Why Does Cannabis Stretch?

Cannabis can increase rapidly in height as the spaces between its nodes lengthen. Growers commonly describe this as stretch, but similar-looking elongation can result from different processes: inherited architecture, the light environment, temperature or the transition into flowering.

Interpreting the change requires its timing and growing conditions. Long internodes throughout vegetative growth are not necessarily explained by the same process as rapid elongation shortly after switching to short days.

Stem growth and internode length

Leaves and branches emerge at nodes along the stem. The sections between them are internodes, and greater cell expansion within these sections increases the distance between nodes. A plant can consequently become taller without producing proportionally more nodes.

Internode length and total plant height are related measurements, but they are not interchangeable. The number of nodes and the overall branching pattern also contribute to the architecture.

Genetic differences in architecture

Cannabis cultivars differ substantially in stem length, branching and internode spacing. In a controlled comparison of three strains, characteristic architectural differences persisted across the tested light spectra (Reichel et al., 2021).

There is therefore no universal amount of flowering stretch. A breeder's description that a plant may double in height after the photoperiod change can inform planning, but should not be treated as a fixed biological rule independent of the environment.

Light intensity during vegetative growth

Moher et al. (2022) grew Gelato plants during a defined vegetative period at approximately 135–1,430 µmol m⁻² s⁻¹ PPFD. Internode length decreased almost linearly as light intensity increased. Plants under higher light generally developed shorter internodes, thicker stems and greater aboveground dry mass, whereas those under lower light had a more open architecture.

Overall height did not follow exactly the same response as internode length. The tested range also does not establish a recommendation to use the highest PPFD in every grow. The guide to PPFD, DLI and plant capacity explains why exposure needs to be considered alongside water supply, temperature and the conditions supporting photosynthesis.

Under limited light, elongated growth can occur without comparable investment in stem thickness or biomass. Greater height alone is therefore not evidence of stronger growth.

Red and far-red light as developmental signals

Leaves absorb red light strongly while reflecting and transmitting more far-red. Light passing through neighbouring vegetation can consequently have a lower red:far-red ratio. Phytochrome photoreceptors detect changes in this spectral environment, which can induce shade-avoidance responses including stem and internode elongation.

Cannabis experiments have reported taller, more elongated plants under spectra with lower red:far-red ratios. However, broad lamp spectra differ in several wavelength regions, so not every height difference can be attributed to that ratio alone.

Far-red supplementation studies have also found changes in height and related traits, with responses depending on genotype and exposure timing (Peterswald et al., 2025; Perotti et al., 2026). Similar conventional PPFD readings do not necessarily mean equivalent spectral conditions: conventional PPFD covers 400–700 nm and does not include the added far-red photons in these comparisons.

Elongation after the switch to short days

Photoperiod-sensitive cannabis is commonly maintained under long days before a shorter photoperiod initiates reproductive development. Alter et al. (2024) described two phases following this transition. During approximately days 5–10 after transfer to short days, the main stem and internodes elongated rapidly. Elongation of newly produced internodes subsequently decreased as the shoot developed the condensed structure characteristic of the inflorescence.

This sequence helps explain flowering stretch as part of a developmental transition, rather than simply a response to inadequate light. The timing describes the plants and conditions in that experiment; it is not a universal timetable for every cultivar.

Gibberellin and inflorescence development

The same study examined hormone changes, including gibberellin GA₄. Concentrations in the shoot apex declined during the transition towards compact inflorescence development under short days. Experimental gibberellin application produced longer stems and internodes and less compact inflorescences.

Returning flowering plants to long days increased gibberellin and auxin levels and triggered renewed internode elongation. These observations connect architecture with photoperiod and hormonal regulation; they do not imply that all stretch has the same cause.

Temperature patterns and humidity

Temperature can also alter elongation. In a controlled hemp experiment, changing the difference between daytime and night-time temperatures substantially affected shoot height and canopy architecture. Some of the tallest plants occurred with relatively warm nights, while the temperature regimes also strongly affected flower production (Bok et al., 2023).

The useful conclusion is that the full temperature pattern matters, not just the maximum reading on a controller. The results should not be reduced to a single temperature combination for all cannabis cultivars.

Water relations influence cell expansion as well. A study of the CBD-dominant cultivar Cherry Berry compared relative-humidity ranges of 37–58% and 78–98%. Plants in the extremely humid treatment became somewhat longer while producing substantially less biomass and much less flower biomass; flowering was also delayed (Corredor-Perilla et al., 2025).

