Greenhouse Temperature: Bad Practices That Stunt Growth

🌡️ There's one thing almost every gardener discovers a little too late: managing the temperature in a polycarbonate greenhouse isn't as intuitive as it seems. We imagine the greenhouse does all the work itself, that all we need to do is plant, water, and let nature do the rest. And then one morning, we open the door to find our tomatoes wilted, our seedlings stunted, or our cucumber plants stuck in a rut for three weeks.

The good news? In 90% of cases, the problem comes from a few very identifiable mistakes. Mistakes everyone makes, from beginners to experienced gardeners. Mistakes that are neither serious nor irreversible, but which, if not corrected, can truly sabotage an entire growing season.

At AtmoSerre, we assist dozens of passionate gardeners every week who contact us with the same questions: "Why aren't my plants growing?", "My greenhouse heats up too quickly in the morning, what should I do?", "It's too cold at night despite the polycarbonate…". And each time, by digging a little deeper, we find the same bad practices related to thermal management.

That's why we decided to write this article. Not to give you a generic list of tips you can find anywhere, but to talk to you really about what happens inside a polycarbonate greenhouse, what influences the temperature, and especially all the things we often do without realizing it that hinder plant growth. 🌿

🌡️ Why polycarbonate greenhouse temperature is a topic in itself

Before even talking about mistakes, one fundamental thing needs to be understood: a polycarbonate greenhouse does not behave like a glass greenhouse, nor like a plastic tunnel. Polycarbonate has very specific thermal properties that make it an absolutely remarkable material… provided you know how to use it.

Twin-wall polycarbonate panels (found on all our greenhouses at AtmoSerre) create an insulating air gap between the two layers of polycarbonate. This air gap slows down thermal exchange between the inside and the outside, which means two important things:

• 🔥 The greenhouse heats up more gradually than a glass greenhouse, but also cools down less quickly at night.
• ❄️ In winter, it retains heat accumulated during the day better, offering natural protection against light frosts.

But precisely, this ability to "retain" heat is a double-edged sword. If you don't actively manage the temperature in your greenhouse, you will either end up with an overheated greenhouse on a summer afternoon (and suffering plants), or a greenhouse that is too cold at night in winter (and stagnant growth).

According to INRAE data, the optimal temperature range for most vegetables grown in greenhouses is between 18°C and 26°C during the day, and between 12°C and 16°C at night. Outside these ranges, photosynthesis slows down, flower fertilization can be compromised, and vegetative growth partially stops. This is a biological reality, not an opinion.

And yet, in practice, many greenhouses fluctuate between 10°C and 40°C in a single day depending on the season, without any regulation. This is where it all comes down to.

❌ Mistake #1: Never ventilating (or ventilating too late)

This is probably the most frequent and most devastating bad practice. Ventilation in a polycarbonate greenhouse is not optional. It is vital. And yet, the number of people who keep their greenhouse hermetically sealed from morning to night is impressive.

Here's what happens concretely when you don't ventilate: ☀️ as soon as the sun hits the polycarbonate panels, the greenhouse effect does its job (that's literally its role), and the temperature rises. Quickly. Very quickly. On a sunny spring day, a closed greenhouse can reach 45°C to 50°C in less than two hours. At these temperatures, the enzymes responsible for photosynthesis begin to degrade. The leaf stomata close to limit transpiration. Tomato flowers abort. Seedlings burn.

🔑 What to do concretely

Open the vents as soon as the temperature inside exceeds 25°C. Not when it's "too hot outside", not when you remember, not when you get home from work in the evening. At 25°C.

Our AtmoSerre greenhouses are equipped with side ventilation windows precisely for this reason. They allow for transversal air circulation which, combined with the front door, creates an effective airflow without creating harsh drafts for the plants.

💡 A very practical tip that many of our customers use: automatic thermostatic wax window openers. These are small cylinders that open automatically when the temperature exceeds an adjustable threshold, without electricity, without programming, without intervention. It's one of the simplest and most effective improvements you can make in a greenhouse.

And the opposite mistake also exists: closing the greenhouse too early in the evening. Many gardeners think they should close it at sunset. But in summer, the temperature can remain high for several hours afterward. Closing too early means trapping this residual heat and creating conditions conducive to fungal diseases. We'll talk about that right after.

❌ Mistake #2: Ignoring humidity in relation to temperature

Temperature and relative humidity are two sides of the same coin in a greenhouse. You can have the perfect temperature and still see your plants suffer if the humidity level is poorly managed. And the two are directly linked.

When you don't ventilate and the temperature rises, the relative humidity also rises. A closed greenhouse in summer can reach 90% to 100% relative humidity. At these levels, plant gas exchange is disrupted, transpiration is blocked, and above all: pathogenic fungi love it. Downy mildew, botrytis, powdery mildew… all these problems often associated with "bad luck" or "bad seeds" are, in the vast majority of cases, problems of thermal and hygrometric management.

