20/4 Vegetative Light Schedule

What it is

A 20/4 schedule means the grow light is on for 20 hours and off for 4 hours in every 24-hour cycle during the vegetative stage. Photoperiod cannabis stays in vegetative growth as long as the daily uninterrupted dark period stays below roughly 10-12 hours, so 20/4 reliably prevents flowering ^[1] Strong evidence.

Autoflowering cannabis is not photoperiod-dependent and will flower regardless of light schedule, so 20/4 is sometimes used for autos as well — but the rationale is different (more daily light, not photoperiod control) ^[2].

Why growers use it

Common reasons cited for choosing 20/4 over the other popular options (18/6 and 24/0):

• Energy savings vs 24/0. Running lights off for 4 hours per day cuts lighting electricity use by ~16.7% compared to 24/0.
• Heat management. A daily off period lets tent or room temperatures drop, which can help in hot climates.
• Plant 'rest' belief. Many growers believe a dark period allows respiration, root growth, or stress recovery. The evidence here is weak — cannabis-specific studies on optimal veg photoperiod are sparse, and general plant physiology shows respiration occurs in light and dark Weak / limited.
• Scheduling. A 4-hour dark window is convenient for running lights-off during the hottest or most expensive electricity hours.

Claims that 20/4 specifically increases yield, potency, or growth rate compared to 18/6 are not supported by published cannabis research No data. This is grower folklore.

When to start

Start 20/4 once your plants are established in the vegetative stage. Typical timing:

• From seed: After seedlings have 2-3 sets of true leaves, usually 10-21 days from germination. Many growers run seedlings at 18/6 first to reduce stress, then move to 20/4.
• From clone: After roots are visible and the cutting shows new growth, usually 7-14 days after taking the cutting.

Keep plants on 20/4 (or whatever veg schedule you choose) until they reach the size you want for flowering. Then switch to 12/12 to induce flowering in photoperiod plants ^[1].

How to do it

1. Get a reliable timer. A digital or mechanical timer, smart plug, or lighting controller. Mechanical timers can drift; check them weekly.
2. Pick your on/off times. Many growers run lights-off during the hottest 4 hours of the day to manage heat, or during peak electricity pricing hours to save money.
3. Set the timer. Example: lights on 4:00 AM, off 12:00 AM (midnight). Or lights on 8:00 PM, off 4:00 PM the next day — the cycle is what matters, not the clock time.
4. Verify the cycle. After setting, watch it complete at least one full on/off transition before walking away.
5. Keep the dark period dark. During the 4-hour off period, avoid light leaks. A short interruption during veg is unlikely to flip plants out of veg (the dark period is too short to trigger flowering signals anyway), but light leaks during the eventual 12/12 flower stage can cause re-vegging or hermaphroditism, so build the habit now Strong evidence.
6. Monitor. Check temperature and humidity through both light and dark periods. Lights-off often means a temp drop of several degrees.

When you're ready to flower, switch directly from 20/4 to 12/12. No gradual transition is needed for photoperiod plants.

Common mistakes

• Believing 20/4 outperforms 18/6 or 24/0. There's no good evidence it does. Pick based on your power costs, heat, and routine.
• Light leaks during dark. Tent zippers, equipment LEDs, and room windows can leak light. This matters less in veg than flower, but check anyway.
• Cheap timers that fail. A timer that resets or sticks can leave lights on continuously or off for days. Use a quality timer and physically verify the schedule.
• Running clones or seedlings at 20/4 with high-intensity light. Young plants don't need full power. Reduce light intensity or raise the fixture, regardless of photoperiod ^[3].
• Switching schedules constantly. Pick one and stick with it. Cannabis tolerates schedule changes fine, but constant tinkering doesn't help.
• Confusing autoflowers with photoperiod plants. Autos don't care about photoperiod for flowering, so 20/4 is just an intensity/duration choice for them ^[2].

Related techniques

• 18/6 veg schedule: The other most common veg photoperiod. Uses less electricity than 20/4, gives a longer dark window.
• 24/0 veg schedule: No dark period. Some growers report faster growth; others see no difference.
• 12/12 from seed: Skipping veg entirely to flower from germination. Smaller plants, shorter total grow.
• Light intensity (PPFD) for cannabis: Often more important than on/off ratio. A weak light running 24/0 underperforms a strong light at 18/6.
• DLI (Daily Light Integral): The total amount of photosynthetically active light delivered per day. DLI is arguably a more useful metric than photoperiod alone for comparing schedules ^[4].

Sources

How this page was made