How Tall Do Cannabis Plants Grow?

How tall do weed plants grow? What optimum height should you be aiming for as a cannabis grower?

When considering the height of your cannabis plant, the factors that you need to bear in mind are your grow space, the strain you are planting and the resources required to reap maximum benefit from your full grown weed plant. Obviously, you want your weed plant to be as big as it can be in order to support as many healthy buds as possible. On the other hand, having it grow too tall, especially in limited grow space can pose its own unique set of difficulties.

Join us as we explore the optimum marijuana plant height and how you can achieve this. 

What Is The Ideal Height For Cannabis Indoor Plants?

If you take the time to examine the final structure of a marijuana plant grown indoors, you will often find that most of the long, fat “colas” are at the top with smaller buds growing below. The further down the plant you go, the smaller the buds will be. 

What does this mean?

When considering the optimum height for your cannabis plant, the availability of adequate light is a huge factor. The height your plant should be allowed to get to should be dependent on the grow light available. The bigger the light you have, the better their ability to support bigger plants, and therefore bigger buds. 

Another very important factor to consider is how big your grow space is in the first place. When growing cannabis plants indoors, space is often a limited resource. If your plants grow too tall, their height can exceed the height of the grow space, leading to them bending or even destroying the structure of the grow space. Luckily, most growers do not let it get to this point, and usually take measures to control the height of their indoor grown cannabis plants. 

Below are some of the measures you can take. Not only will these measures control the height and width of your weed plants, they can also help you increase overall yield:

Low-Stress Training: This involves slightly bending the branches in the direction you want them to grow. This training enables the light to be better distributed throughout the plant, resulting in eventual better yields.

Training using Screen (SCROG technique aka Screen of Green): In this method, instead of simply bending the branches, you use a screen to train the cannabis in the direction in which you want to spread. This method usually results in a more even smooth green canopy.

Topping: In this method, the top of the main stem is completely removed when the cannabis plant is still a seedling. This immediately splits it’s growth into two main stems which allows for more colas to access light even with limited height. 

Defoliation: Though not strictly a method of training, defoliation basically involves removing the biggest fan leaves. This is usually done in the first month of flowering and it encourages the plant to grow bigger buds as more light can get to them. Defoliation can also be done a second time during the flowering stage, which is a time the cannabis plant experiences a growth explosion. 

What do you do if your cannabis plant is not growing tall enough?

If your plant is lagging in growth, you can check the below factors to speed up the growth of your cannabis plants:

• Humidity: How much water your cannabis plants receive affects their overall rate of growth. Marijuana plants thrive in relative humidity of 50-65%. If the growth of your plants is lagging behind, you can either increase the amount of water you are giving the plants or use a humidifier to increase the humidity in the grow area.

• • Temperature: If your indoor cannabis plants are not achieving the height you desire, consider paying closer attention to the temperature in the grow area. Cannabis plants generally flourish in average temperatures of 25°C, so it might help your plants to maintain this.

• Nutrients: Your cannabis plants might not be getting enough nutrients for optimal growth. Depending on the stage of growth your plant is in, provide the optimal nutrients and you should see a marked improvement in the growth of your weed plants.

• Light intensity: Light intensity can be used to control the both the growth speed, and the height of cannabis plants. If your plants are not growing to the height you desire, you can consider increasing the distance of the light from the plant to increase stretch. On the other hands, you can reduce the intensity of the plants if they start getting unruly.

Six weeks into flowering, you can control the growth of the plants by reducing the photoperiod. Increasing the “light-on” period can encourage stem stretching as well as leaf production. Conversely, if you desire to slow down the height of the plant, you can decrease the “lights-on” period to 10 or 11 hours. 

How to Avoid Nutrient Burn

Nutrients are vital in the growing of healthy, cannabis plants. When weed farmers are faced with a nutrient issue, their first instinct is usually to increase the nutrients. This could actually be quite counter-productive as most of the time the plant is suffering from too many nutrients, a condition known as “nutrient burn”. 

When ensuring your cannabis plants get to the optimum height, it is important to ensure that they get enough nutrients, but that they also don’t get too much.

Healthy cannabis leaves will appear green, and be uniform. As long as the leaves on your cannabis plants look like this, and have no signs of discoloration, spots, curling or other issues, you are practically in the green! 

Grow Lights For Optimal Height

The growth patterns of your cannabis plants when using indoor grow lights can be in your “complete control”, and open to your manipulation when your plants are in the vegetative stage. This can allow you to determine how tall you want your full grown weed plant to be, which will also determine eventual yields. 

Unfortunately, once flowering has started, not much can be done to correct uneven cola height without causing damage to your cannabis plant. If your plants are already in the flowering stage, do your best to provide enough light while keeping the plants cool. Adequate light should be able to reach most, if not all of your colas without overheating them or damaging the terpenoids (taste/smell) and cannabinoids(potency)of your buds. 

How much light is too much light for weed plants?