🌿 The right balance to aim for

These values are indicative and vary depending on the cultivated plants, but they provide a good reference base.

💡 Field tip: a simple digital thermometer-hygrometer placed at plant height (not attached to the greenhouse structure!) will give you a real reading of what your crops are experiencing. It's a 15 to 20€ investment that can truly make a difference.

❌ Mistake #3: Underestimating nocturnal variations

This is a point that is really talked about too little. Night cold is often the invisible enemy of greenhouse growth. And in a polycarbonate greenhouse, even if the thermal insulation is much better than with a plastic film or an uninsulated greenhouse, cold nights remain a real challenge.

In France, even in spring and autumn, night temperatures can drop below 5°C in some regions. For plants like tomatoes, peppers, or eggplants, dropping below 10°C at night means:

• 🛑 A near total cessation of growth
• 🌸 Poor fruit set (flowers fall off without producing fruit)
• 🍅 Development of deficiencies, particularly in calcium and magnesium
• ❄️ General weakening that makes plants more vulnerable to diseases

🔥 Concrete solutions for cold nights

First solution: choose the right polycarbonate thickness. Our AtmoSerre greenhouses are available in 4 mm and 6 mm. The 6 mm offers significantly higher thermal resistance, with a lower insulation coefficient (U-value), which means less nocturnal heat loss. If you live in a region with cold winters (Alsace, Auvergne, Alps, Pyrenees…), 6 mm is not a luxury, it's a necessity.

Second solution: supplemental heating. A small fan heater or an electric heater with a thermostat is often enough to maintain the temperature above 10°C on the coldest nights. The idea is not to heat to 20°C all night long (that's useless and costly), but to never drop below the critical threshold for your crops.

Third solution, often underestimated: an interior winter fleece. By stretching a P17 or P30 forcing fleece inside the greenhouse, just above the plants, you create a second thermal barrier that can gain an additional 2 to 4°C. Simple, economical, and remarkably effective.

Our article on managing your greenhouse in winter details all these strategies with much more precision if you wish to delve deeper into this topic. 🌿

❌ Mistake #4: Greenhouse orientation and its thermal impact (often overlooked)

This is rarely discussed in articles on thermal management, and yet the orientation of your polycarbonate greenhouse has a huge impact on how it captures and redistributes heat. This is a mistake made at the time of installation, and it is then very difficult to correct.

An east-west oriented greenhouse (i.e., with the main facade facing south) captures maximum sunlight throughout the day. This is the ideal configuration for most crops, as it allows for progressive and homogeneous heating of the interior space.

A north-south oriented greenhouse, on the other hand, exposes its sides to the sun morning and evening, which generates greater thermal variations and less regular heating.

🌍 In France, depending on your latitude and your immediate environment (nearby buildings, trees, terrain), the ideal exposure may vary slightly. But the principle remains the same: the goal is to maximize solar exposure in winter (when light is scarce) and to plan for protection against overheating in summer (by combining with shade cloth or well-positioned vents).

❌ Mistake #5: Watering without considering ambient temperature

This mistake may seem minor but has direct consequences on thermal management and therefore on growth. Watering at the wrong time, depending on the temperature, can potentially stress your plants and create unfavorable conditions for their development.

The two main watering mistakes related to temperature are:

🌧️ Watering in full heat (between 11 am and 4 pm in summer): cold water on overheated roots creates thermal shock. In addition, water splashing on hot leaves can cause leaf burn. And rapid evaporation abruptly increases humidity, which promotes diseases.

❄️ Watering late in the evening in autumn or spring: the water does not evaporate, humidity remains high all night, and if the temperature drops, perfect conditions are created for botrytis and other fungi. It's no coincidence that gardeners who water in the evening often have more diseases.

💧 The golden rule of greenhouse watering

Water early in the morning, ideally between 7 am and 10 am. At this time, the temperature begins to rise gently, which promotes evaporation without thermal shock. Plants have water all day during their active phase, and leaves have time to dry before night. It's simple, but it really changes everything.

❌ Mistake #6: Placing all your plants without considering thermal zones

This is a subtlety that even experienced gardeners sometimes overlook: the interior of a polycarbonate greenhouse is not thermally homogeneous. There are warmer and cooler areas, and knowing how to use them intelligently can make a real difference in your yields.

Here's what is generally observed in a standard polycarbonate tunnel greenhouse:

🔥 Hottest zone: the center of the greenhouse, at the height of the upper panels that capture maximum solar radiation. This is where it gets hottest in the afternoon.

❄️ Coolest zones: the ends near the gables (front and back walls), especially if they have doors or windows. Cold air enters from the bottom, and these areas remain more temperate.

