What is the ideal temperature for a tomato polyhouse?

The ideal temperature for a tomato polyhouse is usually 21°C to 27°C (70°F to 80°F) during the day and 15°C to 20°C (59°F to 68°F) at night. This range gives tomato plants enough warmth for active growth, while still allowing them to slow down and recover overnight. For many growers, the best practical target is a stable, warm day temperature with a slightly cooler night temperature, rather than keeping the polyhouse hot all the time.

This answer sounds simple, but temperature control inside a polyhouse depends on sunlight, cover material, ventilation, air movement, humidity, and the size of the structure. A tomato polyhouse can warm up very quickly on a sunny day, even when the outdoor temperature feels comfortable. That is why the grower should not only ask what temperature tomatoes like, but also how to keep the crop close to that range throughout the day.

Why this temperature range works for tomatoes

Daytime warmth supports photosynthesis

Tomatoes are warm-season plants. During the day, they use light, water, and carbon dioxide to produce energy through photosynthesis. When the daytime temperature stays around 21°C to 27°C, the plant can grow leaves, develop flowers, and support fruit formation at a healthy rate. If the polyhouse is too cool, the plant works slowly. If it is too hot, the plant may lose water faster than the roots can replace it.

A warm but controlled daytime environment is especially important in a covered structure because the crop is not exposed to the same air movement as an open field. The polyhouse cover protects tomatoes from wind and rain, but it can also trap heat. Good temperature management helps the grower keep the benefits of protection without creating a heat-stress problem.

Cooler nights help the plant balance energy

At night, tomatoes do not need the same level of warmth as they do during the day. A cooler range of about 15°C to 20°C helps the plant reduce respiration and use its stored energy more efficiently. This supports normal flower development, fruit set, and fruit growth. When nights remain too warm, the crop may continue using energy instead of recovering, which can lead to weaker fruit set and lower fruit quality.

Commercial greenhouse recommendations can vary by region, crop stage, and light level. Oregon State University notes that greenhouse tomatoes are commonly managed around 70°F to 75°F during the day, with night temperatures in the low 60s Fahrenheit. The same source also notes that cooling becomes important when day temperatures may exceed 85°F to 90°F. Read the greenhouse tomato temperature guidance from Oregon State University.

What happens when a tomato polyhouse gets too hot?

Flower drop and weak pollination

If the temperature rises too high, the first visible problem is often poor flower performance. Tomato flowers may look normal at first, but pollen can become less effective under heat stress. Flowers may dry, fail to pollinate, or drop before setting fruit. In a polyhouse, this can happen faster than expected because the cover traps solar heat.

High tunnel research and extension guidance also connect tomato pollination with both temperature and humidity. South Dakota State University Extension explains that low night temperatures below 55°F can inhibit pollen development, while high day temperatures of 90°F and above can lead to flower abortion. It also notes that pollination is best supported when humidity is kept in a workable range. See SDSU Extension guidance on tomato pollination in high tunnels.

Wilting, water stress, and uneven ripening

Excess heat also increases water loss. Leaves may wilt during the hottest part of the day, even if the soil still contains moisture. The plant closes its stomata to reduce water loss, but that also reduces gas exchange and slows growth. If this happens repeatedly, fruit size and quality can suffer.

Hot conditions can also affect ripening. University of Minnesota Extension explains that sustained hot temperatures can stress tomato plants and make pollination more difficult. It also notes that tomato maturation is best around 68°F to 77°F, while red pigment development is limited above 85°F. Review the University of Minnesota Extension guide on excessive heat and tomatoes.

What happens if the temperature is too low?

Slow growth and delayed fruit development

When the temperature drops below the ideal range, tomato growth slows down. The plant still lives, but its metabolic processes become less efficient. Roots absorb nutrients more slowly, leaves develop more slowly, and fruit development may be delayed. This is why a tomato polyhouse should not only protect the crop from frost; it should also help maintain a productive growing temperature.

Cold nights can affect early clusters

Cool nights are useful, but cold nights are different. If the night temperature falls too low for too long, pollen development can be affected and early flower clusters may not set well. In small polyhouses, this often happens in spring or autumn when the day becomes warm under sunlight but the temperature drops quickly after sunset. Growers should use a minimum-maximum thermometer to understand this day-night gap.

How to maintain the ideal temperature in a tomato polyhouse

Add shade during peak sunlight

On hot days, ventilation alone may not be enough. Shade cloth can reduce the solar load and slow the temperature rise inside the polyhouse. This is especially useful during summer afternoons or in regions with strong sunlight. The goal is not to block all light. Tomatoes still need light for growth. The goal is to reduce extreme heat while keeping enough brightness for photosynthesis.

