Under-canopy lighting: photons at the floor

Photons at the floor

A top fixture lights a roof, not a plant. By the time photons fight through three or four leaf layers, the lower third of the canopy is sitting at a fraction of the light its buds need to fill out — so it doesn’t. Under-canopy lighting (SCL) and inter-canopy lighting (ICL) put photons where the overhead array can’t, and the published work is now consistent: you don’t necessarily gain gross weight at equal total flux, but you upgrade grade, tighten uniformity, and lift the bottom of the plant from larf to saleable flower.

None of that is free. Every watt you push below the canopy is a watt of heat in the worst-ventilated zone in the room, and you’re now growing dense bud in air that used to be dead space. This paper covers the light, the spectrum, the placement, the training, and — the part most guides skip — the thermal and airflow bill that comes due.

The lower canopy is light-starved

Light attenuates fast through a cannabis canopy. Leaves are extremely good at absorbing the exact wavelengths that drive photosynthesis, so each layer strips the red and blue out of the beam before it reaches the next. A top canopy running a healthy 800–1200 µmol·m⁻²·s⁻¹ commonly drops to 100–200 µmol at the lower bud sites, well under the rough 400–500 µmol floor that cannabis needs to set and fill competitive flower.

The result is the larf you already know: airy, underweight bud on the bottom third that grades B or C, drags your average down, and costs the same in labour to trim as your A-grade tops. The lower canopy isn’t underperforming because the genetics are weak there. It’s underperforming because it’s in the dark.

What the research actually shows

The marketing decks all cite ‘20–60% yield gains’ without naming a source. The honest picture from peer-reviewed and controlled commercial work is more specific, and more useful, because it tells you where the gain comes from.

Hawley et al., 2018 — the Guelph group ran the first controlled cannabis subcanopy trial in HortScience^[1]. Both red-blue and RGB SCL significantly increased yield and THC in the lower-canopy bud. The mechanism: improved light distribution into the lower canopy beats simply raising overhead PPFD — the whole thesis of the field. Detail worth tattooing on the wall: in cycle two they left the bottom growth on (instead of gyping it) and the yield response was stronger. Under SCL, your defoliation logic inverts (Section 7).

2025, TL vs SCL vs ICL head to head — a study in Plants compared traditional top light against SCL and ICL directly^[2]. ICL was the standout, and both methods improved energy-use efficiency — the gain per watt was real, not just gross output bought with more power.

Fluence / Texas Original — the trial that keeps you honest^[3]. At equal total light flux, moving photons into the canopy via ICL did not reliably raise total yield versus top-light alone — but it increased lower-canopy bud size and upgraded the grade (B/C → B/A) with much less variability. In a price-pressured market, consistency and grade are the margin, not gross weight.

The heavy-red trap

Here’s the most expensive mistake in the category. Most under-canopy products are heavy red — cheap, efficient per photon, looks ‘powerful’. It is also the wrong spectrum below the canopy, and the trials prove it.

Two things are happening. First, the mid and lower canopy is already red-rich and far-red-dominant — the upper leaves absorbed the blue and red on the way down. Stacking more concentrated red onto that imbalance is the opposite of what the plant needs. Second, photosystems I and II must be excited roughly equally for efficient photosynthesis; monochromatic red unbalances them.

Use a balanced, broad spectrum below the canopy — closer to what the plant evolved under than to a red space heater. Broad-spectrum minimises bleaching risk while still driving photosynthesis^[4]; Hawley’s red-blue gave consistency, RGB moved terpenes more. Broad-spectrum white with a measured red component is the safe, productive default.

Far-red (700–750 nm) is a scalpel, not a default. Upside: the Emerson effect — FR added to red/white drives photosynthesis synergistically by balancing PSI and PSII, and end-of-day FR raised cannabinoid yield in some cultivars^[5]. Downside: FR is the primary trigger of shade-avoidance — stretch and loose, airy growth, the last thing you want low in the canopy. Dose it deliberately; never ‘more is better’.

PAR targets and how much light to add

The goal isn’t to match top-canopy intensity at the floor — it’s to lift the starved zone over the threshold where bud development becomes viable, without bleaching.

