Artificial Light & CO₂ Targets in Glasshouses

Light and carbon dioxide are the primary drivers of photosynthesis. In protected cropping, both can be manipulated — but only effectively when they are balanced with temperature, water, and nutrition.

This page provides practical reference ranges, not rigid prescriptions.

Why light and CO₂ must be considered together

Photosynthesis is limited by whichever factor is in shortest supply.

Increasing light without sufficient CO₂ → diminishing returns
Increasing CO₂ without light → little response
Increasing both → strong yield and quality response

Temperature controls speed, but light and CO₂ control capacity.

Artificial light: what matters

PAR and DLI (quick recap)

PAR (µmol m⁻² s⁻¹)
Instantaneous usable light
DLI (mol m⁻² day⁻¹)
Total usable light received in a day

Plants respond primarily to DLI, not peak intensity.

Typical target DLI ranges (glasshouse crops)

Crop type	Indicative DLI target
Strawberry	12–20 mol m⁻² day⁻¹
Tomato	20–30 mol m⁻² day⁻¹
Pepper	18–25 mol m⁻² day⁻¹
Cucumber	20–28 mol m⁻² day⁻¹
Ornamentals (general)	10–18 mol m⁻² day⁻¹

Artificial lighting is typically used to top up natural radiation, not replace it entirely.

Light spectrum (practical overview)

Blue light (400–500 nm)
Compact growth
Stomatal regulation
Red light (600–700 nm)
Photosynthetic efficiency
Flowering response
Far-red (700–750 nm)
Stem extension
Can alter flowering and partitioning

Modern LED systems mix spectra to balance morphology and yield.

CO₂ enrichment: why it works

Ambient air: - ~420 ppm CO₂

Most glasshouse crops are CO₂-limited under high light.

Enrichment increases: - photosynthetic rate - water-use efficiency - carbohydrate availability

Typical CO₂ target ranges

Light level	Indicative CO₂ target
Low light / winter	600–800 ppm
Moderate light	800–1,000 ppm
High light / summer	1,000–1,200 ppm

Above ~1,200 ppm: - diminishing returns - increased cost - higher risk if ventilation increases

Interaction with ventilation

CO₂ enrichment is most effective when: - vents are mostly closed - temperature is controlled - humidity is managed

High venting rates rapidly dilute CO₂.

This creates a trade-off between: - temperature control - humidity control - CO₂ efficiency

Common mistakes

Running high CO₂ at low light
Ignoring night-time respiration losses
Assuming “more light is always better”
Not adjusting nutrition to increased growth rate

Practical guidance

Match CO₂ level to available light
Use DLI as the planning metric
Adjust feeding strength when light and CO₂ increase
Expect faster growth — not necessarily earlier phenology

Key principle

Light sets the ceiling, CO₂ raises it, temperature sets the pace.

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