Stretching of indoor cannabis: Techniques to achieve more compact grows

A common challenge faced by cannabis growers (particularly indoors) is the stretching that occurs when plants develop excessive vertical growth, resulting in elongated stems and weak structures. This phenomenon is mainly influenced by factors such as lighting, genetics, temperature, or spacing. In this comprehensive guide, we explore some strategies to prevent and mitigate this problem for growers to achieve more robust and higher-yielding plants.

Cannabis stretching consists of rapid vertical growth that usually occurs when plants move from the vegetative to the flowering phase, when they can double or even triple their height. While a certain degree of stretching is natural and may be a sign of a healthy plant, when cannabis develops excessive vertical growth, it produces long stems and wider internodes that can affect both the stability of the plant and its ability to produce dense and resinous buds.

Despite being a natural behaviour, especially during the first few weeks of flowering, uncontrolled stretching can also result in weak plants, with branches that are unable to support the weight of the buds, and which have an inefficient distribution of light. Also, when plants grow too high, they can reach the grow lights and suffer light stress or even burns, which can affect their overall development.

Nevertheless, stretching is not a phenomenon that’s purely exclusive to the flowering stage, because it can also occur at the beginning of the vegetative period. And of all the stages of plant growth, stretching during the seedling phase is the most undesirable – if a healthy seedling stretches too much and its stem bends and falls over, you’ll have lost everything even before it starts growing.

Factors that contribute to the stretching of cannabis

Cannabis stretching is mainly triggered by changes in exposure to light. When the intensity is low or the light spectrum is not the most adequate, plants tend to lengthen as they look for a more intense light source, in an attempt to maximise their absorption of light radiation.

This behaviour is especially common in grows where the lamps are too far from the canopy, or when lights with the wrong spectrum are used for the growing stage. The lack of blue light, for instance, contributes to more slender growth, as this part of the spectrum promotes a more compact and robust structure.

Similarly, high temperatures, especially when there’s a big difference between day and night, can also stimulate excessive vertical growth. Values above 26 – 28°C during the vegetative phase can induce the plant to stretch, as cannabis tends to expand to improve perspiration and avoid thermal stress in warmer environments.

Another important factor is insufficient spacing between the plants. When there are too many plants in a reduced area, they need to compete for light and oxygen – so in response, they start to stretch out to outgrow each other, resulting in elongated and less robust structures.

But genetics also plays an important role, as some cannabis strains are genetically predisposed to stretch more than others. For example, sativa strains tend to stretch more than indicas, which usually have shorter internodes. Therefore, opting for indica or hybrid varieties with a tendency for more compact growth can be beneficial for grows in limited spaces where you want to avoid excessive stretching.

5 strategies to prevent and control stretching

Leaving aside the genetic component, and based on the previous factors, there are several strategies that growers can implement to minimise stretching and promote more compact growth.

1- Optimisation of the lighting

To prevent plants from stretching, it is advisable to use lights with a balanced spectrum that include a good proportion of blue light during the vegetative phase. It is also possible to gradually change from a bluer to a redder spectrum as the plant matures, encouraging a more robust development.

Ensuring that the light source is placed at an optimal distance is also crucial: lights that are too far away will encourage vertical growth, whereas lights that are too close may cause light stress. Another way to avoid stretching is to use reflective materials on the walls of the grow space, helping to reduce shadows, which in turn makes the plants stretch less because they don’t seek the light as much.

The planning of the light cycle also plays a decisive role. Maintaining an 18/6 photoperiod (18 hours of light followed by 6 hours of darkness) during the vegetative phase is the most common practice. However, some growers like to reduce it to 16/8 in the final week before the flowering to slow down the vertical growth a little before the characteristic phase change stretch. If you’re growing a strain with a tendency to stretch, you can also shorten the vegetative period so that you have more time and space for the plants to stretch out during the flowering stage.

2- Temperature control

As for the temperature, it is advisable to maintain a thermal differential between day and night that is not too drastic – this is extremely important because a key factor in the spacing between nodes is the difference between daytime and night temperatures. Simply put, the closer the daytime and nighttime temperatures are, the less your plants will stretch.

It is also important to provide good ventilation, as the air will help prevent excessive heat that can potentially induce stretching. In addition, a constant airflow can help strengthen the stems as a defence mechanism against the wind, just as in nature. An oscillating fan that simulates the movement of the wind can cause plants to develop thicker and stronger stems.

3- Avoid overcrowding

Efficient use of space is essential to avoid stretching. Plant the seeds or cuttings with enough space between them to ensure that each plant receives adequate light and doesn’t have to compete with its neighbours. Plants which are close together tend to stretch out as they look for adequate amounts of light and oxygen. As a result, some end up growing more than they should.

Depending on the technique and equipment used, an average of 1 to 16 plants can be grown in one square metre. If you want to grow only one plant, either with or without training, a single square metre is recommended. But if the focus is on maximum efficiency and yielding capacity, and you only have one square metre available, then you can place more plants with the intention of training them or keeping them small by inducing the flowering earlier.

4- Application of training techniques

In addition to these environmental adjustments, high or low stress training techniques can be used to effectively control stretching. LST (Low Stress Training), which consists of bending and tying the branches in a controlled way, helps maintain a more horizontal plant structure in order to improve the distribution of light. In the same way, using a mesh (ScrOG) can help control height and encourage flatter growth.

Another useful technique is “Super Cropping”, which consists of strategically bending the branches without breaking them completely, thereby stimulating the development of stronger stems and a more compact structure. Topping (where the main stem is cut to encourage the development of lateral branches) is also an excellent way to limit the height of the plant and increase the production of buds throughout its structure.

This pruning must be done in the vegetative stage and will affect the flow of auxins within the plant. Auxins are the hormones that play an important role in coordinating plant growth. Redistributing natural growth hormones from the main stem to the side stems is a good way to restrict the height and force the plant to develop as a shrub.

5- Use of plant growth regulators

In some cases, the application of specific products can help to control the stretching, but these should always be used carefully according to the manufacturer’s instructions. Substances such as paclobutrazol or daminozide may inhibit vertical growth, but it’s important to note that their use should be carefully controlled, as they can affect the quality of the final product. These plant growth regulators (or PGR) act directly on the action of plant hormones (auxins, gibberellins, cytokinins, ethylene, and abscisic acid), which leads to a reduction in the size of the shoots, or to very compact plants full of flowers.

Many growers opt instead for natural supplements such as silica, which strengthens the cell structure of the plants, making them more resistant without the need for chemicals. Strong and thick cell walls also translate into thicker stems and branches, allowing plants to distribute essential substances (such as water and fertilisers) throughout their tissues a lot more easily.

As you can see, the stretching of cannabis in indoor grows can be a challenge, but you can prevent your plants from overgrowing and ensure more productive and compact crops with proper control. Remember that being successful in controlling the stretching of your plants is highly dependent on constant observation and the application of preventive strategies, to allow plants to grow in a way that’s healthy and well adapted to the specific conditions of your grow.