What Causes Cannabis Plants to Stretch Tall?

As an indoor grower, you do not want your plants to be too small or too lanky, and when this happens, it can be not easy to control as a beginner. In some cases, you may not even know what is causing the plants to become too tall and thin. In this article, I will cover the reasons your plants may begin to stretch during the seedling and vegetative stage and what you can do to prevent it.

Understanding Internodal Growth

As a cannabis plant grows, it produces a stem with nodes along its length. Each node represents a new set of fan leaves and axillary growth sites. The space between nodes is called an internode, and it plays a significant role in the size, shape, and structure of your plants. There are several reasons why a cannabis plant may begin to produce long internodal spacing, which causes plants to become lanky, leggy, and make a weak stem.

• Internodal spacing will determine how short or tall your plant will become
• Long and thin stems will produce weak and lanky plants
• Indica cannabis plants have the shortest internodal spacing
• Sativa cannabis plants are known for having the longest internodal growth

Lanky and Skinny Seedlings

The seedling stage is an essential part of growing strong, resilient and tough plants. To produce healthy, vibrant seedlings that stand up straight without support, you must dial in the environmental conditions. Once you germinate cannabis seeds, you have a short time frame to establish your seedlings, so they do not become too lanky.

• The grow light should be 12-18 inches above the top of the seedlings
• A low-powered LED or CFL works excellently for the first 7 days of the seedling stage
• The temperature should be between 71 and 77 degrees Fahrenheit 
• Humidity levels should be between 65% – 70% during the first 2 weeks
• Heat mats are perfect for keeping seedlings warm during Winter
• Propagators can help raise the temperature and humidity levels

How To Fix The Problem

There will be times when the seedling grows tall and topples over the side of the pot. The stem will feel very thin and delicate; however, all you need to do is use a thin bamboo cane or a long wooden skewer to help stand the stem upright, using a small wire loop every couple of inches.

Long Internodal Length During Vegetation

There is no fixed time a grower should vegetate their plants before transitioning to flower, with the majority choosing a 3-5 week phase. During the vegetative period, your plants focus on producing leaves, stems, side branches, and roots. 

If you notice that your plants are growing tall quickly, there is a good chance you are encouraging it. Below is a list of potential reasons why cannabis plants can produce long internodes during 18/6. 

• Low light intensity will cause plants to grow tall in search of an increase in PAR
• Lights suspended too high above plants will encourage stretching during 18/6
• Cold nighttime or low temperatures can promote stretching
• Hot grow rooms above 82 degrees Fahrenheit will cause plants to stretch 
• Feeding plants too much Nitrogen will promote long internodal spacing
• Plants placed closely together that compete for light will grow taller 
• Sativa cannabis plants have a tendency to grow fast and tall during 18/6

How Plant Training Can Help

There are ways to train cannabis plants during the vegetative stage to influence their shape and structure. Applying the proper plant training technique at the right moment can help you grow stocky, thick, bushy plants with robust side branches. Plant training also helps reduce plant height and unwanted stretching during the vegetative and flowering period.

• LST is the easiest way to immediately lower the height of the plant
• Tying your plants down can be done several times during 18/6
• Topping cannabis plants is also an excellent way to reduce vertical growth
• Fimming during weeks 3 and 5 slows down vertical growth and produces bushy plants

My Top Tips to Prevent Lanky Plants Indoors

There are many downsides when your plants stretch too much and grow close to the top of the grow room or tent. Below are my top tips to help prevent lanky plants and long internodal spacing.

Distance of the Grow Light

When your grow light is set perfectly, your plants are receiving optimal intensity, and PPFD levels are perfect, your plants will not grow tall. The only reason some plants become thin, weak, and lanky during the vegetative or flowering period is due to light intensity. My tip here is to keep the grow light 12-18 inches above the plants.

Cold Temperatures

When the temperature drops, once the lights are off, plants can begin to excessively stretch. The internodal spacing can range from 30 to 60 cm, which can be difficult for growers working in small spaces to cope with. My tip is to use a small, low-powered heater in the grow room at night, combined with a digital hygrometer, and keep an eye on the on-and-off temperatures of the lights. Do not let the lights-off temperature drop below 63 degrees Fahrenheit.

Using Fans

In some cases, cannabis plants can become thin, weak and need support because they have not had a fan blowing at them. You only need a light breeze enough to encourage the seedlings to sway slightly, so the plants become thicker, more durable, and able to stand up without leaning over the side of the pot. A small oscillating fan can make a huge difference, especially during the seedling stage.

Autoflowering Plants

If you are happy to grow feminised autoflowering cannabis plants, then you will not need to worry about plants becoming too tall. Autos are popular due to the fact they do not grow above 120 cm and produce shorter but compact plants in comparison to photoperiods.

Feature photo: A room of plants that are bushy with short internodal spacing. photo credit: Bill Sutherland.