Choose a stable cover and good insulation

The polyhouse cover also affects temperature stability. A good cover helps hold warmth during cool periods and reduces sudden changes caused by wind or rain. In colder weather, a tight cover and well-sealed edges can help prevent heat loss at night. In hot weather, the same structure must still allow enough ventilation, so the design should balance protection with airflow.

A steel frame with a properly tensioned cover can help maintain a more stable shape and reduce gaps caused by loose fabric or poor installation. This matters because air leaks, sagging cover sections, and poorly sealed openings can make temperature control less predictable.

Monitor temperature at plant height

The most useful thermometer is not the one placed near the door or high in the roof. For tomatoes, measure temperature near the plant canopy, where flowers and leaves are actually growing. A minimum-maximum thermometer or digital sensor can show the highest daytime temperature and the lowest nighttime temperature. This gives the grower real data instead of a rough feeling.

Humidity and air circulation also matter

Keep humidity in a safe working range

Temperature should not be managed alone. Humidity also affects tomato health and pollination. The original growing target of around 60% to 70% relative humidity is a practical range for many tomato polyhouses. If humidity stays too high, fungal diseases become more likely. If humidity is too low, plants lose water quickly and pollen may not work well.

Good growers look at temperature and humidity together. For example, a polyhouse can feel comfortable in the morning but still hold too much moisture on leaves. Opening vents early, watering at the right time, and avoiding wet foliage late in the day can help reduce disease pressure.

Improve airflow without drying the crop

Air circulation helps keep temperature and humidity more even inside the polyhouse. Fans can reduce hot spots, move humid air away from the canopy, and lower the risk of stagnant conditions around dense tomato leaves. The airflow should be gentle and steady. Strong direct wind can dry leaves too quickly or damage soft growth, especially in young plants.

Match the polyhouse size to the grower's needs

Small polytunnel for home growers

For a small-scale grower, a 3m x 3m polytunnel can be a cost-effective way to protect tomatoes from rain, wind, and cool nights. It is easy to set up and simple to ventilate by opening the ends. This type of structure suits home gardens, trial planting, and seasonal tomato growing.

Poly tunnel greenhouse kits for regular growing

For growers who want a more complete setup, DELITE green house polytunnel provide a stronger growing space with better control options. The extra volume of air inside the structure can make temperature changes less sudden than in a very small tunnel. These kits are suitable for growers who want a more stable growing environment and easier access for watering, pruning, and harvesting.

Walk-in polytunnels for larger tomato crops

For larger tomato crops, a walk in tunnel greenhouse gives more space, better working height, and more room for airflow. The steel frame provides durability, while the larger structure allows growers to add vents, fans, shade cloth, and organized rows. This makes temperature management more practical for repeated harvests or semi-commercial production.

Practical temperature checklist for tomato polyhouse growers

• Daytime target: keep the tomato polyhouse around 21°C to 27°C when possible.
• Night target: allow a cooler range of about 15°C to 20°C, but avoid cold stress.
• Vent early on sunny days before the structure overheats.
• Use shade cloth during hot periods when ventilation cannot control heat alone.
• Measure temperature at plant height, not only near the door or roof.
• Keep relative humidity around 60% to 70% where practical.
• Use gentle airflow to reduce hot spots and moisture around leaves.
• Watch flowers closely; poor fruit set is often the first sign of temperature stress.
• Choose the polyhouse size and frame strength based on crop volume, local climate, and ventilation needs.

Final answer: the best tomato polyhouse temperature is stable, not extreme

So, what is the ideal temperature for a tomato polyhouse? The best working answer is 21°C to 27°C during the day and 15°C to 20°C at night. This range supports photosynthesis, flower development, pollination, fruit growth, and overall plant health. But the real key is stability. Tomatoes do not perform well when the polyhouse becomes very hot in the afternoon and then too cold at night.

A well-designed tomato polyhouse should help the grower manage that balance. Ventilation removes excess heat. Shade cloth reduces strong solar gain. A stable cover and steel frame help the structure hold its shape. Fans improve air circulation. Humidity control lowers disease risk. When these factors work together, tomatoes can grow in a protected environment that supports both plant health and fruit quality.

If you are planning a tomato polyhouse for home growing, seasonal production, or larger-scale planting, choose a structure that matches your crop size and climate. With the right polyhouse design and a clear temperature routine, it becomes much easier to create the warm, stable, and productive environment tomatoes need.