Controlling the vertical growth of a marijuana plant is a necessary skill for any indoor cultivator dealing with limited space. The natural tendency of cannabis is to grow a single, dominant main stem, often resulting in a tall, Christmas tree shape. This structure is not ideal in a confined grow area where maximizing horizontal canopy space is the primary goal. Managing the plant’s structure, environment, and genetics prevents excessive height and ensures a uniform, productive canopy that utilizes light efficiently.
Implementing Structural Training Techniques
The most direct way to keep a cannabis plant from growing too tall involves physically manipulating its structure to overcome apical dominance. This is the plant’s natural tendency to prioritize growth at the main, uppermost tip, which produces a hormone called auxin that suppresses the growth of lower side branches. Breaking this dominance forces the plant to distribute energy and growth hormones more evenly, resulting in a bushier, shorter plant with multiple main stems.
High-stress training (HST) methods, such as Topping and Fimming, directly remove or damage the primary growth tip, stopping the flow of growth-suppressing hormones to the lower nodes. Topping involves cutting the main stem just above a node, causing the two branches below the cut to become new main colas. Fimming removes about 70-80% of the main tip, which can result in four new main stems, though it is less reliable than a clean top.
Low-stress training (LST) is a gentler approach that involves bending and securing the main stem and branches horizontally without causing physical damage. When the main stem is bent over, the side branches below are exposed to direct light and begin to grow upward, effectively becoming new main stems. LST redirects the plant’s energy sideways, creating a flat, even canopy and preventing unwanted vertical growth.
The Screen of Green (SCROG) technique combines LST with a physical mesh screen placed above the plants. As the plants grow, the branches are woven through the screen, forcing them to spread out horizontally and maintain an even height. This system keeps the canopy flat, ensuring that all potential bud sites receive optimal light intensity, which maximizes yields in a height-restricted environment.
Managing Environmental Factors to Limit Vertical Growth
Adjusting the growing environment significantly controls the rate of vertical stem elongation, especially during the transition to the flowering phase. This period often includes a rapid vertical growth spurt known as the “stretch,” where the plant can double its height. Environmental factors to control are light intensity, light distance, and temperature.
Insufficient light intensity causes etiolation, where the plant stretches excessively as it searches for a brighter light source. Ensuring the light source is powerful and positioned correctly encourages tight internodal spacing. Providing a high Photosynthetic Photon Flux Density (PPFD) level, typically between 400 and 800 µmol/m²/s during the vegetative stage, signals to the plant that it has sufficient light and does not need to stretch.
Temperature differentials between day and night cycles are a powerful tool to regulate height, a technique known as DIF. A larger difference, where daytime temperatures are much warmer than nighttime temperatures (positive DIF), promotes stem elongation. This occurs because the plant produces more growth hormones like gibberellin during the cooler night.
To suppress stretching, growers should aim for a smaller temperature differential or a negative DIF, where the nighttime temperature is slightly warmer or the same as the day temperature. Keeping the night temperature close to the day temperature inhibits the production of gibberellin, resulting in shorter, more compact internodes and a shorter plant structure.
Selecting Cultivars for Compact Structure
The foundational control over a plant’s final height lies in its genetics, which predetermine its growth pattern and size potential. Cultivars are classified by structure, and choosing the right genetic profile minimizes the need for aggressive training and environmental manipulation.
Cannabis indica-dominant strains are characterized by a short, bushy growth habit and broad leaves, having evolved in cooler regions with shorter growing seasons. Their naturally tight internodal spacing makes them well-suited for indoor cultivation with limited vertical space. In contrast, Cannabis sativa-dominant strains are typically tall, lanky, and have an open structure with long internodal spacing, often doubling or tripling in height during the flowering stretch.
Autoflowering varieties offer a built-in height limitation due to their genetics, which include Cannabis ruderalis. These plants transition to the flowering stage based on age rather than the light cycle, resulting in a shorter overall life cycle. Because their vegetative period is fixed and brief, autoflowers do not reach the towering heights of many photoperiod strains, often maxing out between 40 and 100 centimeters.