Whether cannabis plants can grow in the dark depends on the stage of growth and the definition of “dark.” Sustained growth, leading to a healthy, mature plant, requires light as a constant energy source. However, specific, scheduled periods of darkness are necessary for the plant to complete its life cycle and produce flowers. Understanding the balance between light and dark is fundamental to successful cultivation, as each period facilitates distinct biological processes.
The Necessity of Light for Growth
Plants, including cannabis, rely on light to fuel photosynthesis. This process uses the green pigment chlorophyll to capture light energy, converting water and carbon dioxide into glucose, which serves as the plant’s food and building material. Without light, this energy conversion cannot occur, preventing the accumulation of biomass necessary for growth.
The vegetative stage demands extensive light exposure. Growers typically provide photoperiod strains with an 18-hour light and six-hour darkness cycle (18/6) to mimic summer days. This extended light period ensures a continuous energy supply, resulting in robust, vigorous growth. Continuous exposure to light allows the plant to constantly produce sugars, which are then metabolized and stored, driving rapid expansion and development.
Immediate Effects of Total Darkness
If a cannabis plant is kept in constant, prolonged darkness, it cannot photosynthesize and must rely on stored energy reserves. This lack of light triggers etiolation, a survival mechanism where the plant stretches rapidly upward in a desperate attempt to find a light source.
Stems become thin, weak, and elongated. Leaves turn pale yellow or white because the plant stops producing chlorophyll. This resource depletion quickly exhausts stored sugars, leading to severe lack of vigor and structural failure. If total darkness continues indefinitely, the plant will eventually die from energy starvation.
The Essential Role of Scheduled Darkness
While constant darkness is lethal, a scheduled period of uninterrupted darkness is necessary to trigger the reproductive stage in photoperiod cannabis varieties. This phenomenon is known as photoperiodism—the plant’s response to the relative length of day and night. Cannabis is classified as a short-day plant, initiating flowering when the period of uninterrupted darkness exceeds 12 hours.
The plant measures night length using specialized photoreceptor proteins called phytochromes. These proteins exist in two forms; during the dark period, the light-sensitive form slowly reverts. The duration of this uninterrupted conversion signals the plant that autumn has arrived, triggering hormonal changes that shift focus from vegetative growth to flower production.
To force flowering indoors, growers switch to a 12-hour light and 12-hour dark cycle (12/12). This change simulates the shortening days of the fall equinox, instructing the plant to begin creating buds. Even a brief light interruption during the 12-hour dark period can confuse the phytochromes, causing the plant to revert to the vegetative stage or develop undesirable traits like hermaphroditism. The dark cycle is also when the plant metabolizes and transports sugars produced during the day, using this resting period for cell expansion, root development, and strengthening its structure.