• CFLs as well as other fluorescent grow lights can be kept close to your weed plants as they are relatively weak. However, be keen to ensure that the heat is controlled. 
• LED grow lights can generally be kept minimally 12″ away, and both 3W or 5W models being kept 18+” away from the plants for optimal growth. When using LED lights, take into careful consideration the specifications from the manufacturer to guide you on the specific models. As modern LED lights tend to be very powerful, light-burning your plants is a threat you want to avoid. 
• MH/HPS grow lights on the other hand should be kept at least 1-2 feet away. This distance however largely depends on the size of the bulbs. 

Note: It is necessary to have some kind of exhaust or cooling system with all grow lights, which can also serve to bring in fresh CO2 for use by the plants. 

As much as it is important to have adequate lighting for your grow, if you keep increasing the intensity of the light in your grow room, you will get to the point where you max out on how much light your plant can use. 

Plants that have been provided with light that is too bright can experience “light burn” even if the temperatures maintained are cool. It is important to note that some strains are more susceptible to light burn as compared to others. 

When To “Flip” Cannabis Plants For Optimal Height

As a general rule, lights are flipped during the flowering stage, which usually results in the height of the cannabis plants doubling. Below are some general guidelines indicating at which height flipping the lights could be ideal, depending on the grow light you are using:

• CFLs, T5s as well as Other Fluorescents – Especially if you do not have light coming in from the sides, switch when your plants are 6-12″ tall 
• 250W HPS – Switch at 12″ tall
• 400W HPS – Switch at 17″ tall
• 600W HPS – Switch at 21″ tall.
• LEC or CMH Grow Lights – Follow guidelines based on the wattage of your lights. For 315W, you can flip at 15″ tall, while 630W can do for ~22″ tall
• LED Grow Lights – With this method of lighting the parameters vary too widely to give a standard instruction. However, you could use the “half the desired height rule” to determine when to switch. That is, if you desire to end up with plants that are 24″ , you can switch at 12″. 

 “Tall” vs “Short” Strains

Although, as we have described above,  plants will typically double their height after the switch, care should be taken when you are dealing with particularly “short” or “tall” strains as they can stretch more or less than average. 

• For a particularly “tall” strain, the light switch can be made earlier, that is, when the plant has just achieved a third of its desired height. 
• For a “short” strain, instead of switching when the plant is midway of its desired height, wait for it to grow up to three quarters of its projected height then switch. This will allow for extra stretching and better results. 

What is the Optimum Cannabis Height Outdoors?

When dealing with cannabis plants grown outdoors, height is a factor that is mostly dependent on the natural sunlight and how much space the roots have to grow. Outdoor cannabis plants can keep getting taller and taller and some have even been known to get to 20 feet with a breadth of 2 to 3 feet. 

The propensity of cannabis plants to grow means that the grower has to control them where necessary for maximum health and yield.  

How Tall Do Different Cannabis Plants Grow?

A very big percentage of Indica strains have a bushier and more compact appearance and do not grow too tall.  The average marijuana plant height for an indica strain could be about four feet,

Sativa strains on the other hand can grow to gigantic heights. Thys are known to grow really tall with a minimum spread although they can be trained to grow more horizontally. Sativa strains can grow up to 20 feet tall. 

Hybrids tend to be a mix. In this case, it is wise to defer to the instructions given by your retailer on how tall the cannabis seeds you have purchased are capable of growing. 

Autoflowering plants on the other hand are a whole other ball game. Not only do they grow in record time, they rarely exceed 20 inches in height. They typically range between 7 and 15 inches, which makes them practically the shortest cannabis strain. 

So What’s the Best Plant Height?

Ultimately, no definite answer exists as to “best” or “right” height, it is all dependent factors such as the strain and the grow set-up. However, in a nutshell, you can take away the following tips to help you develop ideal growing conditions: 

•  The optimal height of your cannabis plant before flowering should be about the length of the “main” (solid) cola of your plant. This skill might take a bit of time for you to master but with time you will be able to dial in “right on the money”.
• Increasing the overall size of your plant during the vegetative stage, and therefore the number of colas will ultimately increase your yields. 
• Training the canopy of your cannabis plants to be flat, and yet wide enough to occupy the entire light space ensures all the colas get adequate light during the flowering stage.
• Allowing your plants to grow taller than your light can support leads to wasted time and resources in the vegetative stage. 
• Defoliation allows you to produce bigger buds as more light can get further down the plant thereby exposing more bud sites to adequate light. 
• Switching your cannabis plants to the flowering stage when they get to the correct height saves time, resources and leads to better yields.
• Ensure there is adequate circulation of air to avoid mold. Controlling the height of your plants, only to create a musty environment would be counterproductive in the long run.
• Always go back to your last grow and study what you could have done better. After all, there is no better teacher than experience! 

Conclusion

When considering how tall weed plants should grow, it is important to keep in mind that overall height and general size will determine to some extent how many, and how much bigger colas your plant can support. It is therefore important to allow your full grown marijuana plant to get to its potential as far as space and resources allow, while encouraging outward growth for maximum yields. 

You want your cannabis plants to be flat and wide like a table at the top, not skinny and tall!