Short version: Photoperiod plants roughly double or triple in height in the first few weeks after you flip to 12/12 — that's "the stretch." So flip when your plant is about half the final height you can fit, and finish your training before it takes off. Autoflowers skip the decision and flower on their own.
"When do I switch to flower?" is really two questions: when is the plant ready, and how much room do I need to leave for the stretch? Get the second one wrong and you'll be bending colas off your light for a month. Here's how to time it.
What the stretch actually does
When a photoperiod plant flips into flower, it puts on a burst of vertical growth before it settles into budding — commonly 1.5–3× its pre-flip height, depending on genetics (sativa-leaning strains stretch more). This happens mostly in weeks 1–3 of flower. Plan your vertical space around the finished height, not the height on flip day.
When to flip (photoperiods)
Two conditions should both be true:
- Height: the plant is roughly 50% of the maximum height your space allows, so the stretch fills the rest without cooking the tops on your light.
- Maturity: it's bushy and established — generally 4–6+ nodes, a healthy root system, and vigorous growth. Most growers veg 4–8 weeks; longer veg = bigger plant = bigger yield, within your space.
To flip, switch your light cycle to 12 hours on / 12 hours off with a hard, uninterrupted dark period. Light leaks in the dark can stress a plant or cause it to revert.
Autoflowers are different
Autoflowering plants start flowering on their own after a few weeks regardless of light schedule, so there's no flip to time. That also means you can't "veg them longer" to get bigger — with autos, healthy early growth is everything, because whatever size they reach before auto-flowering is close to what you get.
Use the stretch, don't fight it
- Finish training first. Do your topping and low-stress training in veg and early stretch — see fimming vs. topping. A screen (SCROG) filled during the stretch turns that vertical burst into an even, high-yielding canopy.
- Chase the light up. Raise your light as the plants rise to keep it at the right distance.
- Watch node spacing. Very long gaps between nodes can mean the light is too far or too weak — dial in intensity with our DLI calculator.
Once buds are stacking, follow the timeline in our flowering stages week-by-week guide.
Frequently asked questions
How much do cannabis plants stretch after flipping to flower?
Most photoperiod plants gain 1.5–3× their pre-flip height during the first two to three weeks of 12/12, with sativa-leaning strains on the higher end. Always plan your vertical space around the final, post-stretch height.
At what height should I switch to 12/12?
A good rule is to flip when the plant is about half the maximum height your space allows, since it will roughly double. It should also be mature enough — typically four to six or more nodes with strong, healthy growth.
Do autoflowers need a light schedule change to flower?
No. Autoflowers begin flowering automatically after a few weeks regardless of light hours, so there's nothing to flip. Focus on strong, stress-free early growth, because their final size is largely set before they start budding.
