Light Deprivation Automation

What Light Deprivation Automation Is

Light deprivation is the practice of covering a greenhouse (or outdoor plants) each day so they receive 12 hours or less of light, which triggers and maintains flowering in photoperiod cannabis Strong evidence^[1]. Automation replaces the daily human ritual of pulling and unrolling tarps with motors, controllers, and timers that do it on a schedule.

A typical automated system includes:

• A blackout tarp or curtain rated to block essentially all light (often marketed as 99.9%+ opaque).
• Motors and rack-and-pinion or pull-rope drives that move the tarp across the greenhouse structure.
• A controller — either a simple 24-hour timer or a networked climate controller like a Priva, Argus, or Autogrow unit.
• Safety sensors: wind sensors that abort deployment in gusts, and sometimes light sensors to confirm blackout is complete.

The biology is straightforward. Cannabis is a short-day plant, and flowering is initiated by uninterrupted long nights rather than short days per se Strong evidence^[2]. Even brief light interruptions during the dark period can delay or reverse flowering, which is why light-tight construction matters more than tarp specs on paper.

Why Growers Use It

The core reason is harvest frequency. A greenhouse relying on natural photoperiod in the northern hemisphere produces one crop finishing in October. With light dep, you can force flowering in June and July for an early-summer harvest, then run a second cycle finishing in the natural fall window — and in warm climates, a third or even fourth cycle Strong evidence^[3].

Other benefits:

• Avoiding pest and mold pressure that peaks in late season. An early-summer light-dep harvest finishes before botrytis and powdery mildew explode in humid fall weather.
• Market timing. Harvesting when supply is low (early summer) usually means better prices than the fall glut.
• Predictable schedules. You can plan flip dates and harvest dates on a calendar rather than waiting on the sun.
• Energy cost vs. indoor. You get flowering control without paying for full indoor HID or LED lighting through the whole flower cycle.

Automation specifically matters because manual light dep is a 365-day commitment with no days off. A single missed pull — or a tarp pulled 30 minutes late for two days in a row — can stress plants into hermaphroditism or reveg Weak / limited^[4].

When to Start

Timing depends on your latitude and goals. For a first light-dep run in the northern hemisphere:

• Transplant into the greenhouse in April or May once night temps are safe (above ~10°C / 50°F).
• Veg for 3-6 weeks depending on target plant size. Greenhouse plants stretch aggressively, so keep veg shorter than you would indoors.
• Begin pulling when plants are 2-4 feet tall and you want harvest in ~9 weeks.
• Stop pulling when natural nights exceed 12 hours (roughly late August at 40°N latitude). At that point plants flower on their own.

Autoflowers do not need light dep — they flower on age regardless of photoperiod. Running autos in a light-dep greenhouse is a waste of the equipment Strong evidence^[5].

How To Do It: Step-by-Step

1. Build or buy a light-tight structure. This is the step most first-timers underestimate. Every gable vent, roll-up sidewall seam, and door gap must be sealed or covered during the dark period. Use double-layer overlaps and light traps at intake fans. Walk the inside during blackout at noon on a sunny day — any pinhole of light you can see is a problem.

2. Choose your tarp system. Options range from hand-pulled tarps (cheapest, hardest to automate) to retractable roof and wall systems with rack-and-pinion drives (most reliable, most expensive). For automation, retractable systems with dedicated motors per section are standard.

3. Install motors and a controller. Wire motors to a controller capable of astronomical (sunrise/sunset-based) or fixed-time scheduling. Many growers run 12/12 fixed (e.g. cover at 7 PM, open at 7 AM). Others use shorter dark periods (13/11 or 14/10) to reduce heat buildup under the tarp — cannabis still flowers reliably in these ranges Weak / limited^[6].

4. Add safety interlocks. At minimum:

• Wind sensor to prevent tarp deployment in gusts that could tear it.
• End-of-travel limit switches so motors stop at the right position.
• Manual override you can hit from the ground.

5. Manage heat and humidity under the tarp. A closed greenhouse in July can hit 40°C+ in minutes after cover. Options: exhaust fans running through light-trap baffles, an AC system, or a partial pull where sidewalls stay open while the roof covers.

6. Test for two weeks before flip. Run the full cycle on vegging plants to catch light leaks, motor problems, and heat spikes before your crop's flowering is on the line.

7. Monitor every day for the first two weeks of flower. Walk the greenhouse at the start and end of the dark period. Verify tarps are fully deployed. Check corners and vents for leaks. Log any anomalies.

Common Mistakes

• Light leaks. The single most common cause of hermaphroditism and reveg in light-dep crops. Streetlights, security lights, neighboring greenhouses, and even bright moonlit skies through a torn tarp seam can be enough Weak / limited^[4].
• No backup power. If your controller loses power at the wrong time, the tarp doesn't pull and your plants get a 24-hour light day mid-flower. Use a UPS on controllers and consider a generator for motors.
• Cooking plants under the tarp. Pulling at 2 PM in July without ventilation creates greenhouse temperatures that damage flowers and volatilize terpenes. Pull at cooler times or ventilate aggressively.
• Ignoring mold risk. Sealed greenhouses trap humidity. Nighttime RH above 65-70% during flower invites botrytis. Dehumidifiers or nighttime venting through light traps are usually necessary Strong evidence^[7].
• Assuming the timer is running. Verify daily. A drifted schedule, a tripped breaker, or a stuck motor can go unnoticed for days if you only check the plants.
• Overly short dark periods. Some folklore claims 10-hour days speed finishing. Evidence for this is thin, and shortening days too aggressively can stress plants and reduce yield Disputed.

Related Techniques

• Sea of Green (SOG) — pairs well with light dep because short veg times fit the compressed schedule.
• Greenhouse cannabis cultivation — the broader context light dep sits inside.
• Supplemental lighting — the mirror-image technique of adding light in short natural days to keep plants in veg or extend photoperiod.
• Photoperiod manipulation — the underlying biology.
• Autoflower cultivation — an alternative to light dep for growers who don't want the infrastructure.

Sources

How this page was made