Lighting: light quality and quantity for cannabis

TL;DR

• PAR (400-700 nm) is the photosynthetically active radiation.
• PPFD targets: veg 400-600, flower 800-1500 µmol m⁻² s⁻¹.
• DLI 40-65 mol/m²/day maximises yield without photoinhibition.
• LED ~2.5-3.0 µmol/J clearly beats HPS (~1.7 µmol/J).

Spectrum and photomorphogenesis

Blue (400-500 nm) via cryptochromes keeps growth compact, red (600-700 nm) via phytochromes drives stretch and flowering. Far-red (>700 nm) via the Emerson effect can boost yield, UV-B can slightly stimulate trichome formation (Lydon 1987).

LED vs HPS

Modern LED fixtures hit 2.5-3.0 µmol/J, HPS lingers at ~1.7 µmol/J. LED offers tunable spectrum and less heat – lighter HVAC load. HPS is cheaper upfront.

Light quality, PPFD and DLI instead of wattage specifications

For plants, it is not the power consumption in watts that matters, but the photosynthetic photon flux density (PPFD) in µmol/m²/s and the resulting daily light integral (DLI) in mol/m²/day. Reasonable target values are 300 to 600 µmol/m²/s during the vegetative stage and 700 to 1000 µmol/m²/s during flowering; beyond this, yield only increases if CO₂ enrichment and nutrient supply scale accordingly. The DLI is around 20 to 40 mol during the vegetative stage, and 35 to 45 mol during flowering.

Modern LED systems have overtaken high-pressure sodium lamps in efficiency: 2.5 to 3.0 µmol per joule compared to around 1.7 µmol/J for HPS. The practical advantage lies less in power consumption than in the lower waste heat, which makes climate control in small rooms manageable in the first place. Full-spectrum white light with a red component has triumphed over pure blue-red combinations because it allows for the visual assessment of deficiency symptoms.

• Measure PPFD at the canopy, not on the floor
• Note edge drop-off: corners often receive only 50–60% of the average values
• Distance adjustments in steps of 5–10 cm, then observe for 3 days
• Efficiency comparison always in µmol/J, never in watts

Energy costs in the Swiss context

With Swiss household tariffs of roughly 25 to 35 centimes per kilowatt-hour, lighting is the largest variable cost block in indoor production. A more efficient lighting system simultaneously reduces the cooling load, which doubles the real effect. It is precisely this cost structure — electricity, climate control, labor — that explains why Swiss indoor flowers range from CHF 7.80 to 15.60 per gram in retail, while outdoor products start at CHF 3.80 to 6.70.

In Switzerland, the total THC value (THC + THCA × 0.877) alone determines whether a plant is considered industrial hemp or a narcotic. Below 1 %, cultivation is generally permitted; above that, only within the framework of an authorised pilot project under BetmG Art. 8b or with a medical prescription under Art. 8a. Anyone who cultivates themselves bears the burden of proof for the value — the analysis of the harvested batch is decisive, not the variety specification of the seed dealer.

Mounting height and distance regulation in practice

The correct suspension of the light sources determines success in the grow room. During germination and the first vegetative phase, young plants require a gentle supply of light, which is why the distance to the light source should be generous. If the lamp is placed too close, there is a risk of burns and growth disorders due to light stress. Gradually lowering the fixtures continuously increases the intensity and optimally prepares the plants for the demanding flowering phase. Regular checks of the canopy are essential to detect early signs of stress, such as bleaching tips.

During the flowering period, light intensity is maximised, requiring precise adjustment of the mounting height. Since modern LED systems exhibit enormous radiation output despite low heat generation, the optical distance is often more critical than the thermal one. Growers should not rely solely on the hand-warmth method, but should determine the distance based on concrete measurements of photosynthetic photon flux density. By using adjustable rope ratchet suspensions, the height can be flexibly adapted to the daily stretching growth of the plants to ensure uniform illumination of the entire cultivation area.

• Regular adjustment of the suspension to the height growth of the plants to avoid hotspots.
• Use of adjustable rope ratchet systems for millimetre-precise adjustment in everyday practice.
• Observation of the uppermost leaves for bleaching as an indicator of too short a distance.
• Gradual reduction of the distance during the transition from the vegetative phase to the flowering period.

