Field Notes

Cannabis light spectrum — documented effects of red, blue, and far-red wavelengths

Photosynthesis runs on a narrow band of the electromagnetic spectrum, and cannabis is documented to respond differently to each wavelength inside that band. The PAR range — photosynthetically active radiation, 400 to 700 nanometres — covers the visible spectrum from violet through red, and modern horticultural lighting research has spent decades mapping which wavelengths drive vegetative growth, which trigger flowering, and which influence secondary metabolite production. This reference explainer walks through the documented role of each wavelength band, the published McCree curve that anchors the field, and why full-spectrum LED fixtures replaced high-pressure sodium in commercial cultivation between 2015 and 2023. Nothing here is instruction; the goal is to describe what the literature documents.

PAR vs PPFD — what each measurement is

PAR — photosynthetically active radiation — is the band of light from 400 to 700 nanometres that plants are documented to use for photosynthesis. PAR by itself is a wavelength range, not a measurement; what gets measured is PPFD, photosynthetic photon flux density, which counts the number of photons in the PAR band striking a surface per second per square metre. PPFD is expressed in micromoles per square metre per second (µmol/m²/s) and is the metric every modern grow-light spec sheet publishes. Published cannabis lighting research documents the canopy PPFD targets across the plant's life cycle: roughly 200 to 400 µmol/m²/s in seedling and early vegetative growth, 400 to 600 in mid-vegetative, and 600 to 1000 in flower for most cultivars.^[2] CO2-supplemented rooms are documented to tolerate and benefit from PPFD up to about 1500 µmol/m²/s; ambient-CO2 rooms saturate earlier and waste photons above 1000.

The McCree curve — what the research documented

The McCree curve is the foundational dataset in horticultural lighting, published by K. J. McCree in 1972 based on measurements of photosynthetic response across twenty-two crop species.^[1]The curve documents that plants use light across the full 400-to-700 nanometre band but with two pronounced response peaks: one around 440 nanometres (deep blue) and a larger one around 620 to 660 nanometres (red). The relative quantum efficiency drops in the green band (500 to 580 nanometres), though it does not reach zero — green light penetrates deeper into the canopy and contributes to photosynthesis in lower leaves. The McCree curve is documented as the basis for the "PAR" concept itself, and every modern horticultural lighting spectrum is engineered against it.

Blue (400-500 nm) — the documented role

Blue light is documented to drive compact, stocky vegetative growth and to trigger the production of chlorophyll a and chlorophyll b in measurable concentrations. Plants grown under blue-heavy spectrums are reported to produce shorter internodal spacing, thicker stems, and tighter leaf structure compared to plants grown under red-dominant light. Cryptochromes and phototropins — the blue-light photoreceptor proteins in plant cells — are documented to regulate stomatal opening, leaf positioning, and the photoperiodic timing signal that distinguishes a plant's perception of day from night. Cannabis-specific published research documents that a blue ratio of roughly 15 to 25 percent of total PAR produces the most balanced vegetative growth, with structures that hold up to the weight of late-flower buds.

Red (620-700 nm) — the dominant flowering wavelength

Red light is documented as the most photosynthetically efficient wavelength on the McCree curve, and the 660-nanometre peak is the wavelength most consistently associated with flowering response in cannabis. The red and far-red photoreceptor phytochrome exists in two interconvertible forms — Pr (absorbing at 660 nm) and Pfr (absorbing at 730 nm) — and the ratio of these forms is documented to trigger the photoperiodic signal that initiates flowering in short-day plants such as cannabis. High-pressure sodium lamps produce a yellow-orange spectrum heavy in the 580-to-620 nanometre band, which is part of why HPS was historically the documented bloom light of choice; modern LED fixtures replicate and improve on this with targeted 660-nanometre diodes alongside broad-spectrum white LEDs.

Far-red (700-780 nm) — the Emerson effect and shade response

Far-red light sits at the edge of the photosynthetic range and was historically excluded from the PAR definition. Modern research, particularly published work by Bruce Bugbee at Utah State University, documents that supplemental far-red between 700 and 780 nanometres increases photosynthetic efficiency through what is known as the Emerson enhancement effect — the combination of red and far-red light produces more photosynthesis than either wavelength alone.^[3]Far-red is also documented to trigger the plant's shade- avoidance response, leading to stretching and larger leaves. In commercial cannabis cultivation, controlled far-red supplementation during the final hour of the light cycle is reported to shorten the flowering window by three to seven days and to increase finished-flower mass by a measurable margin in published trial data.

UV-B (280-315 nm) — trichome response in the literature

UV-B is the highest-energy band of ultraviolet light that reaches the Earth's surface and is documented to trigger stress-response pathways in plants. Cannabis research from groups including the Lydon and Teramura work originally published in 1987 documents that controlled UV-B exposure during the final two to three weeks of flower increases trichome density and elevates THC concentration in finished flower.^[4]The mechanism is described as a defensive response — trichomes act as a sunscreen layer protecting the plant's reproductive tissue from UV damage — and the secondary metabolites produced in that defensive response include the cannabinoids and terpenes that define commercial cannabis. Excessive UV-B is documented to damage leaf tissue, reduce yield, and accelerate senescence, so commercial protocols describe narrow exposure windows rather than continuous UV-B as the documented best practice.

Why full-spectrum LED displaced HPS

High-pressure sodium lamps were the documented industry standard for cannabis flower from the 1980s through roughly 2018. The transition to LED happened on three measurable axes. The first is electrical efficiency: modern horticultural LEDs are documented to produce between 2.5 and 3.0 µmol of PAR photons per joule of electricity, compared to roughly 1.7 µmol/J for double-ended HPS — a 50 to 75 percent improvement that translates directly to lower electricity costs in commercial operations. The second is spectrum control: LED fixtures can be engineered to deliver precise blue, red, and far-red ratios, while HPS produces a fixed spectrum heavy in yellow-orange. The third is heat output: LED fixtures emit substantially less radiant heat per watt of PAR, which is documented to reduce HVAC load and to permit lights to be positioned closer to the canopy without leaf burn. Fluence by Signify, Gavita Pro, Heliospectra, and California Lightworks are among the manufacturers most frequently cited in published commercial case studies documenting the transition.

Lockbox Seeds publishes reference material about cannabis horticulture and plant science for educational purposes. The legal status of cannabis cultivation varies by jurisdiction; readers are responsible for understanding local law.