Green light is the photon that keeps working below the top leaf layer: on the guide’s 600 umol lettuce feed at 60R/20B/8G/10FR/2UV the bar lands on 486 useful umol, and the 8% green slice contributes 28.8 of that at lettuce weight 0.6. Green light canopy penetration is a depth budget, not a brightness goal — as the canopy closes, the optimal green share falls from 23% at seedling to just 2% at flower, because the leaves green light is good at reaching are the ones left behind. The McCree weighting on this page re-prices every photon by crop weight, so a green-heavy bar is judged by the useful umol it deposits at depth, not by raw watts aimed at the tops.
Key takeaways
- Score photosynthesis against the stage target — Photosynthesis is the weighted PPFD over the stage optimal PPFD scaled to 10, then shaped by CO2 and leaf temperature: seedling 500, veg 800, flower 1000, fruit 800.
- Yellow: mid-band or 20-40% off PPFD.
- Optimal PPFD per stage: seedling 500, veg 800, flower 1000, fruit 800.
- Photosynthesis is weighted PPFD over optimal times 10, shaped by CO2 (800 = 1.0, 2000 = 1.3) and leaf temperature (22 C = 1.0, 35 C = 0.8).
How Light Spectrum Photomorphogenesis Designer McCree + Phytochrome Lab Helps You Master This
- Pick the crop and its McCree weights — Lettuce leans on red at weight 1.0 with blue 0.7 and green 0.6; tomato runs blue 0.8; cannabis pushes far-red to 0.5 and UV to 0.4 because trichome-driven crops spend those photons on secondary chemistry.
- Set the light shares — Enter total PPFD and the red, blue, green, far-red and UV percentages. The engine re-prices every photon by the crop weight so a green-heavy bar still pays its way instead of reading as waste.
- Read the McCree-weighted PPFD — Weighted PPFD is total PPFD times the sum of each percentage times its weight. A 600 umol lettuce bar at 60R/20B/8G/10FR/2UV lands on 486 useful umol.
- Score photosynthesis against the stage target — Photosynthesis is the weighted PPFD over the stage optimal PPFD scaled to 10, then shaped by CO2 and leaf temperature: seedling 500, veg 800, flower 1000, fruit 800.
- Score photomorphogenesis and Pfr — The morphology score subtracts blue and far-red error from the stage optimal (cryptochrome and phytochrome), and Pfr from red over red plus far-red tells you whether the plant is compact or stretching.
- Read the recipe and the safety band — The spectrum recipe prints the stage-perfect mix, target PPFD, photoperiod and Pfr. Green band: on target. Yellow: mid-band or 20-40% off PPFD. Red: off by a mile, UV over 5%, or far-red stretch at seedling.
The Formula Behind the Result
McCree-weighted PPFD (umol) → P_{text{McC}} = Pleft( r,W_R + b,W_B + g,W_G + f,W_F + u,W_U right)Photosynthesis score (/10) → text{Photo} = mathrm{clamp}left( frac{P_{text{McC}}}{P_{text{opt}}},10, f_{CO_2}, f_{T_{text{leaf}}}, 0, 10 right)Yield boost (%) vs 400 umol baseline → Y = mathrm{clamp}Big( frac{P_{text{McC}}-400}{400},100, m_s + E + U, 0, infty Big)Emerson effect bonus (%) → E = 10% Leftrightarrow left( mathrm{flower} land 10% le f le 15% right)UV stress terpene bonus (%) → U = 5% Leftrightarrow 2% le u le 3%Photomorphogenesis score (/10) → M = mathrm{clamp}Big(10 - frac{left| b - b^{star} right|}{b^{star}},5 - frac{left| f - f^{star} right|}{f^{star}},3, 0, 10Big)Phytochrome Pfr ratio from red share → text{Pfr} = frac{R}{R + F}LED energy efficiency (umol/J) by spectrum mix → eta = frac{2.8,r + 2.0,b + 2.5,g + 3.0,f + 1.5,u}{100}
Target Ranges: What the Numbers Should Be
- McCree weights: lettuce red 1.0, blue 0.7, green 0.6, far-red 0.2, UV 0.1. Tomato red 1.0, blue 0.8, green 0.5, far-red 0.4, UV 0.2. Cannabis red 1.0, blue 0.9, green 0.4, far-red 0.5, UV 0.4. Basil red 1.0, blue 0.8, green 0.5, far-red 0.3, UV 0.2. Strawberry red 1.0, blue 0.7, green 0.5, far-red 0.5, UV 0.2.
