PAR & DLI Requirements for Geraniums at Different Growth Stages

Geraniums (Pelargonium spp.) are popular bedding-plants and patio favorites. Their flowering, branching, and foliage quality are strongly influenced by light quantity (DLI) and quality (PAR). Research shows that higher DLI and sustained light levels improve branching, flower count and quality.

---

1. Seedling / Transplant Stage

Metric	Range	Notes
PAR (µmol·m⁻²·s⁻¹)	~150-250	Moderate light helps establish seedlings and roots.
DLI (mol·m⁻²·day⁻¹)	~6-10	Low to moderate DLI supports early vegetative growth before heavy flowering.

Tips:

• Provide bright, indirect or filtered light until seedlings are established.
• Maintain consistent photoperiod (14-16 hours) if using artificial lighting.
• Avoid very low light (<5 mol m⁻² day⁻¹) because quality falls sharply.

---

2. Vegetative / Canopy Development Stage

Metric	Range	Notes
PAR (µmol·m⁻²·s⁻¹)	~300-400	Supports vigorous branching and foliage fill.
DLI (mol·m⁻²·day⁻¹)	~12-15	Research shows significant improvement in development around this range.

Tips:

• Ensure even light distribution across all plants to avoid shading or uneven growth.
• Higher DLI helps shorten time to flowering and improves branch count.
• Maintain moderate temperatures and good air circulation for best results.

---

3. Flowering / Finish Stage

Metric	Range	Notes
PAR (µmol·m⁻²·s⁻¹)	~350-450	Strong light intensity supports flower size, color, and count.
DLI (mol·m⁻²·day⁻¹)	~16-20	Higher DLI correlates with faster flowering and better quality. For example, some geraniums reached best quality when DLI approached 18–20 mol m⁻² day⁻¹.

Tips:

• Ensure full sun or at least 6+ hours of direct light outdoors for optimal performance.
• Under indoor or greenhouse lighting, supplement light to reach target DLI if natural light is insufficient.
• Monitor for signs of too intense light (leaf bleaching, scorch) and adjust if needed.

---

Quick Reference Table

Growth Stage	PAR Target (µmol·m⁻²·s⁻¹)	DLI Target (mol·m⁻²·day⁻¹)	Key Objective
Seedling / Transplant	~150-250	~6-10	Establish roots & foliage
Vegetative / Canopy	~300-400	~12-15	Branching & foliage fill
Flowering / Finish	~350-450	~16-20	Flower size, count & quality

---

Why Monitoring PAR & DLI Matters for Geraniums

Geranium quality (branching, flowering, petal count) improves as DLI increases—studies show that going from ~4-8 mol to ~15-20 mol significantly boosts quality.
By using a PAR/DLI logger like the AquaHorti AH-PARDLI, growers can:

• Track real-time PAR curves throughout the day
• Monitor daily DLI and compare across days or seasons
• Export data for analysis and optimize light regimes

These insights help ensure your geraniums are not just surviving—but flourishing with vibrant flowers and healthy leaves.