A few days ago, a friend asked me when he should start removing leaves on his indoor medical cannabis plants. The tops of the plants were becoming crowded with big healthy leaves; and he was concerned about light penetrating to the lower branches. I was surprised by how many wrong and contradictory answers he had received from other fairly experienced growers. Hopefully I can clarify a few points and help dispel some of the misinformation floating about.
Try to think of leaves like solar panels. They are the plant’s only source of energy. The healthier the leaves, the more energy the plant has to grow and produce huge, resinous buds.
As the leaves on the plant begin to age, they eventually reach a point where photosynthesis peaks and begins to slow. At this point, the leaves are less efficient, and glucose productions declines. New leaves grow in to replace them as the primary solar panels, allowing more efficient photosynthesis to continue. The older, less efficient, leaves begin to work much like the root system, distributing their stored nutrients and plant sugars to wherever it is needed. When they have no more nutrients left to redistribute, these leaves will begin to yellow, and eventually fall from the plant.
This is particularly important to remember if you are experiencing root problems. Older leaves may be sacrificed to allow light to shine on fresh growth–but only if you have a healthy, robust root system. If your roots are not well established or are damaged, you may do more harm than good by removing the older leaves.
I should point out that most of the older leaves will be lower on the plant and therefor unlikely to shade much of the fresh growth. Any fresh growth lower on the plant is probably on shoots and in the shade of fresh foliage that is closer to the light. The principles of light and the inverse square law tell us that artificial light intensity quickly diminishes the further you get from the light source. So, you certainly do not want to sacrifice fresh leaves closer to the light for fresh leaves further away.
You can tell that leaves are beginning to give up their nutrients when they start to lose their dark green color. If these fading older leaves are blocking the light from new growth, feel free to remove them. By the time the leaves become pale green, or begin to yellow, they should be removed–even if they are not blocking younger foliage. Be sure not to remove leaves that have not yet reached their full size, unless they have been damaged by pests. Healthy leaves exposed to the light are facilitating photosynthesis. So your plants require them.
Outside, the Sun’s position in the sky changes through the day, providing light from a wide range of angles. Plants will follow the Sun and lower their leaves in the morning and evening to expose as much surface area as possible to the light. In grow rooms with stationary grow lights, the plants will only receive light from one static angle. This means that lower leaves are often shaded completely once new leaves grow in.
With stationary lighting, the only wavelengths that can penetrate dense leaves is from the far red end of the spectrum. This far red light does not assist photosynthesis, but triggers internal plant hormones called gibberellins. These gibberellins then cause the plants to stretch and create longer internodal spacing. The lower, shaded shoots usually do not amount to much, but may draw a tremendous amount of the plant’s energy stretching towards the light. For indoor gardens, use selective pruning to maximize yield and redirect plant energy to more productive zones.
Removing leaves is not the only method of decreasing shading and increasing light penetration. Using light movers, you can supply light at a variety of angles. This can reduce stretch, and light otherwise shaded leaves. Oscillating fans can be aimed to flutter leaves, allowing light to penetrate deeper into the canopy. Reflective material on the walls will also help to redirect wasted light at angles advantageous to lower leaves. For smaller grow rooms, plants can be set at different levels like stadium seating. In theory, this works great; but it is difficult to implement in larger grow rooms, and virtually impossible with many hydroponic systems.
The best answer for improving light penetration is simply to grow shorter marijuana plants. This is one reason that “See of Green” gardens yield so well. With taller plants, the majority of the bottom branches end up in shade. Even if light did penetrate the upper leaves, the lumens reaching the lower branches are so diminished that they yield poorly anyway.
What have you learned?
- Do not remove leaves indiscriminately.
- Do not remove young leaves unless heavily damaged by bugs or molds.
- Do not sacrifice healthy leaves near the light for the sake of lower leaves.
- Remove leaves that are beginning to yellow.
- Do not remove mature leaves if you have a weak root system.
- Use light movers, oscillating fans, and reflective material to improve light penetration
- Grow your plants short to keep all the branches as close to the lights source as possible
- Use selective pruning to remove sucker shoots that use up valuable plant energy
One final thing to remember: when you remove leaves from your plants, be sure to remove them from the grow room. Piles of leaves in the corner, or in a garbage can, will likely begin to harbor pests or molds. The last thing you want is a waste basket full of botrytis spreading spores through your garden and ruining your harvest.
Removing Cannabis Leaves to Maximize Light,