- Stage optimal spectra: seedling blue 30%, red 40%, far-red 5%, UV 2%, green 23%. Veg blue 25%, red 50%, far-red 10%, UV 2%, green 13%. Flower blue 15%, red 65%, far-red 15%, UV 3%, green 2%. Fruit blue 15%, red 60%, far-red 15%, UV 3%, green 7%.
- McCree PPFD is total PPFD times the sum of each percentage times its crop weight. Optimal PPFD per stage: seedling 500, veg 800, flower 1000, fruit 800. Photosynthesis is weighted PPFD over optimal times 10, shaped by CO2 (800 = 1.0, 2000 = 1.3) and leaf temperature (22 C = 1.0, 35 C = 0.8).
- Yield boost runs from the 400 umol baseline with a stage multiplier (seedling 0.5, veg 1.0, flower 1.2, fruit 1.1). Emerson adds 10% when far-red sits at 10-15% during flower, and UV adds 5% when UV sits at 2-3% for the terpene stress response. The floor holds yield at 0% so the number reads as gain, not loss.
- Photomorphogenesis subtracts blue error (cryptochrome compactness) at 5 points per unit relative error and far-red error (phytochrome stretch) at 3 points, clamped 0-10. Phytochrome Pfr is red over red plus far-red: 0.7 compact baseline, 0.3 stretch trigger, 0.9 very compact.
Use the free Hydroponic System Selector Quiz — Kratky, NFT & Vertical Tower Blueprint to run the numbers on your own setup.
Use the free Nutrient Antagonism Lockout Predictor Mulder Matrix Solver to run the numbers on your own setup.
Use the free Nutrient Cost Optimizer — Bulk Mixing & Recycling Blueprint to run the numbers on your own setup.
Compare every free calculator side-by-side on the MistCulture tools hub — each one keeps your workspace in sync.
Related guides
Worked Example: 8% Green Through a Lettuce Canopy
Take the guide’s reference bar: a 600 umol lettuce feed at 60R/20B/8G/10FR/2UV. McCree-weighted output is 486 useful umol, and the green slice is the share that keeps working after the top leaves have absorbed most of the red and blue: 8% of 600 = 48 raw umol, and at the lettuce green weight of 0.6 that is 28.8 useful umol, or 5.9% of the 486 working photons.
Now thicken the canopy. The stage-optimal recipes cut green from 23% at seedling to 13% at veg and 2% at flower because a closed canopy no longer needs green to rescue shaded leaves. Green light canopy penetration buys you the lower leaves while they exist; once they are gone those photons pay for nothing, so the recipe shrinks the share instead of spending watts on it.
- 48 umol — raw green output at 8% of a 600 umol feed; 28.8 of it stays useful at lettuce weight 0.6
- 486 umol — McCree-weighted total for the 60R/20B/8G/10FR/2UV reference bar
- 23% to 2% — optimal green share from seedling to flower, tracking how much canopy the green photons actually reach
Frequently Asked Questions
References & Science
- USDA National Agricultural Library
- NCSU CALS — Vapor Pressure Deficit resources
- Purdue Extension — hydroponics guide
Frequently asked questions
How much green light does a canopy actually need?
Match the green share to how much canopy the light has to reach: 23% at seedling, 13% at veg, 2% at flower and 7% at fruit. Weight every photon first — green earns 0.6 on lettuce, 0.5 on tomato and 0.4 on cannabis — so a green-heavy bar only pays if the useful umol it deposits below the top leaves beats what red and blue would have bought. On the reference bar the 8% green slice is 28.8 useful umol out of a 486 weighted total as soon as the plant sits past the green line in real use.
What is the biggest mistake when relying on green light for canopy penetration?
Assuming the green share substitutes for coverage. Green penetrates deeper, but the 20% drop rule still applies — flux falls off layer by layer — and the recipe still shrinks green to 2% at flower because a closed canopy shades its own lower leaves. Measure weighted PPFD at depth instead of trusting the fixture rating; the difference is raw watts that never reached a leaf.
Which tools help me measure green light canopy penetration?
The embedded Light Spectrum Photomorphogenesis Designer returns the McCree-weighted PPFD for your green share, and the Light Coverage Architect maps delivered PAR across the real footprint so the canopy-depth reading is measured rather than assumed. Open the PAR / DLI lab to check bottom-leaf values against the 600-umol class targets this guide prices.
References
- Michigan State University Floriculture — Daily Light Integral reference tables
- Bugbee, B. — Toward an optimal spectral quality for plant growth and development (USU)
- USDA NIFA — controlled environment horticulture lighting guidelines
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