Cannabis Grow Lights: PPFD, DLI, and What to Buy

Short answer first: for a 3x3 or 4x4 indoor tent, a full-spectrum LED bar fixture drawing 240 to 320 watts from the wall will take plants from seed to harvest, and it runs cooler and cheaper than the 600W and 1000W HPS lights that ruled basements for decades. The brand matters less than three things you can measure: how much light hits the canopy (PPFD), how long you run it each day, and how evenly it spreads across the tent.

cannabis grow light review 2024

PPFD and DLI are the numbers that move yield

PPFD is usable light landing on a square meter per second, measured in µmol/m²/s. DLI is that same light accumulated across a full day. For photoperiod cannabis, most growers target 300 to 600 PPFD in veg and 600 to 1000 PPFD in flower. Turned into DLI, that lands near 20 to 35 mol/m²/day in veg and 35 to 45 in flower. Push past 45 without added CO2 and a dialed room, and you are mostly buying heat and bleached top colas.

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Autoflowers tend to do better on 18/6 or 20/4 at lower intensity, since they never get a long dark break. Aim for 30 to 40 DLI in flower and treat tacoing leaves as your ceiling.

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Hanging height charts from manufacturers are a starting point, not gospel. A quantum sensor pays for itself by showing where the hot spots sit at canopy level. Phone light meter apps read in lux and get you in the ballpark, but they are not calibrated for plant-usable light.

Cannabis Grow Light Schedule 18/6: Compared With 20/4, 24/0, and 12/12

Spectrum: white LEDs are fine, with one or two tweaks

Plants use roughly 400 to 700 nm, the range called PAR. Modern white LEDs already cover it well. What you shop for is enough deep red near 660 nm and enough blue to keep internodes from stretching.

• Blue (400 to 500 nm): tight node spacing, compact structure, leaf development.
• Green (500 to 600 nm): drives some photosynthesis and reaches deeper into the canopy than red or blue.
• Red (600 to 700 nm): the main driver of photosynthesis and flower bulk.
• Far-red (700 to 750 nm): can speed flowering onset and add stretch. A little helps, a lot makes lanky plants.

LED vs HPS vs CMH

LED

Good bar-style fixtures hit 2.5 to 3.0 µmol per joule, with the best near 3.5. That efficiency is why a 300W LED can match a 600W HPS in output. Drivers run warm, so mount them outside the tent when the design allows. Cheap blurple panels often advertise wattage they never draw, so check input wattage and the PPF rating.

HPS

A 1000W HPS covers a 4x4 to 5x5 footprint and costs less up front. It also dumps heat into the tent, needs a bulb swap every two or three flower cycles, and sits near 1.7 µmol/J. If your space runs cold, that heat is a feature.

CMH

315W ceramic metal halide gives a broad spectrum and solid bud quality in a smaller footprint, usually 3x3. Bulb replacement is the cost to watch.

Wattage per square foot and hanging distance

For HID, plan on 30 to 40 watts per square foot. For LED, 25 to 40 watts per square foot depending on efficiency. A 4x4 tent is 16 square feet, so a 480W bar light is a sensible target and a 240W fixture is undersized for that space.

Distance depends on intensity, not on a chart. Seedlings want 100 to 200 PPFD, early veg 300 to 400, flower 600 to 900. Raise, lower, or dim the fixture to hit those numbers, then watch the plants for a day before you change anything else.

Heat, airflow, and the dark period

LEDs cut heat per photon but do not remove it. Canopy temperature still matters, and leaf surface temperature drives how fast plants transpire. Keep air moving across the tops and measure at canopy height, not at the tent wall.

In flower, an uninterrupted dark period is not optional. A pinprick of light from a zipper, a power strip LED, or a timer that fires at the wrong hour can trigger revegetation and stress that shows up as nanners. Check the tent at night with the room lights off.

Mistakes that cost a harvest

• Buying watts instead of PPF and efficacy.
• Running a fixture too close in late flower, which bleaches tops and pushes foxtails.
• Ignoring DLI while adding CO2 and nutrients.
• Skipping a light meter because the plants look fine.
• Trusting a panel rated 1000W that draws 110 watts.

Buying checklist

1. Measure your tent and set target PPFD for veg and flower.
2. Match fixture wattage draw to square footage.
3. Compare PPF and µmol/J, not marketing wattage.
4. Look at the spread map at your planned hanging height.
5. Confirm dimming, driver placement, and warranty.
6. Budget for a quantum sensor and a reliable timer.

Get those six items right and lighting becomes the least interesting part of the grow, which is exactly where you want it.