Optimal Cannabis Grow Light Schedule: 18 Hours On, 6 Hours Off
Discover the best cannabis grow light schedule for optimal plant health and yield, with a focus on the 18/6 light cycle.
A cannabis grow light distance chart shows how high to hang a lamp above the canopy at each stage of growth. For most LED fixtures, that means 24 to 36 inches above seedlings and clones, 18 to 24 inches in veg, and 12 to 18 inches in flower. HID lamps (metal halide and HPS) hang farther back because they run hotter, at 30 to 40 inches at the start and 18 to 24 inches in bloom.
cannabis grow light distance chart
What Is a Cannabis Grow Light Distance Chart?
It is a table of hanging heights matched to plant stage and lamp type. The goal is simple: give the canopy enough light to build dense flowers without bleaching leaves or cooking the top of the plant. Distance stands in for intensity, so the same chart gives different numbers for a 100W LED and a 1000W HPS.
Grow Light Distance Chart by Stage
These ranges assume a reflective grow tent, one lamp over the canopy, and a plant that fills about half the footprint. Move toward the larger number when your lamp runs at full power.
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- Clones and seedlings: LED 24 to 36 inches, T5 fluorescent 6 to 12 inches, HID 36 to 40 inches.
- Veg, weeks 1 to 3: LED 24 to 30 inches, CMH 24 to 30 inches, HID 30 to 36 inches.
- Veg, week 4 and beyond: LED 18 to 24 inches, CMH 18 to 24 inches, HID 24 to 30 inches.
- Flower, weeks 1 to 3: LED 16 to 20 inches, CMH 18 to 24 inches, HID 18 to 24 inches.
- Flower, weeks 4 to harvest: LED 12 to 18 inches, CMH 15 to 20 inches, HID 12 to 18 inches with air cooling.
Why Distance Changes Light Intensity
Light falls off with the square of the distance. Double the gap between lamp and canopy and the plant receives one quarter of the light. That math explains why a 6 inch shift near the plant matters more than a 6 inch shift near the ceiling.
Heat moves the other way. Radiant heat climbs as the lamp gets closer, so the top colas take the worst of it. A lamp that feels fine at 24 inches can bleach a bud tip at 10 inches.
How Lamp Type Changes the Chart
LED: runs cool at the diode, so the limit is light intensity rather than heat. Most LED charts start at 24 inches and shrink as the plant grows.
HID (metal halide and HPS): pushes out a lot of radiant heat. Air-cooled hoods let you hang closer, while open reflectors need extra room.
CMH and LEC: sits between the two, with strong UV output that can stress leaves at short range.
Fluorescent and T5: weak output, so tubes sit 6 to 12 inches above seedlings and rise as the plant grows.
How to Adjust the Chart for Your Setup
Four things move the numbers: lamp wattage, dimmer setting, tent height, and the spread of the fixture.
- Wattage: a 100W LED hangs closer than a 720W bar light. High-output fixtures need more headroom.
- Dimmer: a lamp turned down to 50 percent can hang closer without burning leaves.
- Footprint: bar-style LEDs spread light across the tent, so the center hotspot is weaker than with a single COB.
- Canopy shape: a flat, even canopy handles closer light than a plant with one tall cola.
Signs Your Light Is Too Close
- Top leaves turn pale or yellow while lower leaves stay green.
- Leaf edges curl up like a taco and tips dry to a crisp.
- Bleached white patches on the highest buds.
- New growth comes out thin and spiky, a trait growers call fox tailing.
Fix it by raising the lamp 4 to 6 inches and checking again in two days. Damaged leaves will not recover, but new growth should come in green.
Signs Your Light Is Too Far
- Long gaps between nodes and thin stems.
- Seedlings lean hard toward the lamp.
- Slow growth in veg and small, airy buds in flower.
Drop the lamp 4 inches at a time. Wait three days between moves so the plant can respond before you change anything else.
Measure Light Instead of Guessing
A PAR meter or a phone light-meter app gives you a PPFD reading at canopy level, and that number beats any chart. Common targets are 100 to 300 PPFD for clones, 400 to 600 in veg, and 600 to 1000 in flower.
No meter on hand? Use the back-of-hand test. Hold your hand at the top of the canopy for 30 seconds, and if it feels hot, the lamp is too close.
Do You Move the Light During Flowering?
Yes. Plants stretch in the first three weeks of 12/12, so a lamp that sat at 20 inches in veg may need to go up to 24. Once stretch stops, lower it in small steps until you reach the flower range.
Keep the lamp at least 12 inches from the tallest cola unless the manufacturer says otherwise. Some modern bar LEDs are rated for 6 inches, so read the spec sheet for your model.
Frequently Asked Questions
Can I hang an LED 12 inches above my plants?
Only if the lamp is dimmed, low wattage, or rated for close hanging. A full-power 500W LED at 12 inches will bleach tops in a few days.
Does the chart change for autoflowers?
No. Autoflowers use the same distances. They flower on their own schedule, so switch to the flower range when you see the first pistils.
What if a plant is too tall for the tent?
Top or train the plant, or dim the light and hang it higher. Running out of headroom is a common reason growers burn their best colas.
Do I need a different chart for a 2x2 tent?
Yes, and the numbers shift up. Small tents hold less air, so heat builds faster and lamps sit closer to the walls. Use the larger distance in each range.
Quick Rules to Remember
- Start high and lower the lamp in small steps.
- Read the plant, not just the chart.
- Raise the light before you blame nutrients when top leaves look bad.
- Check the manufacturer's hanging height for your exact model.