Horticulture reference

Cannabis grow lighting

Published horticulture references describe the light fixture as the engine of the indoor grow, with the choices around which fixture to specify, how far to hang it, and what schedule to run documented as setting the ceiling on every downstream variable. A cheap light over documented genetics is described in published reports as producing an average harvest; a properly specced light over the same seeds is documented as roughly doubling the yield. This reference covers the three fixture types described as worth specifying in 2026, the PPFD and DLI numbers that published references identify as decisive, the documented appearance of light burn, and the photoperiod-versus-auto schedule split as described in horticulture literature.

LED versus HPS versus CMH as documented

High-pressure sodium fixtures are documented as the historical default and still described as producing excellent material, but published references document them as running hot, producing a heavy red-orange spectrum that lacks the blue end the plant is documented as using in veg, and pulling roughly 40% more wattage to hit the same canopy intensity as a modern LED. A 600 W HPS is documented as lighting a 4-by-4 tent adequately, costing the least up front, with bulb replacement documented as required every twelve to eighteen months as the spectrum degrades; the running cost over three years is documented in published reports as higher than an LED of equivalent footprint. Ceramic metal halide is documented as sitting between the two — better spectrum than HPS, longer bulb life, less heat, but still warmer than LED and described in current published material as hard to justify against LED pricing.

Modern quantum-board and bar-style LEDs are documented as running cool, delivering a full spectrum tuned for both veg and flower without a bulb swap, with pricing documented as having dropped enough that a 240 W board for a 2-by-2 tent or a 480 W bar fixture for a 4-by-4 is described in current references as the defensible default.^[3] Cheap LEDs sold under unfamiliar brand names are documented as often misreporting their wattage and PPF — a fixture claiming 400 W from the wall is documented in published teardowns as sometimes actually pulling 220 W and producing roughly half the photon output of a legitimate 400 W board. Diode brand and driver manufacturer are documented as the two specs that predict quality.

PPFD targets per stage in published references

PPFD is documented as the number of photons in the photosynthetically active range hitting a square metre of canopy each second, in micromoles.^[1] Published references describe it as the single number that decides whether plants are light-starved, dialled in, or being damaged. Seedlings are documented as wanting roughly 200 to 400 µmol; pushing harder is documented as stressing the cotyledons and stretching the stem. Vegetative growth is documented as running comfortably at 400 to 600 µmol, with the upper end appropriate from week three onward when the canopy is filled out. Early flower is documented as wanting 600 to 800 µmol, ramping up through the stretch as the plant adds leaf area.

From week three of flower through fat-bud weeks, published PPFD targets in flower are documented around 800 to 1000 µmol·m⁻²·s⁻¹ at the top of the canopy.^[2] Pushing past 1000 µmol without supplemental CO2 is documented as stopping paying back in yield — the plant is documented as unable to use additional photons fast enough to convert them into sugar, with the extra intensity documented as causing heat stress, bleaching, and burnt top colas.^[4] CO2 supplementation at 1200 to 1500 ppm is documented as raising the useful ceiling to roughly 1400 µmol, but adding CO2 to a home tent without sealed-room infrastructure is described in published references as rarely cost-effective. A cheap quantum meter or even a phone app calibrated against a known fixture is documented as sufficient to dial these numbers in.

DLI and the photoperiod math

Daily Light Integral is documented as converting PPFD into a daily total — moles of photons per square metre per day — and is described in published references as the number that actually drives yield. The documented conversion is PPFD × hours of light × 0.0036 = DLI. A vegetative plant running 500 µmol on an 18-hour schedule is documented as receiving a DLI of roughly 32; a flowering plant at 900 µmol on a 12-hour schedule is documented as receiving 39. The documented DLI ranges are 20 to 30 for veg, 35 to 45 for early flower, and 40 to 50 for peak flower. Below 20 the plant is documented as growing but staying leggy; above 50 published reports describe wasted power unless CO2 is in the loop.

The documented implication for autoflowers is that the 20/4 schedule, despite shorter peak intensity, produces a higher DLI than 12/12 at the same PPFD — which is documented as why autoflowers reach respectable yields even with short flowering periods. If a fixture is documented as only delivering 600 µmol, published protocols describe running it for more hours rather than trying to push intensity beyond what the diodes can supply.

Hanging distance and documented light burn

Manufacturer-suggested hanging distances are documented in published references as usually correct but worth verifying with a quantum meter on first hang. For a typical 240 W LED quantum board, documented hanging guidance is 60 to 75 cm above the canopy for seedlings, 45 to 60 cm for veg, and 30 to 45 cm through flower. HPS fixtures are documented as running hotter and sitting further away — usually 45 cm minimum in flower, with a back-of-the-hand test at canopy height as the documented sanity check. If the hand cannot be held at canopy height comfortably for thirty seconds, the fixture is documented as too close.

Light burn is documented in published references as bleached, yellow-white tops on the colas closest to the fixture, often with sharply curled upper leaves and a clear gradient from healthy mid-canopy to scorched top. It is documented as most common in late flower when the plant has stretched into the light without the fixture being raised. The documented response is dimming the light by 10 to 20% if the driver allows, raising it by 10 cm, or both. Bleached tops are documented as recovering colour but with the bud quality on affected sites permanently reduced. Published protocols describe checking fixture height weekly through the stretch and the first three weeks of flower.

Schedules in published references — photoperiod versus auto

Photoperiod plants are documented as reading the length of the dark cycle to decide whether to stay in veg or trigger flowering, with two documented schedules: 18 on and 6 off through veg, and 12 on and 12 off through flower. The flip from 18/6 to 12/12 is documented as the trigger for the move into flower, with the dark period documented as needing to be absolute from that day onward. Even a few minutes of light leakage at night is documented as capable of disrupting the response. A cheap timer with a battery backup is described in published references as the reliable approach; wifi-dependent smart-plug schedules are documented as a known failure mode when a router reboots.

Autoflowering plants are documented as ignoring day length and running their own internal clock, with finish documented in a fixed nine to eleven weeks regardless of schedule. The documented auto schedule is 20 on and 4 off from seed to harvest — long days for maximum DLI with a short rest cycle described as reducing tent heat at night. Some published protocols describe running autos on 24/0 for the first three weeks then dropping to 18/6 for power savings; the documented yield loss is described as small but real, with the documented default being 20/4 unless electricity is genuinely expensive.

Lockbox Seeds publishes reference material about cannabis horticulture for educational purposes. The legal status of cannabis cultivation differs by jurisdiction, and readers are responsible for confirming the law in their own area before acting on this material.