Quick answer
Hydroponic cannabis runs on two core schedules. Use 18 hours of light and 6 hours of dark for vegetative growth, then switch to 12 hours of light and 12 hours of dark for flowering. Autoflowering cultivars skip the flip and perform well on a steady 18/6 or 20/4 from seed to harvest.
- Vegetative photoperiod: 18/6, with 20/4 as a common alternative
- Flowering photoperiod: 12/12
- Autoflowers: 18/6 to 20/4, no light cycle change required
Prerequisites
- Digital timer rated above your light's actual draw
- Dimmable LED fixture or adjustable ballast
- PAR meter or a phone PPFD app for intensity checks
- Stable reservoir temperature between 65 and 72 F
- pH and EC meters calibrated before the run starts
Vegetative schedule: 18/6
Eighteen hours of light gives plants enough daily photons to build roots, stems, and leaf mass without pushing electrical cost past the point of return. Six hours of dark lets the plant run respiration and move sugars out of the leaves. Hydroponic systems recover well from this cycle because the root zone stays oxygenated the whole time.
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When 20/4 or 24/0 makes sense
Twenty hours of light can shorten veg time by several days in a well-dialed room. Continuous 24/0 light works but leaves no dark window for maintenance, and some cultivars show leaf curl or stalled root growth under it. If you try 24/0, watch for calcium spotting on new growth and drop back to 20/4 if it appears.
Flowering schedule: 12/12
Cannabis is a short day plant. Flowering starts when the uninterrupted dark period reaches roughly 12 hours. Any light leak during that dark window can stall flower development, trigger re-vegging, or produce hermaphrodite flowers. Keep the room sealed and check timer accuracy once a week.
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The transition week
When you flip to 12/12, keep nutrient strength at veg levels for the first 5 to 7 days. The plant stretches before it sets flowers, and cutting nitrogen too early stalls that stretch. Move to bloom ratios once you see the first pistils.
Step-by-step: set and hold the schedule
- Plug the light into the digital timer and confirm the timer matches your local time.
- Set the on time and off time to create the cycle you need: 18/6 for veg, 12/12 for flower.
- Run the light through one full cycle before adding plants, then verify on and off with a watch.
- Seal the grow space and turn off any indicator LEDs, controllers, or power strips inside the tent.
- Check the dark period with a light meter or phone camera to confirm zero light leaks.
- Log the flip date when you change to 12/12 and count flowering weeks from that point.
- Inspect the timer weekly and replace batteries in models that use them.
Match light intensity to the schedule
Photoperiod tells the plant when to flower. Intensity determines how much it yields. Aim for roughly 400 to 600 PPFD in veg and 600 to 900 PPFD in flower with CO2 at ambient levels. If you add CO2, you can push flower PPFD toward 1,000 to 1,200. The total light delivered over 24 hours, called daily light integral, matters more than the wattage printed on the fixture.
Common mistakes
- Switching to 12/12 before the plant fills at least 60 percent of the trellis
- Leaving a small LED on a power strip inside the tent during dark hours
- Running 24/0 in veg and then flipping straight to 12/12, which shocks the plant
- Raising PPFD during the flip instead of waiting for flower set
- Trusting a mechanical timer that drifts by 15 minutes per week
Convenience drives most of these choices. Autoflowers and app-controlled timers remove the flip entirely, and for many growers that tradeoff beats a slightly higher yield from a hand-managed photoperiod crop.