Causes of Cannabis Plant Stretching and How to Control It

Heady Vermont Staff 8 Nov 2020
Learn what causes stretching and how you can reign in cannabis growth spurts.

Stretching is a natural growth-spurt for cannabis plants, but is known to adversely affect the outcome of a crop. We delve into what causes stretching in the first place and how to prevent it from taking over your grow room and creating unstable plants.

There are few things more frustrating than an overcrowded grow room. “Stretching,” the term used to describe the rapid growth of cannabis stems, is a natural part of the vegetative stage and is influenced by plant genetics, among other variables.

Unfortunately, stretching is a common perpetrator of low yields and lanky plants, resulting in teetering, physically unstable crops. Despite the threat it poses, stretching can be controlled by first examining the factors influencing its growth, then creating protocol to sidestep these issues. Controlling cannabis stretching requires an understanding of how plants interact with both their internal and external environments.

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Why Do Plants Stretch?

There are several reasons why plants stretch, one of which has to do with the strains themselves. Genetics plays a pivotal role in determining the eventual height of sativas, indicas and hybrids. Whereas most indica strains are bred to grow shorter and bushier, sativas often experience significant stretching, sometimes growing six feet or higher in some breeds.

While stretching isn’t harmful in and of itself, dramatic stem growth carries the potential of significantly lowering the crop’s final yield between 20-30% overall. When considering strains for your next home grow, research the common height of each variety you plan to grow in order to gauge whether or not the strain experiences a dramatic flowering stretch.

Light Exposure

One of the main culprits of inordinate stretching of cannabis plants is lack of light exposure. Plants that are too far away from an adequate light source will respond by spurring stem growth to move themselves closer to the lamp. Especially in the presence of many other plants, sparse lighting can easily cause a grow room to become severely overcrowded, throwing off the steady progress of your crop and the ratio of distance between bulbs and plants.

In response to this issue, be sure to provide enough light to quell stem growth. At the same time, you don’t want to position your lights too close to your plants, as this will also result in overstretched stems and in some cases, lost yield.

In addition to distance between the source and the plant, the kind of light being used also holds influence over the amount your crop will stretch. Orange and red light encourages stretching and results in thinner, taller stems. Conversely, blue light stimulates thicker stem growth and a shorter height therein. When a strain is ready to enter the vegetative stage and undergo its most drastic period of stretching, metal halide lamps can be used to discourage extra-long stems.


Heat is another pivotal element that can determine how much plants will stretch during vegetation. Temperatures reaching over 27 degrees push stems to grow longer and longer and will propel sativas to their full height potential. Heat lamps that are positioned too close to the plants creates an intolerable environment that will rouse tall, wobbly plants, which are likely to fall over and potentially lose flowers in the process.

Other Causes of Stretching

There are numerous variables that can lead to plants stretching beyond what is normally expected from the strain. Significant environmental stressors resulting from transplant may cause the plant to go into shock. This will then trigger a reaction, causing it to stretch. Cannabis plants that are not properly cultivated under decent conditions or aren’t receiving satisfactory nutrition will respond in a number of adverse forms, including stretching.

Crops that are not spaced far away enough from one another are likely to stretch as a result of competition for resources. Due to extreme proximity, plants will fight each other to reach the light, forcing growth throughout the crop.

SARP is term used among growers that is short for “shade avoidance response phenotype.’’ This is when a plant stretches to reach light that is being stolen by a neighboring plant. Of course, temperature and the timing of the growth cycle play a role in the phenomena as well; so, the overall environment is still critical.

How To Control Flowering Stretch


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Flowering stretch, you guessed it, takes place when you switch your plants over from veg to bloom. This is a completely normal response as your plants prepare themselves to support the weight of their buds.


How much a plant stretches at the beginning of flowering varies a lot; some plants might double in height while others only grow a few centimetres. There are two main variables, however, that can give you at least some idea of whether your plants will stretch during bloom, and how much. These are:

  • Strain: Genetics have the biggest impact on your plant’s size. Generally speaking, sativa strains stretch more than indicas and tend to develop long, lanky stems.
  • Lights: Certain lights, like HPS, are more likely to encourage stretching. Moreover, large distances between your canopy and your lights will cause plants to stretch as they attempt to get closer to the light source.

The flowering stretch usually lasts for the first two weeks of the phase. To minimise stretching, keep your lights between 10 (for CFL) and 30 centimetres (for HID) from your canopy. Also, stick to indica strains if you’ve got a smaller grow space.

How To Prevent Stretching

In many cases, you can account for at least some stretching to occur in the flowering stage of your cannabis plants. With this in mind, it’s helpful to establish a plan in case your strain ends up growing higher than you imagined. If the information is available, obtain the average reported height of your strain and compare it to your grow room dimensions, considering the necessary distance between bulbs and plants.


While outdoor grow-ops benefit from the presence of the open air to naturally limit stretching, indoor operations are tasked with the tough assignment of maintaining air circulation inside a closed space. Sufficient airflow helps cannabis stems strengthen and grow thicker, rather than taller. This way, plants still grow vigorously, without reaching nerve-wracking heights.

Manual Intervention

For those cultivators looking to get their hands dirty, there are physical intervention methods that can be taken to reduce over-stretching. Similar to the natural effect wind has on cannabis plants, manually bending the leaves and stems will cause tiny tears in plant tissue that will cause the plant to focus its attention on stem regeneration instead of vertical growth.


Topping is a form of manual intervention on cannabis to affect its yield, shape or size. In essence, topping is the process of cutting off a new, actively growing node from your plant in order to reduce its size and create a “v” shape that will then form two colas. Topping can be an effective measure for combating stretching, but it’s important not to top once the flowering stage begins.

In most cases, unexpected stretching from cannabis plants won’t destroy your yield entirely. In fact, stretching can actually help to increase your yield. For cultivators with sizable grow rooms that can sustain tall plants, stretching may actually increase a strain’s total yield with more vertical surface area for colas to form. Either way, stretching can be easily prepared for in advance and there are a number of different solutions to help with this all-too-common problem.

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