🌱 What we concretely recommend:

• Tomatoes, peppers, eggplants (heat-loving plants): in the center, in the warmest areas
• Lettuces, spinach, arugula (cold-tolerant plants): towards the ends or along the side walls
• Seedlings and young plants: sheltered from drafts, slightly away from the gables

This is called thermal zoning, and it's a practice widely used in professional market garden greenhouses, but still largely unknown to amateur gardeners.

❌ Error #7: Forgetting the thermal inertia of the soil

Temperature management in a greenhouse is not just about the air. The soil plays an absolutely fundamental role in the thermal regulation of your greenhouse. And it's a dimension that many gardeners completely ignore.

Bare, sandy, or very well-drained soil has very low thermal inertia: it heats up quickly in the morning but cools down just as quickly in the evening. Conversely, soil rich in organic matter and slightly moist has a much higher thermal inertia: it absorbs heat during the day and gradually releases it at night, helping to reduce temperature fluctuations.

🪴 Practices that improve soil thermal inertia

• Mulching: 5 to 8 cm of straw, shredded dead leaves, or ramial chipped wood (BRF) on the ground retains heat, reduces evaporation, and stabilizes root temperature. This is one of the simplest and most effective practices you can adopt in a greenhouse.
• Wooden raised beds: they provide natural insulation for the substrate from cold soil. Especially in winter, a substrate in a wooden bed remains several degrees warmer than a substrate placed directly on the ground.
• Black water barrels: an ancient but remarkably effective technique. 20 to 50 liter barrels painted black, filled with water, and placed along the interior walls of the greenhouse absorb solar heat during the day and slowly release it at night. Studies have shown that such an installation can maintain the nocturnal temperature 2 to 3°C higher than a greenhouse without this device.

❌ Error #8: Not adapting thermal management to the season

This is perhaps the most general error, but also the one with the biggest consequences: many gardeners apply the same thermal management practices all year round, without adapting their behavior to the specificities of each season.

However, thermal management in a polycarbonate greenhouse is radically different whether it's March, July, or November. The stakes are not the same, the risks are not the same, and the solutions are not the same either.

☀️ Spring: the season of sudden heat excesses

In spring, days lengthen and the sun rises higher in the sky, but nights remain cool. The greenhouse can go from 5°C to 40°C in a few hours on a beautiful April morning. This is when vigilance is highest.

Spring priorities:

• 🌬️ Active ventilation from morning (open before leaving for work)
• 🌡️ Thermometer-hygrometer to monitor peaks
• ❄️ Close in the evening to retain accumulated heat against late frosts

☀️ Summer: fighting overheating

In mid-summer, overheating is the number one enemy. A greenhouse that regularly exceeds 35°C inside will suffer greatly. Mediterranean crops (tomatoes, peppers, etc.) tolerate heat, but not the stagnant, dry heat of a closed greenhouse.

Summer priorities:

• 🌬️ Maximum ventilation (door + side windows permanently open in hot weather)
• 🌑 Shade net on the upper part to reduce direct radiation
• 💧 Water the soil early in the morning to maintain slight coolness

🍂 Autumn and Winter: the art of heat retention

Here, the objective is completely reversed. It's about maximizing heat input and minimizing losses. This is also the period when the thickness of the polycarbonate panels makes all the difference.

Autumn-winter priorities:

• 🔒 Airtight closure at sunset
• 🌿 Internal forcing fleece on the most sensitive crops
• 🔥 Supplemental heating if nighttime temperatures drop below 5°C outside

❌ Error #9: Thinking "more heat = more growth"

This misconception is perhaps the most difficult to uproot because it seems logical on the surface. We tell ourselves that plants love heat, that the tropics are hot, so the hotter, the better. This is wrong, and this belief can really ruin a season.

Plant growth is not a linear curve. It's a bell curve, with an optimal peak and two stress zones on either side.

🌱 Here are some very concrete examples:

• Tomatoes ideally flower and fruit between 20°C and 27°C. Below 10°C, fertilization is compromised. Above 35°C, pollen becomes sterile and flowers abort. Result: you have a beautiful green plant, but no fruit. And you don't understand why.
• Lettuce is a cool-season plant. It grows perfectly between 15°C and 20°C, bolts (goes to seed) as soon as temperatures regularly exceed 25°C, and becomes bitter. In an overheated greenhouse in May, your lettuces will bolt before they are even harvestable.
• Cucumbers, on the other hand, like heat and tolerate up to 30°C, but suffer from cool nights below 12°C. It's a plant that prefers thermal stability over high temperatures.

Each plant has its optimal thermal range, and going outside this range is costly in terms of growth. That's why we always emphasize knowing the specific needs of each crop.

