Why Your Plant Shoots Up After 12/12 (The Science)

Watch the episode · 4:34

You switch the lights to flower, and your plant shoots up. That's the stretch. In one study, White Widow plants were about 81 cm tall when the lights switched to 12 hours; by the end, they stood 114 cm. So why does a plant that should be busy making buds keep climbing? And the trick many growers use to stop it? A trial found it costs you flower.

Source · Hesami et al., Plants (2023)

The flip and the stretch ▶ 0:27

When the nights get long enough, the plant gets the signal: time to flower. But the tip doesn't stop growing. It keeps adding new nodes, each carrying flower sites, and the stem between them grows long. That lengthening is driven by a growth hormone called gibberellin.

Block it, and the stretch shrinks. In a 2026 study, cannabis treated with a gibberellin blocker grew into semi-dwarf plants. That is a research tool, not a recommendation.

Source · Heer et al., Plant Cell Physiol. (2026)

In that White Widow study, the plants added about a third of a metre after the flip: 40% on top of everything they grew in veg.

When it stops ▶ 1:02

Watch the very tip of the main stem. In a study of female cannabis plants, about 8 to 10 days after the first clusters of pistils appeared at the top, the tip itself turned into a flower. A stem whose tip has become a flower can't grow any taller. From then on, the plant fills in buds instead of climbing.

Source · Spitzer-Rimon et al., Front. Plant Sci. (2019)

So the stretch has a built-in end. It belongs to the first weeks of flower, while the top is still growing.

Why some stretch more ▶ 1:31

Genetics, first. In that same study, one month into short days, one variety stood 52 cm tall, another 63 cm, and a third 106 cm. Twice the height.

A 2026 study of 25 native cannabis populations found the same pattern: the late-flowering ones grew tallest, with the longest gaps between nodes. It's why long-flowering, sativa-leaning strains like Haze have a reputation for a big stretch.

Source · Babaei et al., Sci. Rep. (2026)

Then there's light. Plants read far-red, the deep red at the end of the day, as a sign of shade. In a 2025 study, adding far-red at the end of the day made plants taller in all three varieties tested. It's the same shade switch that makes seedlings stretch.

Source · Peterswald et al., Sci. Rep. (2025)

And the popular fix, more blue light? In a Utah State study that changed the share of blue light, the effect on height wasn't statistically significant.

The fix

"More blue light stops the stretch"

Not ProvenWestmoreland et al., PLoS ONE (2021)

Taming the stretch ▶ 2:33

Now, that trick. Crop steering with a saltier, stronger feed is a popular way to hold plants short. A 2025 Utah State trial tried it on two varieties. The stressed plants were 15% shorter, but flower yield fell by about 20% in most of the stressed groups. In the authors' words, it's difficult to shrink the plant without shrinking the harvest.

Source · Allred, Fatzinger & Bugbee, J. Cannabis Res. (2025)

Topping and SCROG ▶ 3:00

Topping does better. Topping means pinching out the main growing tip while the plant is still in veg. And remember: a stem without a growing tip can't climb. In a 2026 hemp trial, plants topped above the fourth node grew 17 to 25% shorter, and made more biomass, not less. An indoor medical cannabis trial found the same direction: topped plants made more flower than untopped ones.

Source · Chavalina et al. (2026); Crispim Massuela et al. (2022)

Then there's the screen of green, or SCROG: a net stretched over the plants before the flip. As they stretch, you tuck the tips under the net, so the growth spreads sideways into one flat, even canopy. That's grower practice; we found no trial of it yet.

So plan for it. Flip early, and leave room for the plant to grow by around half again, more for a late-flowering strain. Know your strain. Go easy on far-red at the end of the day. And top in veg, then let a screen catch the stretch.

The stretch isn't a fault. It's the last growth spurt: long nights start it, gibberellin drives it, and it ends when the tip turns into a flower. Top in veg, net the stretch, and you can plan for it.