Grow Light Distance Guide: How Far Should Lights Be From Plants?
Getting your grow light height right is one of the easiest ways to improve plant growth. Too close causes light burn, too far causes stretching. This guide shows you exactly where to position your lights for optimal results.
Quick Reference Guide
| Growth Stage | LED | HPS | Fluorescent |
|---|---|---|---|
| Seedlings | 24-36" | 30-36" | 6-12" |
| Vegetative | 18-24" | 24-30" | 6-12" |
| Flowering | 12-18" | 18-24" | 4-8" |
These are starting points - adjust based on your specific light's intensity and plant response.
Why Light Distance Matters
Light intensity follows the inverse square law: when you double the distance from your light, intensity drops to 1/4. This means small height changes have big effects on how much light reaches your plants.
The Inverse Square Law in Action
- At 12 inches: 1000 PPFD
- At 18 inches: ~444 PPFD (55% reduction)
- At 24 inches: 250 PPFD (75% reduction)
- At 36 inches: ~111 PPFD (89% reduction)
Moving lights just 6 inches can dramatically change intensity!
LED Grow Light Distance by Growth Stage
Seedlings & Clones: 24-36 inches
Young plants have small root systems and limited ability to handle high light. Start high (36") and gradually lower as they develop. Target PPFD: 200-400 umol/m2/s.
- Week 1-2: 36 inches, dimmed to 25-50%
- Week 2-3: 30 inches, dimmed to 50-75%
- Week 3-4: 24 inches, 75-100%
Vegetative Stage: 18-24 inches
Plants in veg can handle more light. Lower gradually as they grow stronger. Target PPFD: 400-600 umol/m2/s.
- Early veg: 24 inches
- Mid veg: 20-22 inches
- Late veg: 18-20 inches
Flowering Stage: 12-18 inches
Maximum light during flower drives bud development. Target PPFD: 600-1000 umol/m2/s (higher with CO2).
- Transition/stretch: 18 inches
- Peak flower (weeks 3-6): 12-16 inches
- Late flower: Can raise to 16-18" to reduce stress
Important: Check Your Light's Specs
These are general guidelines. A 200W LED needs to be closer than a 600W LED for the same PPFD. Always check your manufacturer's recommended hanging height and adjust from there.
HPS Light Distance Guidelines
HPS lights run hotter than LEDs, so they need more distance to prevent heat stress. The primary concern is infrared heat rather than light intensity.
| HPS Wattage | Seedling | Veg | Flower |
|---|---|---|---|
| 250W | 20-24" | 14-18" | 10-14" |
| 400W | 24-30" | 18-24" | 14-18" |
| 600W | 30-36" | 24-30" | 18-24" |
| 1000W | 36-48" | 30-36" | 24-30" |
The Hand Test: Hold your hand at canopy level for 30 seconds. If it's uncomfortably hot, the light is too close. Your plants feel the same heat.
Signs Your Lights Are Too Close
Watch for these symptoms of light stress (also called light burn or photo-bleaching):
Light Burn Symptoms
- Bleached tops: White or yellow patches on leaves closest to light
- Leaf curling: Leaves curl upward ("taco" or "canoe")
- Brown spots: Burnt patches that start at leaf tips
- Crispy texture: Dry, brittle leaves at the top
- Foxtailing: Abnormal, airy bud growth (cannabis)
What to Do
- Raise lights 4-6 inches immediately
- Dim lights if possible (start at 75%)
- Check temperature at canopy level
- Increase airflow over the canopy
- Remove severely damaged leaves
Signs Your Lights Are Too Far
Insufficient light causes different but equally problematic symptoms:
Light Deficiency Symptoms
- Stretching: Long, thin stems reaching toward light (etiolation)
- Wide internodes: Large gaps between leaf nodes
- Pale leaves: Light green or yellowing foliage
- Leggy seedlings: Tall, weak stems that can't support themselves
- Slow growth: Stunted development and delayed flowering
- Small flowers/fruits: Reduced yields and potency
How to Find Your Perfect Height
Step 1: Start with Manufacturer Recommendations
Quality light manufacturers provide PPFD maps at various heights. Start at their recommended height for your growth stage.
Step 2: Observe Your Plants
After 3-5 days at a new height, check for:
- Healthy green color (not pale, not dark)
- Compact growth (not stretchy)
- No leaf curling or discoloration at top
- Active new growth
Step 3: Make Gradual Adjustments
Move lights 2-4 inches at a time, then wait 3-5 days before adjusting again. Sudden large changes shock plants.
Step 4: Measure if Possible
A PAR meter or smartphone app (like Photone) gives you actual PPFD readings. Target ranges:
- Seedlings: 200-400 PPFD
- Vegetative: 400-600 PPFD
- Flowering: 600-900 PPFD (up to 1200 with CO2)
Adjusting for Different Light Wattages
Higher wattage lights need more distance. Here's a rough guide for LED grow lights:
| LED Wattage | Seedling Height | Flower Height |
|---|---|---|
| 100-200W | 18-24" | 8-12" |
| 200-400W | 24-30" | 12-16" |
| 400-600W | 30-36" | 16-20" |
| 600W+ | 36-42" | 18-24" |
Pro Tips for Optimal Light Positioning
Use Adjustable Hangers
Ratchet hangers make height adjustments easy and keep lights secure. They're cheap insurance against dropped lights.
Account for Plant Growth
Plants can grow inches overnight during stretch. Check daily and raise lights as needed to maintain distance.
Consider Canopy Uniformity
Taller plants get more light. Use training (LST, SCROG) to create an even canopy so all plants get similar intensity.
Use Dimming Before Raising
If plants show light stress, try dimming 10-20% before raising. This maintains coverage while reducing intensity.
Key Takeaways
- Start high (36") for seedlings and gradually lower as plants mature
- LED lights can be closer than HPS due to lower heat output
- Watch for light burn (bleaching, curling) or deficiency (stretching, pale)
- Adjust in small increments (2-4") and wait 3-5 days to assess
- Higher wattage lights need more distance
- Use a PAR meter or app for precise measurements
Calculate Your Optimal Light Setup
Our free calculator determines the perfect wattage and provides hanging height recommendations for your specific grow space.
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