Think in added flux at depth, not bar wattage. A modest contribution — on the order of 25–60 W·m⁻² of installed sub-canopy fixture depending on geometry — is usually enough to clear the threshold. Past the bleaching point you’re paying in heat and pigment loss for negative return.

The leaf was built by yesterday's light

A leaf is not a fixed solar panel. While it expands it builds its own photosynthetic hardware — chloroplast structure, electron-transport capacity, Rubisco, photoprotection — calibrated to the light it experiences during development. A shade-grown leaf builds thin, low-capacity, lightly-defended hardware; the same genetics under bright light build a thicker, higher-capacity leaf. Rodriguez-Morrison, Llewellyn & Zheng (2021) measured this in cannabis^[6]: leaves acclimated to ~91 vs ~1,238 µmol differed by about 50% at high intensity. Same cultivar, different light history, different machine.

One — don’t trust a single leaf to predict the canopy. The same study found leaf-level photosynthesis saturates well below where whole-plant yield keeps climbing: dry inflorescence yield rose linearly to 1,800 µmol (the highest tested) while a single leaf’s curve flattened far earlier. Don’t size under-canopy targets off leaf-saturation logic.

Two — the lower leaves you’re about to light grew up in shade. Their installed capacity is shade-grade. Throw sudden high intensity at them and you risk photoinhibition before production, because the photoprotective machinery was never assembled. Newly-developing leaves re-acclimate far better than mature shade leaves — the mechanistic case for ramping intensity rather than slamming it on (Section 10).

Fixture placement and mounting

SCL — bench / floor mount. Bars on pot rims or low rails, throwing light upward into the basal bud sites:

• Aim up and in. Simplest to install and clean; lowest disruption to your canopy.
• Standoff distance. Keep enough gap that you’re not scorching the nearest bud — bleaching risk scales with proximity × red fraction. Broad-spectrum tolerates closer placement.
• Coverage uniformity. Daisy-chained thin bars with overlapping throw beat a few point sources. Map it (Section 10), don’t eyeball it.

ICL — in-canopy mount. Bars hung within the branches at basal and middle tiers:

• More effective for tall, unpruned genotypes — exactly the architecture that benefits most. This is where the strongest study numbers came from.
• Plan plant count down. Bars in the canopy need lanes — the ‘reduce plant count’ accommodation the field data assumes.
• Waterproof, cleanable, daisy-chainable. These live in the humid, sprayed, trimmed zone — IP-rated housings and DLC certification (rebate eligibility) are baseline.

Plant training changes when you light from below

This separates operators who get the 30% from operators who bleach their bottoms and rot their cores. Under-canopy lighting and your training regime are one system, not two.

The gyping reversal. Standard practice strips the bottom 20 cm — gyping, lollipopping — because that growth is shaded, contributes nothing, and invites rot. Once you light it, that logic flips. Hawley’s second cycle left the lower growth on precisely because SCL made those previously-useless leaves and bud sites productive. If you light the bottom then strip it, you’ve paid for fixtures to illuminate bare stem.

• Keep: lower bud sites and the leaves directly feeding them — now lit and earning.
• Remove: large fan leaves that shade newly-lit bud sites or trap humidity against them. Selective, not scorched-earth.
• Tuck before you cut where you can — redirect shade without removing photosynthetic area.
• Even canopy / SCROG: a trellised, evenly-spread canopy lets ICL bars thread through and lets air move. Packed hedges trap air.

Timing: run structural defoliation around the usual windows (~day 21, a lighter pass ~day 42 if still dense), but re-target it — you’re opening airflow lanes and removing shade onto lit bud sites, not clearing dead zone. Go light late in flower; aggressive late defoliation swings transpiration unpredictably in a zone you’ve made humid on purpose.

The heat bill: a watt is a watt

Ignore the marketing about LEDs ‘running cool’. For HVAC sizing it’s false in the way that matters. In a sealed room, essentially all the electrical power you feed a fixture ends up as heat your HVAC has to remove. A 600 W LED and a 600 W HPS impose the same cooling load. The LED’s advantage is hitting your target PPFD at fewer watts — you install fewer watts, not cooler ones.

That 1,706 BTU/hr sits on top of your overhead array, dehumidifier (nearly 100% of its wattage becomes in-room heat), equipment and people (~400 BTU/hr each). Lighting is typically 70–85% of total room cooling load before SCL — so a retrofit is a direct, calculable increase to your single largest load. Size it deliberately; don’t assume your AC has the margin.

