How Far Should Cannabis Grow Lights Be From Plants?

Quick answer

For a modern LED fixture, keep the light 12 to 24 inches above the top of the canopy. For HID lamps, use 18 to 36 inches depending on wattage. Fluorescent tubes and CFLs sit much closer, often 2 to 6 inches above the leaves. These ranges are starting points, not laws. The number that matters is the light intensity at canopy level, measured in PPFD (photosynthetic photon flux density). A weak light hung too high starves the plant. A strong light hung too low bleaches it.

read more

What you need before you start

• The wattage and type of your fixture (LED, HID, CMH, fluorescent)
• A PAR meter or a phone light-meter app that reads PPFD
• The current growth stage of the plants
• A thermometer placed at canopy height

Typical distance by light type

• LED, 100 to 200 watts: 12 to 18 inches
• LED, 300 to 600 watts: 18 to 30 inches
• HID or HPS, 400 watts: 12 to 18 inches
• HID or HPS, 600 watts: 18 to 24 inches
• HID or HPS, 1000 watts: 24 to 36 inches
• CMH or LEC, 315 watts: 18 to 24 inches
• T5 fluorescent: 2 to 6 inches
• CFL: 2 to 4 inches

Typical distance by growth stage

Seedlings and clones need the least light. Keep PPFD around 200 to 400 micromoles per square meter per second and hang fixtures at the high end of the range. Vegetative plants do well at 400 to 600 PPFD. Flowering plants want 600 to 1000 PPFD, and rooms running added CO2 can push to 1200 or higher. When you move a plant into a new stage, increase intensity over several days instead of all at once.

how many watts of grow light for cannabis

How to set the right height

1. Measure the gap from the lowest point of the fixture to the tallest bud site.
2. Write down the PPFD target for the current growth stage.
3. Hold the meter at canopy level and read the center of the footprint.
4. Read the four corners of the footprint and note the lowest number.
5. Raise or lower the fixture by 2 inches.
6. Wait 24 hours before you take the next reading.
7. Repeat the adjustment until the center and corners both land inside the target band.
8. Place the back of your hand at canopy height for 10 seconds.
9. Raise the light if your hand feels uncomfortable.
10. Record the final height and dimmer setting for the next run.

Signs the light is too close

• Top leaves turn pale yellow or white
• Leaf edges curl upward like a taco
• Tips turn crisp and brown while lower leaves stay green
• Buds stretch into thin foxtails

Signs the light is too far

• Stems stretch long between nodes
• New growth comes in thin and pale
• Buds stay small and airy
• Flowering takes longer than the breeder's window

Adjust in small steps and give the plant a day to respond. Cannabis tissue reacts slowly to light changes, so a fix made this morning shows up tomorrow.

Cannabis Grow Lights for Autoflowering Strains: A Practical Guide