Topping is a horticultural technique used to manipulate a plant’s structure, moving away from its natural vertical growth habit. The primary purpose is to redirect the plant’s energy and resources into developing a bushier, wider canopy. By interrupting the plant’s single main stem growth, the goal is to create multiple growth points, maximizing harvestable sites and improving light use. Does this manipulation reliably translate into a net increase in total yield?
The Practice of Topping
Topping is a form of high-stress training that involves the physical removal of the plant’s main growing tip, known as the apical meristem. This action is typically performed using a clean, sharp tool to make a cut just above a node, which is the point where a leaf or side branch emerges. The immediate visible result is a halt to the plant’s vertical extension.
The plant’s energy, previously channeled into upward growth, is then redistributed laterally. Instead of continuing to grow as a single, tall stalk, the plant’s structure changes to a more sprawling or bushy form. The two lateral branches directly below the cut begin to develop and grow outward, effectively replacing the single main stalk with two new ones.
Biological Basis: Breaking Apical Dominance
The reason topping works lies in a natural process called apical dominance, where a plant’s main stem suppresses the growth of its side branches. The apical meristem, located at the top of the plant, is the primary production site for the plant hormone auxin. This hormone is transported down the stem, and its high concentration inhibits the growth of the dormant lateral buds, also known as axillary buds.
When a plant is topped, the source of this inhibitory auxin is removed entirely. This sudden drop in auxin concentration allows the axillary buds nearest to the cut to activate and begin to grow. The removal of the apical meristem also triggers a change in the balance of other growth regulators, such as cytokinin, which promotes the growth of these lateral shoots.
Consequently, the single dominant growth point is replaced by two or more vigorous lateral shoots that now compete to become new main stems. This hormonal redistribution shifts the plant’s focus from producing one central leader to producing multiple leaders, forcing the plant into a more branched growth pattern.
Influence on Harvestable Yield
The direct answer to whether topping increases yield is that it often leads to a net positive increase, achieved through a structural trade-off. An untopped plant typically produces one very large central production site (often called a cola) with smaller sites on the lower lateral branches. Topping converts this structure into a plant with multiple, though individually smaller, production sites.
By creating a wider, more uniform canopy with multiple new main stems, topping significantly improves light distribution across the plant. This allows more of the plant’s total leaf surface area to receive optimal light exposure, maximizing photosynthesis and energy conversion. The increased number of upper growth points leads to a greater total biomass and a larger overall harvest.
However, the practice requires the plant to divert energy from growth to repair the wound, which temporarily slows development. The recovery time means that topping generally extends the necessary vegetative growth stage by one to two weeks. For the potential yield increase to be realized, the plant must fully recover and the added vegetative time must be accommodated in the growing cycle.
Practical Factors for Success
The success of topping as a yield-enhancing technique is heavily dependent on the grower’s timing and environmental control. Topping should be performed during the plant’s vegetative stage, well before it begins to flower, to allow sufficient time for recovery and development of the new branches. A common guideline is to wait until the plant has developed at least three to five nodes, ensuring it has enough energy reserves to withstand the stress and heal quickly.
The health of the plant is also a factor; only vigorous, disease-free specimens should be subjected to this high-stress technique. The cut itself must be clean and precise, and sterilizing the cutting tool is important to prevent the introduction of pathogens into the open wound.
Different plant species and varieties respond uniquely to topping; those with a strong tendency toward vertical growth generally show the most dramatic structural change and yield improvement. Ultimately, topping must be viewed as a tool that creates the potential for higher yield by optimizing light exposure and increasing the number of production sites, but this potential is only fulfilled when combined with proper care and sufficient recovery time.