This is not evidence that high humidity is a useful height-management tool. Nor does an extreme treatment describe every modest humidity increase, or establish the lower treatment as a universal prescription. Elongation can accompany reduced overall performance.

Far-red effects beyond plant height

Far-red photons can contribute to canopy photosynthesis when combined with shorter wavelengths. Manipulating the spectrum can therefore influence both architecture and production, but flower biomass, cannabinoid yield and height remain separate outcomes.

The Peterswald study did not find an overall increase in flower biomass from its far-red treatments, while cannabinoid responses differed with genotype and treatment schedule. Results from different spectral comparisons should not be combined into a general yield claim. Whether additional elongation is useful depends on the cultivation objective and available space.

Canopy density and training

Internode spacing affects light penetration, air movement, leaf arrangement and flower distribution. Some elongation during early flowering is normal, and maximum compactness is not necessarily the best architecture for every crop.

Topping, pruning and branch training redistribute growth across shoots and canopy space. They can change where elongation is expressed without eliminating the developmental response to flowering; pruning can also influence growth signalling. The article on lollipopping, pruning and below-ground responses examines these interventions separately.

Allowing space before flowering

If plants already fill the available vertical space before the switch to short days, normal early-flowering elongation may create problems with lamp distance, canopy uniformity, air movement and light distribution. The remaining space needs to accommodate the particular cultivar's response, rather than a standard doubling assumption.

The guide to deciding when to switch cannabis to flowering develops that planning question. Records of how a cultivar responds in the same growing system can help place general descriptions in context.

Assessing unexpected elongation

Long internodes throughout vegetative growth warrant a review of genetics, light intensity and spectrum. Rapid elongation soon after the switch to short days may be part of normal flowering development. Elongation concentrated in shaded branches points towards local light conditions and canopy competition as factors to investigate.

Where height increases while biomass production remains poor, assess the wider environment rather than interpreting the change as vigour. A consistently tall cultivar may retain that architecture across a range of reasonable growing conditions.

Managing stretch begins with distinguishing these patterns. Light exposure, spectral signals, temperature, water relations, genotype and reproductive development can all contribute, and an appropriate response depends on which processes are relevant to the plant and space.

References

Moher, M., Llewellyn, D., Jones, M. & Zheng, Y. (2022). Light intensity can be used to modify the growth and morphological characteristics of cannabis during the vegetative stage of indoor production. Industrial Crops and Products, 183, 114909. https://doi.org/10.1016/j.indcrop.2022.114909.

Reichel, P. et al. (2021). Impact of Three Different Light Spectra on the Yield, Morphology and Growth Trajectory of Three Different Cannabis sativa L. Strains. Plants, 10, 1866. https://doi.org/10.3390/plants10091866.

Alter, H., Sade, Y., Sood, A., Carmeli-Weissberg, M., Shaya, F., Kamenetsky-Goldstein, R., Bernstein, N. & Spitzer-Rimon, B. (2024). Inflorescence development in female cannabis plants is mediated by photoperiod and gibberellin. Horticulture Research, 11, uhae245. https://doi.org/10.1093/hr/uhae245.

Peterswald, T.J. et al. (2025). The effects of far-red light on medicinal Cannabis. Scientific Reports. https://doi.org/10.1038/s41598-025-99771-6.

Perotti, F. et al. (2026). Optimizing Light Spectra for Cannabis: Effects of End-of-Day and Continuous Far-Red on Plant Morphology and Flower Induction. Horticulturae, 12, 456. https://doi.org/10.3390/horticulturae12040456.

Bok, G. et al. (2023). Optimizing Indoor Hemp Cultivation Efficiency through Differential Day–Night Temperature Treatment. Agronomy, 13, 2636. https://doi.org/10.3390/agronomy13102636.

Corredor-Perilla, I.C., Kwon, T.-H. & Park, S.-H. (2025). Elevated relative humidity significantly decreases cannabinoid concentrations while delaying flowering development in Cannabis sativa L. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2025.1678142.

Featured image: Alter et al. (2024), Figure 1, reproduced without alteration under CC BY 4.0. Source. A–B: shoot tips after one week under long days (LD, 18 h light/6 h dark) or short days (SD, 12 h light/12 h dark). C: stem elongation over 30 days; D: lateral branch lengths at selected nodes. E–F: plants after 30 days under LD or SD. Scale bars differ: A–B, 2 cm; E, 20 cm; F, 10 cm. Do not compare plant heights from photograph size alone. The figure illustrates responses under the study conditions, not a universal stretching timetable.