How to Flip Cannabis to Flower (the 12/12 Switch)

3 min read

A grower setting a grow light timer to a 12-hour on, 12-hour off flowering schedule

The flip is the single switch that turns a leafy green plant into a flowering one — and the timing of it decides whether the rest of your grow is calm or a fortnight of panic bending branches off the light. Most beginners flip too late, get caught by the stretch, and spend flower fighting headroom. Here’s how to do it right.

The short version:

  • Flipping means changing the light from 18 hours on to 12 on, 12 off
  • That shorter day signals the plant to stop growing and start flowering
  • Plan for the stretch — most plants double in height, some triple, in the first 2–3 weeks
  • Flip when the plant is roughly 30–40cm in a standard tent, not when it’s already tall
  • Autoflowers ignore all this — they flower on age, not light

Want the full breakdown? Keep scrolling.

What does flipping to 12/12 do?

You’ve been running 18 hours of light while the plant built her body — roots, stems, branches. Switch to 12 hours on and 12 hours of uninterrupted dark, and the plant reads it as the days getting shorter, autumn coming, winter on the way. Her last mission is to reproduce before she dies, so she stops bulking out and starts making flowers. Everything she does from here makes sense through that lens — she’s not growing for you, she’s growing to reproduce, and that desperation is your harvest. The dark period must be genuinely dark and unbroken; light leaks during those 12 hours are a leading cause of stress and hermaphrodites, so seal the tent.

When should I flip?

This is the decision that saves your grow. Most indoor plants double in height in the first two to three weeks of flower; sativa-leaning ones can triple. That stretch is coming whether you’re ready or not, so you flip before the plant fills the tent, not after. Do the maths: tent height, minus pot height, minus the light’s hanging distance, minus at least 40cm of clearance between light and canopy — what’s left is your maximum height at flip. In a standard 1.6m tent with a 20cm pot and an LED wanting 40cm clearance, that means flipping at around 30–40cm tall. The Optimist fills the tent in veg, flips at 18 inches, watches her hit 36 in ten days, and spends the whole grow supercropping branches that are already flowering. Flip earlier than feels comfortable and let the stretch have its room.

What about autoflowers?

None of the timing applies. Autos flower on their internal clock around week three or four regardless of light — you can’t choose when they flip, only support them well when they do. Keep them on a long day (18–20 hours) right through. The environment and feeding advice for flower still holds for autos; only the when-to-flip decisions are photoperiod territory. So if you’re on autos, there’s no flip to plan — just be ready for the stretch to arrive on its own schedule.

FAQ

What light schedule flowers cannabis? 12 hours on, 12 hours of uninterrupted darkness. The long, unbroken night is the signal; light leaks during the dark period can stress the plant or cause it to hermie.

When should I switch to 12/12? When the plant is roughly 30–40cm in a standard tent, leaving room for it to double during the stretch. Work it out from your tent height minus pot, light distance and clearance.

Do autoflowers need 12/12 to flower? No. Autos flower on age, not light, so they’re kept on 18–20 hours throughout. Only photoperiod plants need the 12/12 flip.

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