Topping is a central and fundamental plant training practice. It enables you to train your plants to grow horizontally and make the most efficient use of the light. In this article we explain the science of apical dominance and the reasons why you should top cannabis plants. We provide simple instructions for when to top cannabis plants to avoid stunting and get the best results. Topping is actually rather simple and any grower can master it. We offer useful advice for how to top cannabis plants.

Topping is a “high-stress” training technique which entails removing the top growth tip from the main stem or a branch. Many growers are concerned that cutting the top off their young plants may slow their grow down, however, when done properly, topping is a critical method to fill the canopy faster and reduce vegetation times.

Be sure to read our article, “Why You Should Train Cannabis Plants”

Why Top Your Plants:

Cannabis plants typically grow tall and skinny, much like a Christmas tree. Without training, they have minimal lateral growth and concentrate most of their resources in one stem with one main cola. They display a phenomenon called “apical dominance”, which is the key factor that determines the directional growing characteristics of plants. Different types of apical dominance lead different plants to produce their patterns of both vertical and horizontal growth. This programming in plants is an evolutionary adaptation to their typical growing environments.

The landrace varieties that are the source of modern cannabis genetics are typically tall fast-growing annuals that are in competition with other plants for canopy space. Thus, they grow in a shape that is effective for that environment. However, we are not growing them in that environment!

There are other plants that grow flat and broad because they are adapted for environments where light is scarce. These plants serve as a better model for us to think about when trying to make the most effective use of overhead light. Ideally, we want low broad plants that harvest every photon of light energy available to them.

Apical Dominance and Auxins in Cannabis

Cannabis naturally grows like a Christmas Tree because its apical dominance is set for tall skinny growth. It is possible to achieve some degree of lateral growth dominance with nothing but low-stress training (LST), but the single most effective way to break the apical dominance in cannabis is to cut off the apex, by topping the plant.

The top or end shoot, which is the main growing point in plants, contains the hormones which control the functions for apical dominance. Auxins, are one hormone that appears to play a particularly significant role. The apical growth tip produces auxins, and they are transported through the phloem, always downward, where they inhibit the growth of lateral growth tips lower on the plant. The auxins prevent elongation of the growth-tips below the apex, however, they don’t travel long distances through the plant. Therefore, as the plant gets taller, the lowest branches become freed from the inhibiting effects of the auxins and begin developing. This produces the classic “Christmas tree” shape.