Stretch: Why do plants stretch so quickly when switched to flowering, and how can you control it?

During the transition from the vegetative phase to flowering, plants go through a short period of rapid growth, which we call the “stretch”. During this time, over the course of several days to weeks, plants can “stretch” to as much as three times their original height. This phenomenon is more noticeable in indoor growing, where the change in day length happens abruptly, whereas outdoors the photoperiod changes gradually. If you want to avoid an overcrowded grow box or plants outgrowing a greenhouse, it is important to learn how to estimate the final size of plants in advance.

It used to be commonly said among growers that once switched to flowering, plants would immediately begin to grow rapidly, and this would continue until the first buds formed. However, modern research suggests that the reality is somewhat more complex. It has been found that approximately between the fifth and tenth day after switching to a short day, plants go through a several-day period of internode elongation (the distances between individual nodes). At the same time, during the first two weeks after switching, plants grow faster than during the vegetative phase. However, this does not mean that growth stops immediately when the first buds appear. The overall height and growth rate after the transition to flowering depend on the environment and, above all, on the plant’s genetic make-up.

Differences in the morphology of different types of varieties

How much a plant stretches before its growth stops completely depends on the type of variety. In general, the longer the flowering time, the greater the stretch and the taller the plants. The shortest and least stretched are broad-leaved indicas. By contrast, the longest growth period and tallest plants can be expected with pure sativas. The truth is that most cultivars grown today are hybrids with sativa and indica genes and may show different growth characteristics. Quality seed sellers usually provide information on the expected plant size for their varieties.

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Sativa varieties

Sativas are varieties originating from equatorial and tropical regions of Asia, Africa and the Americas. They are characterised by narrow leaves, a slender structure and large gaps between internodes. Pure sativas can grow to a height of three metres or more and flower for longer than 12 weeks. Sativas begin flowering more slowly and stop growing later than other varieties, so even an experienced grower may find them difficult to manage.

• Height: 1.5–3 m or more
• Leaf shape: long narrow blades
• Stretch: pronounced, 200–300% of height

Indica varieties

Indicas are varieties from the mountainous regions of Afghanistan, Pakistan and northern India, where the growing season is short and the length of the day changes significantly throughout the year. In addition to rapid ripening, their structure also reflects their adaptation to this environment. They are shorter and more robust, with distinctly broad leaves and short internodes. Compared with sativas, they enter flowering more quickly and their growth stops relatively fast.

• Height: 0.5–2 m
• Leaf shape: short broad blades
• Stretch: moderate, 50–100% of height

Autoflowering varieties (ruderalis)

Autoflowering varieties were created by crossing classic varieties with the ruderalis subspecies, which originates from the northern regions of Europe and Asia. Unlike photoperiod varieties, they begin to flower independently of day length, making their growth much more predictable. Stretch in autoflower varieties is usually less pronounced, and plant growth typically stops very quickly after the transition to flowering.

• Height: 0.5–1.5 m
• Leaf shape: depends on the specific variety
• Stretch: small to moderate, 30–80% of height

How to predict stretch and practical tips

An experienced grower can predict stretch and plant growth to some extent. Especially in indoor growing, it is important to think ahead and train plants so that they fill the growing space properly after switching. In grow boxes with a standard height of 200 cm, we recommend switching to flowering when the plants are approximately 30 to 60 cm tall.

Base your approach on what you can expect from the variety. Indicas usually only need to be topped once or twice, and you will end up with short, bushy plants and plenty of side branches. Sativas are often better pruned more than once or woven into nets.

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Remember that bending and other methods of shaping and training plants can only be carried out during the vegetative phase and at the beginning of flowering. As plants begin to ripen, their stems become less flexible and it will be more difficult to bend them without breaking.

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Another factor that affects plant height is temperature. High temperatures generally encourage internode elongation. In the summer months, when temperatures in many grow rooms rise to 30 °C, plants may stretch more than in winter, when it is cooler.

You can find more guides and practical tips for indoor and outdoor growers on the Higarden blog!