PAR & DLI Requirements for Lettuce at Different Growth Stages

Lettuce is a staple leafy green often grown in controlled environments like greenhouses and indoor farms because it responds well to precise lighting. Matching light intensity and daily light exposure to each growth stage helps optimize yield, texture, and quality.

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1. Germination / Seedling Stage

Metric	Typical Range	Notes
PAR (µmol·m⁻²·s⁻¹)	50–150	Gentle light to avoid stress on young leaves
DLI (mol·m⁻²·day⁻¹)	4–10	Enough light to support root and leaf initiation

Tips:

• Use diffuse white or cool-white LED lighting.
• Keep lights at moderate distance to prevent hotspots.
• Monitor leaf development and adjust light if seedlings stretch.

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2. Vegetative / Leaf Growth Stage

Metric	Typical Range	Notes
PAR (µmol·m⁻²·s⁻¹)	200–400	Supports robust leaf expansion and chlorophyll development
DLI (mol·m⁻²·day⁻¹)	12–18	Common target range for many lettuce cultivars

Tips:

• Use a balanced spectrum (some blue + some red) to support healthy leaf structure.
• Ensure uniform lighting across the canopy to avoid shading of lower leaves.
• Monitor leaf color — pale leaves may indicate insufficient light or nutrients.

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3. Maturation / Harvest Stage

Metric	Typical Range	Notes
PAR (µmol·m⁻²·s⁻¹)	300–500	Higher light intensity helps maintain leaf color, crispness, and density
DLI (mol·m⁻²·day⁻¹)	18–25	Helps maintain growth speed and leaf quality as plants approach harvest

Tips:

• Avoid light stress—if temperature is high, slight shading during midday can help.
• Use supplemental lighting on cloudy days to uphold DLI targets.
• Monitor for bolting (premature flowering) — this often occurs under fluctuating light and heat stress.

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Why These Are Guidelines, Not Absolutes

• Lettuce variety, leaf form (romaine, butterhead, crisphead, looseleaf) can vary in light tolerance.
• Growing environment (greenhouse glazing, spacing, shading) affects actual light available.
• Other growth factors (CO₂, temperature, humidity, nutrients) must be in sync with light to avoid limiting yields.

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Using AH-PARDLI to Optimize Lettuce Lighting

With a PAR/DLI logging device like AH-PARDLI, you can:

• Track real-time PAR peaks to ensure your setup meets target intensity
• Monitor daily DLI and verify whether you’re hitting your growth stage goals
• Compare performance across days, weeks, or seasons with logged data
• Export CSV files to analyze production efficiency under different lighting regimens

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Summary Table

Growth Stage	PAR Target	DLI Target
Germination / Seedling	50–150 µmol·m⁻²·s⁻¹	4–10 mol·m⁻²·day⁻¹
Vegetative / Leaf Growth	200–400 µmol·m⁻²·s⁻¹	12–18 mol·m⁻²·day⁻¹
Maturation / Harvest	300–500 µmol·m⁻²·s⁻¹	18–25 mol·m⁻²·day⁻¹

By following these light targets and continuously measuring actual conditions, you can push your lettuce to perform at its best — with crisp texture, bright color, and steady growth.