Quick answer: A greenhouse blackout curtain creates complete darkness so you can control day length for photoperiodic crops (for example short-day crops such as poinsettia, chrysanthemum and kalanchoe), and stops artificial light escaping at night where light pollution rules apply. Choose a light-tight fabric (often black on one side and white or aluminised on the other), a drive system that seals edges and overlaps, and controls that open a gap for ventilation. The common problem is heat and humidity trapped under a closed curtain: a transpiring crop can push the air under the curtain to saturation within about an hour unless air is exchanged or dehumidified.
Quick answer
- Uses: short-day treatments, long-night control, light-emission limits for lit greenhouses.
- Key specs: light-tightness, edge sealing, reflectivity, drive reliability.
- Plan for humidity: gaps, fans or dehumidification under the curtain.
What blackout curtains are for
| Use | How it works |
|---|---|
| Short-day treatment (inducing flowering) | Pull the curtain in the late afternoon to give the crop a long, uninterrupted dark period |
| Holding crops vegetative | Prevent unwanted flowering by controlling night length |
| Light emission control | Keep supplementary light inside at night where regulations or neighbours require it |
| Energy (secondary) | A closed curtain also reduces heat loss at night |
Purdue Extension explains how to manage greenhouse photoperiod, including artificial short days with blackout systems (Purdue Extension HO-253). The dark period needed depends on crop and cultivar; follow crop guidance.
Choosing a blackout system
| Factor | What to check |
|---|---|
| Fabric | Light transmission close to zero; durability; black side down or up per design; white or aluminised top reflects heat in summer |
| Edges and overlaps | Side curtains, gutter seals and overlaps that block light leaks |
| Drive | Wire or rack-and-pinion; speed; reliability under daily cycles |
| Porosity | Some fabrics pass a little air and moisture; fully closed ones trap more humidity |
| Controls | Scheduled close/open; ventilation gap ("cracking") on high temperature or humidity |
| Combination | Separate energy screen and blackout screen, or one combined system |
Heat and humidity under a closed curtain
A closed blackout curtain seals the crop into a small air volume. Plants keep transpiring, and in summer the curtain is often pulled while the house is still warm.
Worked example: a 1,000 m² bay with about 3 m of air under the curtain (about 3,000 m³, roughly 3,600 kg of air), at 20 °C and 70 % RH. Assume crop transpiration of 15 g per m² per hour.
- Water added: 1,000 × 15 g = 15 kg per hour.
- Moisture content rise: 15 kg ÷ 3,600 kg ≈ 4.2 g/kg per hour.
- Air at 20 °C / 70 % RH holds about 10.3 g/kg; saturation at 20 °C is about 14.7 g/kg.
- Result: saturation in about an hour if no air is exchanged and nothing removes water.
The transpiration rate is an assumption for illustration; the conclusion holds for most actively growing crops: humidity under a closed curtain rises fast.
How to manage it
- Crack the curtain (open a small gap) when RH or temperature passes a set limit, if the dark requirement allows. A light-tight gap design avoids light leaks.
- Run horizontal airflow fans under the curtain so the canopy stays at air temperature.
- Exhaust or vent above the curtain where the system design allows air exchange without light entry.
- Dehumidify under the curtain when air exchange is not allowed or outside air is humid. Refrigerant dehumidifiers return the condensation heat to the crop zone. See greenhouse dehumidifiers.
- Avoid irrigating late before the curtain closes.
For energy screens and humidity, see greenhouse humidity under thermal screens, and for the disease risk of humid nights, see greenhouse humidity and disease.
Blackout vs shade curtains, and light-deprivation greenhouses
| Blackout curtain | Shade curtain | |
|---|---|---|
| Light blocked | Nearly all (about 99.9 % or more) | Part, e.g. 30–70 % |
| Purpose | Photoperiod control: short days for flowering, or keeping light in at night | Reduce heat and light on bright days |
| Use timing | Closed for set hours each day | Closed in strong sun |
A light-deprivation greenhouse is a standard greenhouse fitted with a blackout system so the grower can shorten the day for short-day crops out of season.
Commissioning and maintenance
- Check for light leaks at night from inside and outside, especially at gutters, posts and overlaps.
- Log temperature and RH under the curtain for a week after installation.
- Inspect fabric, wires and drives seasonally; repair tears promptly.
- Keep the curtain dry when stored open; folded wet fabric can grow mould.
- Clean the fabric as the maker allows, usually with low-pressure water and mild detergent, and let it dry fully before storing; avoid solvents that damage coatings.
- Expect a service life of several years; UV, moisture, abrasion and repeated folding shorten it. Replace when light leaks or tears can't be repaired.
FAQ
Frequently asked questions
What is a greenhouse blackout curtain used for?
To give crops a controlled dark period for flowering (short-day crops), to keep crops vegetative, and to keep light inside lit greenhouses at night.
Do blackout curtains cause humidity problems?
They can. A closed curtain traps a small volume of air with a transpiring crop, so RH can reach saturation within about an hour without ventilation or dehumidification.
How do I control humidity under a blackout curtain?
Open a ventilation gap when allowed, run circulation fans, and use dehumidifiers when air exchange is not possible.
How do I clean and maintain blackout curtains?
Inspect seasonally, clean gently as the maker allows, dry fully before storing, and repair tears and light leaks promptly.
What should I evaluate when choosing a blackout curtain?
Light blocking, fabric durability and UV rating, how it seals at edges and gutters, drive system, and how it lets heat and moisture escape.
Blackout curtain vs energy screen: what is the difference?
A blackout curtain blocks nearly all light; an energy screen mainly reduces heat loss and lets some light through. Many greenhouses have both.
Sources
- Purdue Extension, Managing greenhouse photoperiod (HO-253-W).
- ASHRAE Handbook—Fundamentals, Psychrometrics.
