Cannabis grow lights are lamps that give an indoor plant the light intensity and spectrum it needs to photosynthesize and build flower. Three specs decide the result: PPFD (how much light hits the canopy), spectrum (blue for leaf and stem, red for flower), and coverage (the square feet the fixture lights well). For most home grows, a full-spectrum LED that hits 400 to 600 PPFD in veg and 700 to 1,000 PPFD in flower does the job.
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What Do Cannabis Grow Lights Do?
Light is the raw material for photosynthesis. The plant converts photons into sugars, and those sugars become stem, leaf, and bud. Starve the plant of light and it stretches toward the source, yields less, and finishes with airy flower.
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Indoor growers control light in a way outdoor growers cannot. You set the hours, the intensity, and the color. That control is the main reason indoor flower can match or beat outdoor quality.
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Which Type of Grow Light Should You Buy?
Four light types cover almost every home grow. Each one trades cost, heat, and efficiency against the others.
LED Grow Lights
LEDs are the default choice for most home growers. A good bar-style LED runs at 2.5 to 3.0 µmol per joule, lasts 50,000 hours, and pushes little heat into the tent. A 240 to 480 watt fixture lights a 4x4 space. The tradeoff is price: quality LEDs cost more up front than HPS.
HPS (High-Pressure Sodium)
A 600 watt HPS is the old workhorse for a 4x4 tent. The gear costs less than a comparable LED, and the red-heavy spectrum flowers plants well. Downsides: 600 watts of heat, a ballast, and a new bulb every two or three grows.
CMH and LEC
Ceramic metal halide, sold as CMH or LEC, sits between LED and HPS. A 315 watt lamp covers about a 3x3 area and gives a fuller spectrum than HPS. Heat is lower than HPS but higher than LED.
Fluorescent (T5 and T8)
T5 and T8 tubes are cheap, cool, and weak. They work for seedlings, clones, and mother plants kept small. They will not carry a plant through flower on their own.
What Specs Matter Most on a Cannabis Grow Light?
PPFD and DLI
PPFD measures photons that land on the canopy each second, in µmol/m²/s. DLI (daily light integral) is the total light delivered over a day, in mol/m²/day, and it is the number plants respond to. A fixture with high PPFD run for 12 hours can deliver less total light than a weaker fixture run for 18.
Spectrum
Blue light (400 to 500 nm) drives compact growth and strong stems. Red light (600 to 700 nm) drives flowering and bud weight. White full-spectrum LEDs mix the two and let you read plant color with your eyes.
Wattage Draw vs Advertised Wattage
Ignore product names that promise huge wattage. Check the wall draw in watts. A fixture labeled 2000W that pulls 200 watts from the outlet is a 200 watt light.
Coverage and Efficiency
Every fixture has a footprint where light spreads even. Corners can drop 30 to 40 percent below center. Efficiency in µmol per joule tells you how much light you get per watt, and it is the number that shows up on your power bill.
How Much Light Does Cannabis Need at Each Stage?
- Seedlings and clones: 100 to 300 PPFD, 18/6 or 24/0, DLI 10 to 15.
- Vegetative: 400 to 600 PPFD, 18/6, DLI 25 to 40.
- Flowering: 700 to 1,000 PPFD, 12/12, DLI 35 to 50.
- With added CO2: 1,000 to 1,500 PPFD, and only if temperature, humidity, and nutrients keep pace.
Those values are canopy averages, not the reading under the hot spot at the center. Raise the light or dim it if leaves taco, bleach, or show burnt tips.
How Far Should You Hang the Light?
Height depends on the fixture, not the plant label. Start with these numbers and adjust from there.
- LED bar and panel: 12 to 24 inches above the canopy.
- Air-cooled HPS 600W: 18 to 30 inches.
- CMH 315W: 18 to 24 inches.
- T5 fluorescent: 6 to 12 inches.
The hand test works for any light. Hold the back of your hand at canopy height for 30 seconds. If it feels hot, the light sits close enough to stress the plant.
How Do You Match a Light to Your Grow Space?
Measure the floor area, then aim for watts per square foot. LEDs want 25 to 40 watts per square foot, HPS wants 40 to 60, and CMH sits near 35 to 45. A 4x4 tent is 16 square feet, so a 480 watt LED or a 600 watt HPS fits.
Tents smaller than 3x3 need less. A 2x2 space takes a 100 to 150 watt LED. Buying light for a bigger space than you own wastes power and can cook a small plant.
Heat, Airflow, and Power
Light produces heat, and the heat has to leave the room. An LED adds little, so a small oscillating fan often does the work. HPS and CMH need an exhaust fan sized to swap the air in the tent every one to three minutes.
Check the circuit before you buy. A 600 watt HPS with a ballast pulls near 660 watts, plus fans and pumps. Most US household circuits run 15 amps at 120 volts, which is 1,800 watts, and you should stay under 80 percent of that.
Common Mistakes That Cost Yield
- Trusting the number in the product name instead of the wall draw.
- Hanging LEDs too close, which bleaches tops and stunts growth.
- Running 12/12 too early and capping plant size.
- Leaving light leaks in the tent during flower, which can trigger male flowers or revegetation.
- Buying one light for a 2x4 veg tent and a 4x4 flower tent. Two lights fit two jobs.
Do You Need CO2 or UV Add-Ons?
Sealed rooms that run 1,200 to 1,500 PPFD can use added CO2 to push growth. Open tents vent the gas out before it helps, so most home growers skip it. UV and far-red supplements have small effects on resin and stretch, and they are the last upgrade, not the first.
Cannabis Grow Light FAQ
What is the best grow light for cannabis?
A full-spectrum LED that draws 30 to 40 watts per square foot of canopy is the best pick for most home growers. It runs cool, lasts years, and matches or beats HPS on yield per watt.
How many watts per plant do I need?
Think in area, not plants. Budget 25 to 40 LED watts per square foot. Four plants in a 4x4 need the same light as one plant in a 4x4, because the canopy fills the same footprint.
Can I use an ordinary household LED bulb?
You can, but you need many of them and the light spread is poor. A horticulture fixture covers the canopy with even PPFD and costs less to run for the same light output.
How long should cannabis grow lights stay on?
18 hours on and 6 off for veg, 12 on and 12 off for flower. Autoflowering plants usually run 18/6 or 20/4 from start to finish.