Greenhouse cannabis: definition, types, light deprivation and cultivation

A greenhouse is a structure with transparent walls (glass, polycarbonate, plastic) that allows plants to be grown in natural sunlight, while protecting them from the vagaries of the outdoor climate. For cannabis, it's the ideal compromise between indoor cultivation (total control, high cost) and outdoor cultivation (free light, exposure to risks).

Greenhouse, indoor, outdoor: comparison

Types of greenhouses

Plastic tunnel Semicircular structure covered with UV plastic film. Most economical solution (a few hundred euros), easy to assemble, effective weather protection. Limited lifespan of plastic (2-5 years depending on quality). The basic model for small-scale growers.

Glass greenhouse permanent structure with glass walls and roof. Better light transmission, excellent durability, attractive appearance. A heavier investment, but optimal over the long term for an established grower.

Polycarbonate greenhouse Glass: semi-rigid honeycomb panel, better thermal insulation than simple glass, lighter and less fragile. The right compromise between glass and plastic for semi-permanent use.

Light deprivation greenhouse See dedicated section below.

Light deprivation: the key greenhouse technique

The light deprivation (light dep) is the technique that fundamentally distinguishes the greenhouse from other cultivation methods. It involves manually or automatically covering the greenhouse with an opaque black tarpaulin to artificially limit sunshine duration and thus induce flowering earlier than the natural photoperiod would allow.

Photoperiodic cannabis enters flowering when it receives less than 12 hours of light per day. Outdoors, this occurs naturally from late July/early August in the northern hemisphere, requiring a single harvest in the autumn. With light dep, the grower can trigger flowering as early as June or July by covering the greenhouse in the early evening:

Light dep in greenhouses allows for 2 to 3 harvests a year, whereas outdoor cultivation produces only one, while maintaining the quality of natural light.

Parameters to be monitored in the greenhouse

The greenhouse protects from the elements but creates its own climatic challenges:

Temperature A greenhouse in full sun can reach 45-50°C in summer, well above the critical threshold for cannabis (>30°C = heat stress). Solution: lateral ventilation, partial shading during the hottest hours, forced air circulation.

Humidity Humidity builds up quickly in enclosed spaces. During flowering, keep humidity below 50% to avoid Botrytis, the gray mold that destroys colas. Ventilate and dehumidify if necessary.

Ventilation Ventilation: essential for the renewal of CO₂ (necessary for photosynthesis) and the prevention of fungal diseases. As a minimum: openings in the facade and ridge to create a natural draught.

Pests The greenhouse reduces but does not eliminate risk. Visit red spiders and thrips find their way easily. A program of integrated pest management remains indispensable.

Varieties suitable for greenhouse cultivation

The choice of variety must take into account the size of the greenhouse and the latitude:

For light dep (early flowering desired) Photoperiodic fast-flowering varieties: Early Skunk, Critical, Amnesia Express, Quick One.

For greenhouses in temperate climates varieties resistant to humidity and temperature variations: Northern Lights, White Widow, Jack Herer, OG Kush mold-resistant.

For autoflushers the autoflowering varieties do not depend on photoperiod and are ideal for an early first cycle in the greenhouse as early as April, without the need for light dep.

For large greenhouses Sativa varieties with high outdoor yield potential: Amnesia Haze, Super Silver Haze, Durban Poison. Allow for available height. An unpruned greenhouse sativa can exceed 3 meters.

Advantages of greenhouse cultivation

In short, the greenhouse combines the best of both worlds:

Natural sunlight, with a richer light spectrum than the most efficient HPS or LED lamps, guarantees flower quality superior to that obtained indoors at a marginally higher energy cost than outdoors. Climate protection means that the season can start and end earlier and later than outdoors, and light dep means that cycles can be multiplied.