Let’s talk about marijuana plant height in your indoor grow room. If you grow your marijuana plants too tall, it can create too many problems to manage your operation and can cause severe damage to plants.
Indoor grow spaces, by the very nature of growing inside of a structure, have ceilings. On average, the majority of houses and indoor spaces will provide around eight feet in height to work with.
Your marijuana plants will be grown in some type of medium (ex. soil in a container, hydrotron in a hydroponics system, using a soilless method such as coco noir, etc.) and grow marijuana plants no higher than four (4) feet tall from the medium.
The reason why indoor plants aren’t grown taller than four (4) feet is that your HID lighting system will hang from the ceiling and your plants will burn from the light’s heat if they are too close.
Another big concern, when you grow tall plants, is that they will fall over easily, as they have a higher center of gravity. So as your plants bud the height and weight can cause the plant to collapse. A two (2) foot marijuana plant in a five-gallon container will produce a few pounds of moist buds (not dried weight, of course) and it is easy to grow without any concerns.
However, if that very same marijuana plant was a six (6) foot plant with the same weight of buds, but it has long and stretching branches that can easily tip the whole plant completely over along with the container. Marijuana plants that are grown indoors will typically have flimsy stems and branches because it doesn’t have the outdoor wind to “toughen” the plants.
Once the marijuana plants’ buds put on some weight, you will have to do a bunch of staking, caging, and tying of branches to maintain the integrity of the plant’s branches. The taller the marijuana plant is, the more stem length you will have in between the branches and nodes, which presents two issues:
1) The plant needs an exorbitant amount of energy to grow these lengthy branches; energy it could have used to grow large, dense buds.
2) Carbon Dioxide (CO2) is denser than air and it will settle close to the floor, this means the marijuana plants’ roots (which need oxygen) are picking up much more CO2, than the leaves and flowers (which need CO2).
Lighting Intensity
When marijuana plants receive inadequate lighting, their branches will start to extend because the gibberellins (plant hormones) cause lengthier and quicker cell growth in-between the plant’s nodes.
To avoid the production of this hormone, you will need to make sure that you have sufficient lighting for your indoor grow room. It is best to keep the lights as near the top of the marijuana plants as possible without burning the buds and leaves.
To ensure that all of the marijuana plant’s branches are under direct, intense light; you will want to effectively space the plants evenly apart. Also, to make ensure that the marijuana plants are receiving enough light, you may want to purchase a light meter.
Air Circulation
Using oscillating fans in your indoor grow room imitates the outdoor wind that naturally blows between the leaves and stems. Air movement will disrupt the plant cell elongation, triggering the cells to exchange lengthier growth for thicker cell walls.
Essentially, marijuana plants naturally make modifications to withstand the wind/breeze as they would outdoors. This is to simulate what plants do in nature so that they are not easily bent over or flattened by a windy day. As an included benefit, oscillating fans stir up CO2 from off the floor and also help reduce pest infestation problems.
Growing Temperature
The development of many plants, cannabis included, is impacted by variations or changes in temperature. If the temperature throughout the day (lights on) is less than during the night (lights out), this is called an “unfavorable differential” or “negative differential”.
You can use this to keep your plants short and stocky, by raising the temperature during the dark to a few degrees warmer compared to the lights “on” period. Some strains will respond to negative differences greater than others, depending on the climate the strain is originally from.
You have to track temperature level and humidity anyway so make sure you have a good thermostat and humidistats to monitor your indoor grow room.
Topping Cannabis Plants
This is when the “top” of a marijuana plant is cut off. Topping marijuana plants (removing a sizeable portion of the main stalk) assists by keeping them short and stocky in two different ways:
1) The most important reason to top your plants is that auxins (plant hormones) accumulate in the tips of the leading branches and that is what drives development.
2) When you squeeze (or ‘pinch’) these tips off, the focus of the auxins hormone is distributed to the ends of other branches.
By forcing these auxins to spread throughout the branches of marijuana plants it will help promote a shorter, bushier plant development.
Cannabis Plant Training
You can “train” your marijuana plants to be much shorter, bushier plants. Branches and plant tops can be pulled down and forced plants to grow flat (or vertical) using strings, trellises, or cages.
Using a small amount of tension to training your marijuana plants is not an ability that will be mastered overnight. You will likely crack/break some branches until you gain enough experience.
Not all indoor grow rooms will easily accommodate the training of marijuana plants. Tall marijuana plants will also cast shadows inside grow rooms, which will reduce the light that is readily available to other plants who need to share the light.
When marijuana plants are grown tall, the distance between the lower branches and the lighting system that is hanging from the ceiling increases so they receive less light. Based on light intensity principles, as the distance from the leaves to the light source increases the number of lumens that reach the leaves greatly diminishes.
On tall plant marijuana plants, the leaves on the higher branches often shade the lower branches and their leaves. The only light that can pass through the plant’s foliage to its lower branches is Far-Red Light Waves, which are between about 760 – 800 Nanometers (Nm).
Far-Red Light Waves activate Phytochrome Far Red (PFR), which will send signals to the lower, shaded branches and the leaves to start to lengthen so they can receive more light.
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