Commercial greenhouse pepper runs cooler than most hobby guides claim. Alberta Agriculture's production guide holds the day at 21°C (70°F) and the night at 16 to 17°C (61 to 63°F). The 24-hour mean lands at 20 to 21°C, which the same guide names as the optimum average for yield. Humidity sits at 70 to 80 percent, carbon dioxide at 800 to 1,000 ppm. Those four numbers do more for a crop than any structure upgrade.

What temperature should a pepper greenhouse run?

Split it by stage. For the first week after transplanting, Johnny's greenhouse protocol holds 23°C (73 to 74°F) by day and 21°C (69 to 70°F) at night. Seven to ten days in, drop the night to 18 to 20°C (64 to 68°F) and hold the day at 21 to 24°C (70 to 75°F). Alberta's guide then settles into a steady 21°C day and 16 to 17°C night for the rest of the crop, and keeps the root zone at 20°C. The 24-hour average is what the plant integrates, so a hot afternoon needs a cooler night to balance it.

SettingTargetSource
Day air21°C (70°F)Alberta Agriculture.
Night air16 to 17°C (61 to 63°F)Alberta Agriculture.
24-hour mean20 to 21°CAlberta Agriculture.
Root zone20°C (68°F)Alberta Agriculture.
Relative humidity70 to 80 percentAlberta Agriculture.
Carbon dioxide800 to 1,000 ppmAlberta Agriculture.
Feed EC2.5 to 3.5 mmhosAlberta Agriculture.
Feed pHAbout 5.8Alberta Agriculture.

How much light do greenhouse peppers need?

Michigan State's greenhouse lighting guidance targets 25 to 30 mol per square meter per day for high-wire fruiting vegetables. It sets a floor of 15 for vine crops, with more than 20 preferred. The useful rule of thumb from the same source: a one percent gain in DLI buys roughly a one percent gain in production. Supplemental intensities of 150 to 200 µmol per square meter per second are standard for crops in this bracket. North of about 40 degrees latitude, winter sunlight alone will not carry a fruiting pepper crop.

Plant density and training

Alberta puts density at 3 to 6 plants per square meter, trained to two stems each in a V system, for 6 to 7 stems per square meter. Johnny's arrives at the same stem count from 3 to 3.5 plants per square meter pruned to two to four stems on string. Stems per square meter is the number that matters, not plants. Hobby growers work in beds rather than on wires, often with a bell such as California Wonder. Hold 60 to 90 cm (2 to 3 ft) between rows there. Airflow, not root space, is the real constraint at that scale.

Irrigation and feed

Drip delivers consistency and keeps foliage dry, which cuts disease pressure directly. Alberta targets around 5 percent overdrain early in the season, rising toward 30 percent in summer, and watches root-zone EC at 3.5 to 4.0 mmhos against a feed of 2.5 to 3.5. Feed pH around 5.8 keeps the micronutrients available. In soil beds rather than substrate, aim for pH 6.0 to 6.8. Flush container systems periodically, since salts build quietly and blunt uptake long before anything visible happens.

Disease, and why airflow is the whole game

Two problems dominate enclosed pepper crops. Botrytis cinerea, gray mold, moves on dead tissue and free moisture, so it follows sloppy leaf removal and long wet periods. Powdery mildew in pepper comes from Leveillula taurica, which behaves unlike most mildews. The white sporulation appears on the underside of the leaf while the upper surface shows only chlorotic blotches. It works from the older leaves upward. It thrives on hot dry days followed by cool humid nights, which is exactly what a poorly vented house produces, and it can take half a crop.

Run fans continuously, vent through the day, and keep no stagnant corners. Clear fallen fruit and pruned leaves rather than leaving them on the floor. Monitor with sticky cards and a weekly walk. Quarantine incoming plants before they reach the main house, because one infested transplant reseeds a season of work. Details on the insect side sit in the pest guide.

Do I need to add carbon dioxide?

For a sealed commercial house, yes: Alberta's guide specifies 800 to 1,000 ppm, roughly double ambient. A tight structure with a full canopy draws ambient carbon dioxide down through the morning, and the crop slows accordingly. For a hobby greenhouse that vents freely, enrichment leaks straight out and the money goes further into light and temperature control.

Overwintering and second-year plants

Under glass, a pepper is a perennial. Cut watering back hard, stop feeding, hold above 10°C (50°F) and give just enough light to keep some leaves green. Prune in spring once new growth appears, not in fall, because cutting a dormant plant back invites dieback and disease. A second-year plant carries a full season's root system under minimal top growth, which is why it reaches size in roughly half the time of a seedling and starts cropping weeks earlier. C. chinense and C. pubescens both reward the effort, and a Habanero kept for several years becomes woody and heavy-bearing. The full method sits in the overwintering guide.

Sources & Further Reading