, ROI and Grower Reviews: https://growershouse.com/blogs/lighting/under-canopy-lighting-led-grow-lights-reviews-testimonials
Hydrobuilder, Under-Canopy Lighting: Yields, Setup & ROI Guide: https://learn.hydrobuilder.com/under-canopy-lighting-boost-yields-bud-quality/
FAQ
When should I turn on under canopy lights?
Once stretch has finished and the canopy has closed, usually between day 14 and day 21 of flower. A 2025 cannabis trial switched them on at day 22.
Should under canopy lights run on the same schedule as top lights?
Yes. Run them on the same 12/12 photoperiod with the same dark period, but on a separate dimming group so they can ramp up on their own schedule.
Should I run under canopy lights in veg?
Generally no. In veg the lower canopy isn't shaded enough for the extra light to pay back the power.
How far below the canopy should under canopy bars be?
A common guideline is about 8–12 inches below the lowest leaves you want to light, pointing up, on both sides of each row. Adjust as the plants fill in.
Do I need to change how I defoliate?
Usually. Keep the lower bud sites you're now lighting, and remove only the large fan leaves that block light from reaching them.
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