Pruning affects the height, shape and density of cannabis foliage. It is the process of cutting, bending or pinching growth shoots to affect canopy shape and structure.
Pruning is a stress that can either help or hinder overall plant growth. It works because it triggers the production of auxin, a hormone that causes tissue elongation at the remaining growth shoots. Physical and hormonal manipulation of cannabis gives gardeners control over the shape and density of the plant. Removal of top shoots promotes side branching, and vice versa.
When you let a cannabis plant grow unpruned, you get to see what natural, un-checked growth looks like. Cannabis has an instinct to grow tall—too tall for many indoor grows. Some plants naturally grow bushier; others are slender, or pine-tree shaped. Unpruned plants are beautiful, but they usually are not the most efficient use of space and light. Indoor growers often want wider, not taller plants.
The Sweet Spot
The goal of indoor plant training is to maximize the usable light in the room. Even the best grow lights are relatively dim compared to the sun. They are also fixed overhead, so the angle of the light never changes—it is eternally high noon or midnight in an indoor garden.
Even the best grow lights only penetrate about 12” into the plant canopy. This means that the focal point of the light fixture is the only “sweet spot” with good lighting. Higher up is too bright; lower is too dim. For a 1000w light, this sweet spot is a 4’x4’ horizontal plane a few feet below the light fixture.
The goal of canopy management is to create many top shoots, each the same height and distance from the light fixture.
Growth happens in meristematic tissue cells. Plants elongate from apical meristem and grow thicker from lateral meristem. The top of a plant or tip of a branch is the dominant apical meristem. Lower branches are subordinate apical meristem.
When the dominant apical meristem is bent or cut, subordinate branches multiply and grow tremendously in an attempt to replace the damaged tissue. This makes cannabis bushy and dense with lots of bud sites— sympodial structure.
When lower branches are removed, hormones and energy shift even more toward the dominant apical meristem(s). This causes plants to grow vertical and slender with a single large cola at the top—monopodial structure.
Only remove dominant apical meristem in the vegetative phase. Subordinate branches can be trimmed up until the second week of flower. Branches and budsites that do not recieve direct light need removal to focus growth toward more productive areas of the plant.
Plants need time to recover from pruning, especially when the apical meristem is removed. Over-pruning causes shock, stunting and self-pollination. Never remove or stress more than 25% of the foliage. Allow two weeks recovery before pruning again.
Low Stress Training
Picture a tree that long ago fell to the ground, exposing side branches up toward the sun. Low Stress Training forces the main trunk stem to grow in a similar, horizontal manner. The stem is never cut, kinked or broken. Instead, bend and tie it down. After a few days the new growth will reach back up toward the light. Repeat the process until the side branches are dominant and even in height.
LST can be as simple as staking and tying branches of the plant to the ground or bamboo stakes. Many growers favor horizontal trellis netting, sometimes referred to as SCROG (Screen of Green). The trellis netting is woven cord, similar to a hammock. Stretch and anchor it across the canopy to create a horizontal plane at the desired height. As the plant grows, slowly weave it into the netting to create a flat-topped hedge of cannabis flowers.
Supercropping or "High Stress Training(HST)" is a more aggressive form of bending top stem shoots. It discourages vertical growth and encourages branching without cutting or tying the plant. Instead, crush and severely bend top stems at a spot 2-3 nodes (where branches emerge) below the tip.
Supercropped stems kink and hinge downward, like an elbow. The damaged stem will eventually heal into a strong, gnarly bulge. While it is healing, auxin surges to the lower branches causing them to lengthen and shoot upward.
The top shoot eventually rights itself but is now surrounded by elongated lower branches. The result is a bushier plant. Repeat HST throughout veg to encourage more branching and bud sites. Only supercrop dominant apical meristem.
Topping is true pruning. Cut one dominant shoot and two will replace it. Cut the stem 2-3 nodes below the tip instead of bending it. Use clean, sharp scissors to prevent infection to the wound. After topping, auxin will surge to the two branches closest closest to the wound. These will become two new dominant top shoots.
Topping is best done as early as possible so that plants are able to heal from the trauma before flowering. It is only applied to the most dominant shoots of the plant. Only vigorous plants should have their top growth cut off, and only in moderation. Too much topping shocks and stunts plants, so don’t remove more than 25% of the plant.
Removing Leaves and Unders
An unintended consequence of pruning the dominant apical meristem is excessive bottom branch growth, aka “unders”. These lower branches are in the shade of the top shoots. They do not get enough light to produce quality flowers. Instead, bottom branches stagnate and inhibit sub-canopy airflow. They also attract disease. Prune unders repeatedly during veg and the first two weeks of flower. Cut off any branch, or sub-branch that is shaded and trying to grow up and around the dominant canopy. Removing the lower branches redirects hormones and energy back up to the dominant top shoots.
SOG and Lollipopping
Sea of Green is a radically different canopy management technique. Instead of one bushy plant, the grow room is packed with a large number of small, single-stem plants with monopodial structure. The top of the plant is untouched while lower branches are removed. This pushes all hormones and energy upward to one single dominant shoot. These techniques make large buds, minimize veg time and use space efficiently; however, they are not an efficient use of legal plant count.
Easy Does It
There is no need for new growers to rush into major canopy management actions. In fact, unpruned plants are a great point of reference to work from. When you do get the courage to cut or bend your plants, take it easy at first. Overzealous pruning can quickly destroy months of growth. Work slowly and stay mindful of the moment as you prune your cannabis plants.
Pruning is one of the most significant things a gardener can do for her crop. Once confident, you will likely find the plants vigorous, resilient and interactive. Sculpting and shaping cannabis plants is a joy for any gardener.