Greenhouse Temperature Guide for Plants — Celsius and Fahrenheit by Season

A greenhouse extends your growing season, but only if the temperatures inside stay within the right ranges. Too cold and plants stall or die; too warm and they bolt, wilt, or suffer heat stress. The ranges vary significantly by crop type, and what works for tomatoes will cook your lettuce.

The Temperature Converter converts between Celsius and Fahrenheit if your thermometer or heating system uses a different scale than your reference.

The Three Greenhouse Temperature Zones

Most greenhouse growers think in three categories:

Cool greenhouse: 7–13°C (45–55°F) minimum overnight. Suitable for frost-sensitive crops that can tolerate cool temperatures but not freezing. Think overwintering dormant plants, growing hardy annuals, or starting cold-tolerant vegetables.

Temperate greenhouse: 13–18°C (55–64°F) minimum overnight. The most common target for productive year-round growing. Supports a wide range of crops including tomatoes (at the warmer end), peppers, cucumbers, and most herbs.

Warm greenhouse: 18–24°C (64–75°F) minimum overnight. Needed for tropical plants, orchids, and crops from warm climates. Expensive to heat in winter; mostly relevant in climates where nighttime temperatures drop below 18°C regularly.

The "minimum overnight" is the critical number — plants can tolerate much wider temperature swings during the day, but the nighttime low determines whether they survive and whether growth continues.

Optimal Temperature Ranges by Crop

Tomatoes

• Daytime: 21–27°C (70–80°F)
• Nighttime minimum: 16–18°C (60–65°F)
• Notes: Below 10°C (50°F), tomatoes stop setting fruit. Above 32°C (90°F), pollen becomes sterile and fruit set fails. High temperatures are the more common greenhouse problem in summer — ventilation matters more than heating for tomatoes.

Cucumbers

• Daytime: 24–28°C (75–82°F)
• Nighttime minimum: 18°C (65°F)
• Notes: Cucumbers are warm-season crops that genuinely need warmth. Temperatures below 13°C (55°F) cause chilling injury. Consistent humidity (70–80%) at these temperatures reduces powdery mildew risk.

Peppers

• Daytime: 21–29°C (70–85°F)
• Nighttime minimum: 18°C (65°F)
• Notes: Similar to tomatoes but somewhat more heat-tolerant. Below 13°C (55°F) growth essentially stops. Peppers take longer to mature than tomatoes and benefit from a longer heated season.

Lettuce and leafy greens

• Daytime: 16–21°C (60–70°F)
• Nighttime minimum: 4–7°C (40–45°F)
• Notes: Lettuce is a cool-season crop and tolerates near-freezing temperatures in a cool greenhouse. Above 24°C (75°F) it bolts (goes to seed) and becomes bitter. This makes lettuce incompatible with tomatoes in a single unpartitioned greenhouse.

Herbs (basil, parsley, chives)

• Basil: 18–24°C (65–75°F) daytime, 16°C (60°F) minimum. Highly cold-sensitive.
• Parsley: 15–21°C (60–70°F); tolerates cooler nights down to 7°C (45°F).
• Chives: Very hardy; fine down to 0°C (32°F) if hardened off, though growth slows significantly.

Orchids (Phalaenopsis)

• Daytime: 21–29°C (70–84°F)
• Nighttime: 16–18°C (60–65°F) — cooler nights trigger flowering
• Notes: Phalaenopsis orchids are common because they're the most adaptable. More exotic orchids have tighter requirements; check the specific genus.

Succulents and cacti

• Active season: 18–30°C (65–86°F)
• Dormant/winter: 5–15°C (40–60°F)
• Notes: Most succulents are more cold-tolerant than people think and actually prefer a cool winter dormancy. Keeping them warm year-round can shorten their lifespan and prevent flowering.

Seasonal Temperature Management

Winter

The goal is maintaining minimum nighttime temperatures without excessive heating costs. Key strategies:

Thermal mass: Water barrels, stone floors, or concrete block walls absorb heat during the day and release it at night. A greenhouse with good thermal mass can hold 3–5°C (5–9°F) warmer than one without on cold nights with no additional heating.

Double-layer covering: Double-wall polycarbonate panels or adding a layer of horticultural fleece inside the greenhouse on very cold nights significantly reduces heat loss. A single layer of glass or plastic loses heat rapidly; double glazing cuts losses by 40–50%.

Zone heating: If you only have a few tender plants, a small electric propagator or heat mat under those specific plants is more efficient than heating the entire greenhouse.

Monitoring: A min/max thermometer (or a wireless sensor you can check from inside) tells you the actual low reached overnight. This is more reliable than assumptions — a greenhouse with inadequate insulation can drop 10–15°C (18–27°F) below outdoor temperatures overnight in cold climates.

Summer

Overheating is as dangerous as cold. Greenhouses can easily reach 40–50°C (104–122°F) on a hot sunny day with inadequate ventilation — temperatures that stress or kill most crops within hours.

Roof vents: Heat rises. Roof vents are far more effective than side vents at cooling a greenhouse. Aim for ventilation area of at least 15–20% of the floor area, with most of that at roof level.

Shade cloth: 30–50% shade cloth reduces solar gain without blocking all light. Critical for lettuce and cool-season crops in summer; less critical for tomatoes and peppers which want more light.

Evaporative cooling: In dry climates, wetting the floor and allowing evaporation can reduce temperatures by 5–10°C. This also increases humidity, which may help or hurt depending on the crops.

Automatic vent openers: Wax-cylinder automatic openers open vents at a set temperature (typically 17–25°C, adjustable) without electricity. They're inexpensive and prevent overheating if you're away during the day.

Spring and Autumn

The challenge in shoulder seasons is temperature swings: warm days requiring ventilation and cool nights requiring heat. A thermostatically controlled heater and automatic vent openers work well in combination — the vents open when it's warm enough, the heater kicks in when it's too cold.

Temperature Conversion Quick Reference

Use the Temperature Converter for any specific value not in this table. The formula for Celsius to Fahrenheit is °F = (°C × 9/5) + 32; for Fahrenheit to Celsius: °C = (°F − 32) × 5/9.

Thermometers for Greenhouse Use

A basic greenhouse needs at minimum a min/max thermometer placed at plant height (not mounted on the wall near a vent or heater, which will give unrepresentative readings). Reading the minimum temperature each morning tells you whether your heating is working.

For more monitoring, a wireless temperature sensor that sends readings to a phone app alerts you to problems — a heater failure overnight, an unnoticed cold snap — before it kills your crop. These sensors typically cost $20–50 and can save a season's worth of plants.

Soil temperature matters too, particularly for germination. Most seeds germinate well at 18–24°C (65–75°F) soil temperature. A soil thermometer or heat mat with a thermostat is useful if you're starting seeds in cool conditions.