Vegetative stage: how cannabis grows

TL;DR

• Photoperiodic strains stay vegetative at ≥14 h light/day.
• Photosynthesis peaks around 30 °C and 1500 µmol PAR (Chandra 2017).
• VPD sweet spot: 0.8-1.2 kPa – stomata open, transpiration efficient.
• Pre-flowers at the nodes reveal sex at 4-6 weeks.

Photosynthesis in cannabis

Cannabis is a C3 plant with high light saturation. Chandra et al. (2015, 2017) measured net photosynthesis up to 40 µmol CO₂ m⁻² s⁻¹ at 30 °C and 1500 µmol PAR – well above typical houseplants. Hence the heavy light and CO₂ demand.

Dioecious: male vs female

Cannabis is largely dioecious. Females produce resinous inflorescences (cannabinoids, terpenes). Males are removed to avoid seeding. Feminised seeds yield ≥99 % female offspring.

Vegetative Phase: Light hours, growth control and typical mistakes

During the vegetative phase, the plant builds up foliage and root systems — both directly determine the yield later on. Under 18 hours of light and 6 hours of darkness, photoperiod strains reliably remain in vegetative growth; autoflowering genetics are independent of this and transition based on age. Optimal conditions are 24 to 28 °C at 60 to 70% relative humidity, which corresponds to a vapor pressure deficit of around 0.8 to 1.1 kPa.

Growth control is more effective in this phase than any subsequent correction. Topping and fimming break apical dominance and produce multiple equivalent main shoots; low-stress training bends shoots horizontally without damaging tissue, delivering more predictable results when height is limited. Every cut costs three to seven days of regeneration — therefore, the final training measure must be carried out at least one week before the time of flower induction.

• Target vegetative vapor pressure deficit: 0.8–1.1 kPa
• Adjust air movement so that leaves gently sway — not flutter
• Root zone well-rooted before the transition to flowering, but not root-bound
• Last cut at least 7 days before the light change

Swiss Classification

Industrial hemp in field cultivation goes through the vegetative phase without light control; the indoor parameters described here come from controlled production, as used by licensed Swiss operations for CBD flowers. It is precisely this process control that explains the price difference between indoor and outdoor products in retail.

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 requirements and substrate dynamics during growth

During the vegetative phase, the plant requires a specific nutrient profile that differs significantly from the subsequent flowering phase. The focus here is on nitrogen, which is essential for the formation of chlorophyll, proteins, and new shoots. A stable foundation in the substrate ensures that the plant builds the necessary structure to support heavy flowers later on. In addition to nitrogen, phosphorus plays a key role in root growth, and potassium is crucial for regulating the water balance. A deficiency in this phase significantly delays development and reduces yield potential.

Substrate maintenance requires continuous monitoring of the pH value and salinity. Since nutrient uptake depends heavily on the pH value of the irrigation water, an incorrect value quickly leads to nutrient lockouts, even if sufficient fertilizer is present. In soilless cultivation such as coco or hydroponics, this control is even more critical than in organic soil, which possesses buffering capacities. A balanced ratio of wet and dry phases in the substrate also promotes oxygen supply to the roots. This prevents waterlogging and encourages the fine root hairs to actively spread throughout the entire pot in search of water.

• Nitrogen-rich fertilizer to promote leaf and stem growth
• pH value control of the irrigation water depending on the substrate type (soil vs. hydroponics)
• Avoiding over-fertilization by regularly measuring the drain EC
• Maintaining dry cycles to promote root growth

Root health and pot selection for optimal growth

The invisible foundation of a healthy vegetative phase lies entirely beneath the soil surface. A strong, widely branched root system is the basic prerequisite for efficient water and nutrient uptake in the later flowering phase. In conventional plastic pots, roots tend to grow in circles as soon as they hit the pot wall. This phenomenon, also known as root circling, severely limits the active absorption surface and weakens the plant in the long term. Modern cultivation methods therefore increasingly rely on alternative container concepts that regulate root growth through natural air contact on the outer sides.

Fabric pots or special Air-Pots utilise the effect of so-called air pruning. As soon as a root tip comes into contact with air, it dries out, which stimulates the plant to form new, finer branches inside the pot. The result is an extremely dense and vital root network that makes optimal use of the substrate. In addition, the high breathability of these pots significantly improves the oxygen content in the root zone. Oxygen is essential for the cellular respiration of the roots and for beneficial soil microorganisms. A well-aerated substrate also minimises the risk of root rot caused by anaerobic bacteria.

• Use of fabric pots to promote natural air pruning
• Prevention of root circling through timely repotting
• Oxygen enrichment of the substrate through structurally stable additives such as perlite
• Preventive protection against root rot by avoiding standing water in the saucer

Integrated pest management in the vegetative phase

The vegetative phase offers the ideal window of opportunity for preventive plant protection measures, as no flowers are present yet. Pests such as spider mites, fungus gnats, or thrips can multiply rapidly under the warm and humid conditions of the grow room. Early infestation weakens the photosynthetic performance of the leaves and delays overall development. Since no sticky flower resins are formed during this phase, biological plant protection agents and beneficial organisms can be used particularly effectively and without leaving residues. Regular inspection of the undersides of leaves and the use of yellow sticky traps help to detect pests at an early stage.

In addition to animal pests, fungal diseases such as mildew also pose a risk, especially with high humidity in the dense canopy. Targeted defoliation in the lower third of the plant improves air circulation and stabilises the microclimate. The use of beneficial microorganisms such as mycorrhizal fungi or beneficial bacteria in the substrate also strengthens the plant's systemic resistance to pathogens. A robust and pest-free transition into the flowering phase lays the foundation for a successful harvest, as later treatments of the flowers are extremely limited and often risky for the final quality.

• Regular inspection of the undersides of leaves with a magnifying glass for pests
• Use of yellow and blue sticky traps for the early detection of flying insects
• Application of beneficial organisms such as predatory mites or nematodes at the first signs
• Improving air circulation through the targeted removal of leaves close to the ground

Legal grey areas and controls in Swiss home cultivation

Anyone cultivating cannabis in Switzerland must strictly comply with the stringent legal limits regarding THC content. As long as the total THC value of a plant remains strictly below 1 %, it does not fall under the Narcotics Act and is considered legal industrial hemp. Although the THC content of young plants is usually still extremely low during the vegetative phase, the legal classification is based on the potential final value of the mature flowers. The private cultivation of CBD strains is therefore permitted, but requires a careful selection of certified seeds. Swiss authorities and cantonal laboratories conduct random checks to verify compliance with these limit values on the market and in private facilities.

A major risk for home growers is that many internationally offered seeds exceed the Swiss limit of under 1 % THC in the flower. If a value of 1 % or more is measured during an official inspection, the entire facility is legally considered an illegal cultivation of narcotics. This usually leads to the confiscation and destruction of the plants as well as criminal consequences. For the legal cultivation of THC-rich cannabis for recreational use, there are currently only strictly limited, scientifically monitored pilot trials in selected Swiss cities. Outside of these projects, the cultivation of THC cannabis remains illegal without a corresponding exceptional permit from the BAG.

• Strict compliance with the Swiss limit of under 1 % total THC for legal cultivation
• Use of certified CBD seeds to avoid legal risks
• Risk of confiscation and prosecution if the limit value is exceeded
• Participation in licensed pilot trials as the only legal option for recreational THC cultivation

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