Switch cannabis plants to flower when they are healthy, sexually mature, and about half the final height you have room for. Indoors, that usually means flipping the lights to a 12/12 schedule after 4 to 8 weeks of vegetative growth. Outdoors, photoperiod plants switch on their own as daylight shortens in late summer, so you mostly control when you start them rather than when they flip.
How Many Hours of Light During Cannabis Vegetative Stage
What Actually Tells a Cannabis Plant to Flower
Cannabis is a short-day plant. It measures the length of uninterrupted darkness, not the length of the light period. Once the dark period reaches roughly 10 to 12 hours each night, the plant shifts hormone production toward flowering.
cannabis vegetative stage light schedule
- Indoors: you control the trigger with a timer. Most growers use 12 hours light and 12 hours dark to start flower, and some drop to 11/13 late in the cycle.
- Outdoors: the sun controls the trigger. Flowering begins as nights lengthen past a certain threshold for your latitude.
Any light leaking into the tent during the dark period can delay or stall the transition, so check zippers, vents, and nearby lamps before you flip.
Cannabis Vegetative Stage Temperature and Humidity Guide
Signs Your Plant Is Ready to Flip
Sexual maturity matters more than a calendar date. A plant flipped too young will flower, but it produces less.
- Alternating nodes: branches stagger on the stem instead of growing in matched pairs.
- Preflowers: small calyxes with two pistils appear at the nodes.
- Age: typically 4 to 6 weeks from seed, or a few weeks after a clone roots well.
- Health: steady green growth, no active pest issue, pH and feeding dialed in.
- Recovery: at least several days since topping, transplanting, or heavy defoliation.
Match the Flip to Your Ceiling
Most strains roughly double in height during the first two to three weeks of flower, and sativa-leaning plants can stretch more. Measure your usable vertical space, subtract the distance your light needs from the canopy, and flip while the plant still fits the math.
- Short tent: flip earlier, around one third of the available height.
- Tall room: flip later and use topping or low stress training to fill the space sideways.
- Indica-dominant plants: stretch less, so they can veg longer in the same footprint.
When to Flip Outdoors in the US
Outdoor photoperiod plants begin flowering as nights get longer, which usually lands between late July and September depending on latitude. Southern growers often see the switch later than northern growers. Porch lights, street lamps, and security lights can interrupt the dark period, so a plant that seems stuck in veg outdoors is often just sitting under light pollution.
Autoflowers Are the Exception
Autoflowering cannabis does not wait for a light change. It flowers on its own timeline, generally a few weeks from sprout, no matter what schedule you run. Do not flip the lights for autos. Most growers keep them on 18/6 or 20/4 for the whole grow.
Pre-Flip Checklist
- Confirm the plant is mature and pest free.
- Set the timer and verify it with a light meter or your eyes at lights-off.
- Feed for the transition and plan your bloom nutrient shift.
- Trim lower growth that will never see light.
- Install a trellis or stakes before the stretch starts.
- Write down the flip date so you can count flowering weeks later.
Common Timing Mistakes
- Flipping at two weeks old and wondering where the yield went.
- Waiting until the plant touches the light, then flipping and running out of height.
- Switching a stressed or sick plant, which slows flower set.
- Counting harvest from the flip instead of from the first real flower clusters.
What Happens After the Flip
The first two to three weeks are a transition and stretch phase. Vertical growth slows once true flower clusters form, and most photoperiod strains finish 8 to 11 weeks after the switch, depending on genetics. If you keep notes on flip date, height at flip, and final height, your next grow gets easier to time.