❌ Error #10: Choosing a polycarbonate greenhouse that is too small, and therefore thermally unstable

We conclude this tour of bad practices with an error we often see that is fundamentally linked to thermal management, even if it's not immediately obvious: choosing a greenhouse that is too small.

A small greenhouse means a small volume of air. And a small volume of air heats up incredibly quickly and cools down just as fast. The daily temperature range is much greater in a 4 m² greenhouse than in a 15 m² greenhouse, all other things being equal.

The larger the volume, the greater the thermal inertia of the air, and the more temperature variations are dampened. This is why professional greenhouses are large structures: not only for the growing area, but also for the thermal stability that volume provides.

At AtmoSerre, we offer greenhouses (our collection of robust and bright polycarbonate garden greenhouses) in sizes starting from 6 m² and going up to 36 m², precisely to adapt to everyone's needs. If you have intensive use, if you live in a region with strong sunshine or harsh winters, thinking "big" from the start means investing in the thermal stability of your greenhouse. And it's often much cheaper than buying a larger greenhouse two years later.

Our model L'Intemporelle is perfectly representative of this philosophy: available from 6 to 36 m², in 4 mm or 6 mm polycarbonate, with a robust steel structure that resists winds up to 180 km/h and 90 cm of snow. It is designed to last and to offer a thermally stable environment for your crops, season after season. 🏡

🛠️ Summary table: errors and their solutions

❓ FAQ — Most frequently asked questions about greenhouse temperature

🌡️ What is the ideal temperature in a polycarbonate greenhouse?

Most common vegetables ideally grow between 18°C and 26°C during the day and between 12°C and 16°C at night. It is within this range that photosynthesis is optimal, fertilization proceeds well, and growth is regular. Outside these values, every degree of difference has a measurable impact on plant development.

🌬️ How can I prevent my greenhouse from getting too hot in summer?

Three combined levers: active ventilation (door + side windows open from morning), shade net placed on the upper part to filter 30 to 50% of direct radiation, and morning watering of the soil to maintain a slight coolness. If you have automatic window openers, even better.

❄️ Does my polycarbonate greenhouse really protect against frost?

It depends on the polycarbonate thickness and external conditions. 6 mm polycarbonate offers significantly better insulation than 4 mm and can keep the inside of the greenhouse 3 to 5°C above the outside temperature. By combining with an internal forcing fleece and supplemental heating, you can cultivate continuously all year round, even in the coldest regions of France.

🍅 Why aren't my tomatoes producing despite a nice greenhouse?

In the vast majority of cases, it's a thermal or pollination problem related to temperature. If nights are too cold (below 10°C), fruit set is compromised. If days are too hot (above 35°C), pollen is sterile. Check your thermometer-hygrometer and compare your readings with the optimal ranges for tomatoes. Our complete guide to growing tomatoes in a polycarbonate greenhouse will give you all the keys to diagnose and correct the problem. 🍅

📊 What temperature difference is normal between day and night in a greenhouse?

A difference of 10 to 15°C between day and night is completely normal and even desirable for certain crops (tomatoes, in particular, appreciate a slight nocturnal coolness). However, differences of 25°C or more are a sign of a poorly insulated, poorly ventilated, or poorly positioned greenhouse. It is this type of amplitude that 6 mm polycarbonate and good thermal management practices can significantly reduce.

💡 Is a thermometer enough to manage temperature in a greenhouse?

A simple thermometer is a good start, but a thermometer-hygrometer with min/max recording is much more useful. It allows you to see the maximum temperatures reached during the day and the minimums at night, which is essential for diagnosing thermal management problems. Expect to pay 15 to 25€ for a reliable model. It is one of the most cost-effective tools you can have in a greenhouse.

🌱 Conclusion: thermal management is 80% of greenhouse success

If you were to take away only one thing from this article, it would be this: temperature in a polycarbonate greenhouse is not something that is managed "by intuition" or "by feeling." It is a parameter that requires attention, observation, and a few simple tools.

The bad practices we have reviewed in this article are not errors made by "bad" gardeners. They are very natural errors that everyone makes when starting out. The good news is that they are all easily correctable, and the results of good thermal management are seen very quickly: more vigorous plants, fewer diseases, and more regular and abundant harvests.

At AtmoSerre, we design our polycarbonate greenhouses precisely to facilitate this thermal management: well-positioned ventilation windows, quality polycarbonate available in 4 or 6 mm, robust and stable structures that withstand the most demanding conditions. But the greenhouse, however well-designed, does not replace good gardening practices.

🌿 Are you curious to discover which greenhouse would best suit your region, your garden, and your crops? Visit our AtmoSerre website and explore our range. Our teams are available Monday to Friday from 10 am to 6 pm and Saturday from 10 am to 5 pm to answer all your questions. 🧑🌾

Happy growing to all! 🍅🌿☀️