How to measure your actual load: sum measured wattage from driver labels and a clamp meter (not marketing specs); apply ×3.412; add dehumidifier watts (~1:1 to heat), people and ventilation infiltration; then validate against reality — log the lights-on temp-rise rate; if the room heats faster than your BTU math predicts, you’ve under-counted a load.

The lower-canopy microclimate

You have deliberately created flower in the worst-ventilated zone in the room. The lower canopy is where air stalls, humidity pools, and botrytis germinates inside dense colas from the inside out. Before SCL that zone was sparse larf or bare stem. Now it’s dense, transpiring bud. The airflow problem isn’t a side note — it’s the direct consequence of doing this at all.

• Stagnant dead spots. Overhead HAF fans sweep the top; the basal zone sits in still air below the airstream.
• Added latent load, low down. New bud mass transpires into the zone with the least air movement. Moisture has nowhere to go.
• Cold floor, warm air. Condensation forms near pots and the lowest leaves first — exactly where you’ve put your new flower.

That covers bulk exchange. It does not solve the sub-canopy microclimate, because room-average airflow says nothing about the dead zone at the bottom. You need dedicated low-level air movement:

• Dedicated low fans aimed through the lower canopy — the single highest-leverage move once you light the bottom.
• Sweep, never blast. Gentle turbulent movement that flexes leaves, not a jet at the buds — direct blasting dries trichomes and causes wind burn.
• ~one oscillating fan per 4–6 plants as a starting density, biased to the lower tier.
• Open the structure (Section 7) so air can thread through — fans can’t fix a packed hedge.

VPD and dew point at depth: manage VPD where the bud is, not just at the room sensor. Reasonable flower targets ~0.8–1.2 kPa early, tightening to 1.2–1.6 kPa late. Keep leaf temperature ~6–8 °C above dew point, especially at lights-off. Put a sensor in the lower canopy — the basal zone reads wetter than room average, and that delta is exactly the risk SCL introduces.

Commissioning: how to dial it in

Don’t install to a spec sheet and walk away. Install, measure, adjust, log. The whole value of SCL is in the lower-canopy numbers, so that’s where you measure.

1. 1
   Baseline map

   Before fitting bars, take PPFD at apical, middle and basal strata across a grid — quantum sensor, three directions at each point. This is your ‘before’.
2. 2
   Install for uniformity, not peak

   Overlapping daisy-chained bars beat hot-spotted point sources. Set standoff generous initially; you can always move closer.
3. 3
   Re-map

   Confirm middle/basal land in the target bands (500–700 / 300–600 µmol). Hunt for hot spots near fixtures — those are your bleaching risks.
4. 4
   Ramp, don't slam

   Bring sub-canopy intensity up over several days — to catch early bleaching and because shade-developed leaves must re-acclimate before they can use the light (Section 5).
5. 5
   Re-map the air

   Drop a temp/RH sensor into the basal zone, compare to room average, add low fans until the delta closes and you hold the VPD / dew-point buffer at depth.
6. 6
   Re-check thermal

   Log lights-on temp-rise against your BTU prediction. Confirm cooling and decoupled dehumidification hold through a full lights-off transition.
7. 7
   Hold flux constant for the first run

   Evaluate grade and uniformity against your geometric change before deciding to add net flux.
8. 8
   Log per cultivar

   Response is cultivar-dependent — bleaching threshold, stretch under FR, grade uplift. Dense, tall, unpruned genotypes gain most; some tight cultivars gain little and rot easily.

The economics, framed honestly

The return on under-canopy lighting is mostly a grade story, not a gross-weight story. The clearest financial mechanism in the research is converting B/C-grade lower bud into A/B-grade saleable flower, plus reduced variability — worth more in a price-pressured, quality-led market than raw biomass.

The realistic 25–35% average improvement is achievable^[7] — conditional on cultivar fit, plant-count discipline and the climate accommodations. Model it on your grade spread and your power and HVAC costs, not a vendor’s headline. If most of your lower canopy is already saleable, the upside is smaller. If you’re throwing away larf every harvest, that larf is the prize.