Risk factor: waste heat and climate control

Although modern LED fixtures emit significantly less radiant heat than classic sodium vapor lamps, they also generate substantial waste heat via their drivers and circuit boards. This heat accumulates in the upper area of the cultivation room and must be continuously dissipated to prevent heat build-up. An uncontrolled rise in temperature not only impairs photosynthesis but also promotes evaporation, causing relative humidity to rise massively. Without a coordinated exhaust air system, mold can quickly form in the dense bud structure, which can jeopardize the entire harvest.

Controlling the indoor climate therefore requires precise coordination between light output and ventilation capacity. In the Swiss winter months, the waste heat from the lamps can be used in a targeted manner to save heating costs in the cultivation room. In summer, on the other hand, the combination of high outdoor temperatures and lamp waste heat often leads to critical values, which is why placing the LED drivers externally outside the tent is advisable. Continuous monitoring of temperature and humidity ensures that plant transpiration remains within the optimal range.

• Relocation of LED drivers outside the cultivation room to significantly reduce the thermal load.
• Dimensioning of the exhaust ventilation to match the actual power consumption of the installed lighting.
• Use of circulation fans to prevent stagnant heat and moisture pockets under the lamps.
• Use of climate controllers that automatically adjust the fan speed to temperature peaks during the light phase.

Legal safeguarding and measurement obligations in home cultivation

For indoor cultivation in Switzerland, strict compliance with the legal framework is of central importance. Since the limit for legal cultivation is below one percent total THC (according to the Narcotics Act, BetmG SR 812.121), growers must ensure that their plants do not exceed this threshold at any time. Excessively intense lighting combined with stress factors can unexpectedly influence resin production and thus the active ingredient content. Anyone who disregards the statutory provisions of the Narcotics Act risks legal consequences, which is why careful strain selection and controlled lighting management are essential.

For legal home cultivation, it is advisable to use only certified seeds and not to expose the plants to extreme light conditions that could trigger unpredictable stress reactions. Since the cantons handle the enforcement of the Narcotics Act individually, growers should inform themselves about local guidelines. In the event of an official inspection, the burden of proof regarding the THC content lies with the operator of the facility. Complete documentation of the cultivation conditions and the lighting equipment used can, in case of doubt, help to prove the professionalism and legal compliance of the project.

• Strict compliance with the Swiss limit of less than one percent total THC to avoid criminal liability.
• Avoidance of extreme light stress to prevent uncontrolled fluctuations in the phytochemical profile.
• Use of certified genetics that remain stably below the THC limit under the chosen lighting conditions.
• Documentation of light cycles and facility configuration to safeguard against cantonal inspections.

Typical mistakes in lighting design for home cultivation

A common mistake when setting up indoor lighting is the assumption that more light automatically leads to better results. Without a proportional adjustment of nutrient supply, irrigation, and especially the carbon dioxide content in the air, plants cannot process extremely high light intensities. Instead of the hoped-for increased yield, the plants react with stunted growth, leaf burns, or disrupted nutrient uptake. A well-balanced equilibrium between all growth factors is therefore far more important than simply maximizing the amount of photons on the canopy.

Another planning mistake concerns the uneven distribution of light in the cultivation room. Often, the radiation is concentrated directly under the lamp, while the peripheral areas and corners remain in the shade, leading to uneven growth. Neglecting wall reflection also significantly reduces the efficiency of the overall system. By using low-quality foils or dirty surfaces, valuable light energy is lost that should actually benefit the lower parts of the plants. Thoughtful positioning of the lamps and highly reflective walls optimize light yield without additional power consumption.

• Avoiding light oversaturation without simultaneous optimization of CO2 levels and nutrient supply.
• Regular cleaning of reflectors and lamp covers to maintain maximum light yield.
• Use of highly reflective wall coverings for better illumination of the peripheral areas in the cultivation room.
• Avoiding abrupt changes in the light spectrum to minimize plant stress reactions.

FAQ