Flowering: photoperiodism & trichome ripening

TL;DR

• 12/12 light/dark triggers flowering via the phytochrome pathway.
• Duration: Indica 7-9, Sativa 10-14, hybrid 8-10 weeks.
• Trichomes: milky = peak THC, amber = peak CBN/sedating.
• Cannabinoid synthesis happens in capitate-stalked glandular trichomes.

Photoperiodism: the switch

An uninterrupted dark period (>10 h) activates the floral meristem. A brief light break at night can delay flowering or trigger re-vegging.

Trichomes and cannabinoid synthesis

Cannabinoids (THCA, CBDA, CBGA) are made only in capitate-stalked glandular trichomes on flowers and adjacent leaves (Livingston 2020, Plant Journal). Terpenes come from the same trichomes – hence the aroma/effect link.

Flowering phase: process, climate control and maturity assessment

Photoperiod plants transition to flowering under 12 hours of uninterrupted darkness. The first two to three weeks are characterized by the stretch, during which the height can double depending on the genetics. After that, the plant shifts its resources to bud formation and resin production. Climatically, conditions are kept drier now: 20 to 26 °C and 45 to 55 % relative humidity, tending lower towards the end, as dense buds otherwise encourage botrytis.

Maturity is not read from the calendar, but from the trichomes. Clear glandular heads mean immature, milky-cloudy mark the peak of active ingredients, and amber-coloured indicate the beginning of degradation. The pistils are only a rough indicator — they also change colour due to mechanical stimuli. For a balanced harvest, around 70 to 90 % milky and 5 to 15 % amber trichomes are considered a guideline.

• Target humidity late flowering: below 50 %, no condensation at night
• Trichome assessment with a 60× loupe, always on buds, never on sugar leaves
• Dark phase strictly light-tight — light leaks cause hermaphroditism
• Thin out dense canopies: air circulation beats any fungicide

Why the flowering phase determines legality

The THC content increases over time during flowering, while CBD plateaus earlier. For industrial hemp varieties, a harvest delayed by two weeks can therefore be enough to push the total THC value above the 1 % threshold — turning legal material formally into a narcotic. For this reason, licensed Swiss producers perform preliminary analyses during flowering and determine the harvest time accordingly.

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.

Nutrient management and physiological change

With the transition to the generative phase, the physiological needs of the plant change fundamentally. While nitrogen dominates structural development during the vegetative phase, the plant now requires increased amounts of phosphorus and potassium. Phosphorus supports energy transfer and flower development, while potassium regulates the water balance and strengthens tissue integrity. A common mistake is adjusting the feeding schedule too late, leading to an oversupply of nitrogen. This delays flower formation and makes the tissue susceptible to pests.

In the final weeks before harvest, a reduction in nutrient supply, often referred to as flushing, is recommended. During this process, only pure water or a very mild solution is supplied to the plant so that it mobilises stored nutrient reserves in the leaves. This prevents an unpleasant taste in the final product caused by remaining mineral salts in the plant tissue. Precise monitoring of the EC value in the runoff helps to detect salt concentrations in the substrate in good time and prevent over-fertilisation.

• Gradually reduce nitrogen supply to slow down vegetative growth.
• Increase phosphorus and potassium as needed to support flower mass and resin production.
• Regularly measure the EC value of the runoff water to prevent salt accumulation in the medium.
• Stop fertilising in the final one to two weeks before harvest to break down nutrient residues.

Risk management for bud rot and mould

The increasing density of the buds in the advanced flowering phase creates a microclimate that highly favours the development of bud rot. Since the flowers transpire a lot of moisture, the risk of stagnant pockets of moisture inside the plant increases. Without sufficient air movement, this water condenses on the surfaces at night as temperatures drop. Once the fungus is established, it spreads rapidly inside the buds, rendering the entire harvest unusable.

Effective prevention is based on strategic defoliation and optimised air circulation. Removing excess foliage in the lower third of the plant significantly improves air circulation throughout the entire canopy. Fans should be positioned to gently move the air between the buds without causing direct wind burn. Since mould spores are ubiquitous, consistent control of the relative humidity below the critical limit remains the most effective tool indoors.

• Defoliate the lower third of the plant to maximise vertical air circulation in the grow room.
• Align oscillating fans in such a way that no stagnant air zones are created in the canopy.
• Consistently keep relative humidity in late flowering below the critical limit of fifty per cent.
• Immediately and generously remove affected plant parts and disinfect tools to prevent the spread of spores.

Swiss practice: THC monitoring and legal compliance

For players in the Swiss market, precise monitoring of the flowering phase is not only a question of quality, but of legal survival. Since the limit for legal status is below one per cent total THC, producers must time the harvest precisely. The Narcotics Act provides for strict sanctions if this limit is exceeded. Therefore, licensed businesses regularly take samples to have the content of tetrahydrocannabinol and its precursor tetrahydrocannabinolic acid determined in the laboratory using high-performance liquid chromatography.

Strict quality requirements, which are monitored by the Federal Office of Public Health, also apply within the framework of the state-approved pilot trials in accordance with Article 8b of the Narcotics Act. Here, not only is the active ingredient content controlled, but freedom from pesticides, heavy metals, and mycotoxins is also guaranteed. For consumers and patients who obtain medical cannabis via a medical prescription, this controlled cultivation during the flowering phase ensures a standardised and clean end product that complies with the requirements of the Pharmacopoeia.

• Carry out regular HPLC analyses in accredited Swiss laboratories to monitor the total THC value.
• Strictly comply with the legal limit of below one per cent total THC to maintain the status as industrial hemp.
• Observe the quality standards of the BAG and Swissmedic regarding the absence of contaminants in medical flowers.
• Seamlessly maintain documentation obligations over the entire flowering process for commercial producers.

Odour development and immission control in urban areas

As maturation progresses and terpenes are intensively synthesised in the trichomes, cannabis flowers develop an extremely intense odour. These volatile organic compounds are characteristic of the respective strain, but can quickly lead to conflicts in urban areas of Switzerland. Swiss neighbour law prohibits excessive immissions, which includes long-lasting odours. Insufficient filtration of exhaust air in residential areas therefore regularly leads to official investigations or complaints from neighbours.

The technical solution requires the use of activated carbon filters that are precisely matched to the capacity of the exhaust fan. Relative humidity also plays a decisive role here: if it rises above seventy percent in the late flowering phase, the activated carbon loses its binding capacity, allowing unfiltered odours to escape. Continuous negative pressure in the cultivation room ensures that no unfiltered air escapes through cracks or doors, maintaining the privacy of the cultivation.

• Dimension activated carbon filters to match the volume flow of the exhaust fan in order to completely bind odours.
• Keep relative humidity in the filter area below seventy percent so as not to impair the carbon's function.
• Permanently maintain negative pressure in the cultivation room so that no unfiltered air escapes in an uncontrolled manner.
• Check filter media for saturation before the start of each new flowering phase and replace them if necessary.

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