Plants in the greenhouse that are not growing: causes & solutions

By the AtmoSerre team

You've invested in a beautiful polycarbonate greenhouse, you've carefully sowed seeds, watered regularly… and yet, nothing happens. Plants stagnate, yellow, or worse, die before they even really get started. This is one of the most frustrating situations for a gardener, and we completely understand.

At AtmoSerre, we regularly receive these types of questions from our customers. And the good news is that in the vast majority of cases, the causes are identifiable and the solutions are at hand. A polycarbonate greenhouse offers ideal conditions for year-round cultivation, provided you understand how it works and avoid some common mistakes.

In this article, we will dissect all the reasons why your greenhouse plants are not growing, from temperature problems to watering errors, including substrate quality, light, ventilation, and diseases. Each point will be accompanied by concrete, tested, and truly effective solutions.

Because a greenhouse should not be a source of disappointment. It should be your best ally 🌿.

🌡️ Temperature in a polycarbonate greenhouse: beginner gardeners' #1 mistake

This is probably the most frequent cause of plants not growing in a polycarbonate greenhouse. And yet, it's also the one that's least clearly talked about. So let's clear things up.

A greenhouse heats up quickly. Very quickly. 🔥

Contrary to what many imagine, the problem in a polycarbonate greenhouse is not often cold… it's excessive heat. As soon as the sun hits the panels, the temperature can rise dramatically. In summer, it's not uncommon to reach 40 to 50°C inside an unventilated greenhouse in the middle of the day.

And at these temperatures, most vegetable plants simply stop growing. Tomatoes, for example, stop fruiting above 35°C. Cucumbers suffer above 32°C. Lettuce bolts above 28°C. These are not theoretical figures; this is what we observe in the field, season after season.

Symptoms of excessive heat 🌡️

• 🍂 Leaves curl up
• 🟡 Foliage yellows from the top
• 🚫 Flowers fall off before forming fruit
• 😴 Plants seem to "stagnate" for no apparent reason
• 💧 Soil dries out in a few hours despite watering

But cold is also an enemy 🥶

Conversely, in winter or early spring, a poorly insulated polycarbonate greenhouse can allow nocturnal temperatures to drop too low. Most vegetable plants need a minimum temperature of 10 to 12°C to maintain their growth. Below that, they enter a kind of dormancy. They don't necessarily die, but they stop growing altogether.

6mm polycarbonate, with its double-walled hollow structure, offers much better insulation than 4mm. For regions with harsh winters (northern France, mountainous areas, high altitudes), this is often the difference between plants that survive and plants that thrive.

Concrete solutions 💡

• Install a max-min thermometer in your greenhouse (less than 10€, essential)
• Open windows and the door as soon as it reaches 20°C to prevent overheating
• Use a shade net in summer to filter light and reduce heat by 5 to 10°C
• Mulch the soil to retain coolness and moisture
• In winter, add a miniature indoor greenhouse or winter fleeces for the most fragile crops

💨 Ventilation: the silent problem everyone underestimates

Among the causes of poor growth in a greenhouse, insufficient ventilation is undoubtedly the most underestimated. We often think that hermetically sealing the greenhouse protects plants. This is a mistake.

Why ventilation is vital 🌬️

Plants need CO₂ for photosynthesis. In a closed greenhouse, CO₂ is rapidly consumed in the early morning, and growth slows abruptly from 10-11 am. Without air renewal, plants are essentially "running out of fuel."

Furthermore, a stagnant, warm, and humid atmosphere is a paradise for fungi and diseases like botrytis (gray mold), powdery mildew, or damping-off. These diseases can decimate a crop in just a few days.

Signs of poor ventilation ⚠️

• 🍄 Appearance of white or gray spots on leaves
• 💦 Permanent condensation on polycarbonate panels
• 🌿 Long, soft, and spindly stems (etiolation)
• 🐛 Increased presence of pests (aphids, whiteflies)
• 🌡️ Stifling heat and a stuffy smell upon entering

Practical solutions 🛠️

• Open roof windows and the door every morning as soon as the temperature exceeds 18°C
• Invest in thermosensitive automatic vent openers (they open on their own above a certain temperature, which is very practical when you're away)
• Create a cross-draft by opening two opposite sides simultaneously
• In winter, even in cold weather, ventilate for 30 minutes a day in the middle of the day
• Avoid overcrowding your greenhouse with plants: space between plants is as important as root space

We have also dedicated a full article to this topic: Garden greenhouse ventilation: advantages and mistakes to avoid 📖, if you want to delve deeper into this specific point, it's a truly useful read.

☀️ Light: more complex than you think in a polycarbonate greenhouse

This is a topic that deserves proper explanation. Polycarbonate diffuses light evenly, which is a huge advantage over glass (no burning from concentrated rays). But it also means that the light that penetrates is slightly filtered.

Greenhouse orientation: crucial and often overlooked 🧭

A greenhouse oriented east-west (with the ridge in this direction) maximizes sunlight throughout the day. A poorly oriented greenhouse can lose up to 30 to 40% of available light. That's a lot.

If your greenhouse is already installed and the orientation is not ideal, you can compensate:

• By using white floor paints or reflective tarps to reflect light
• By eliminating any shade cast by nearby trees or buildings
• By regularly cleaning the polycarbonate panels (dust, algae, and moss deposits significantly reduce light transmission)

Lack of light in winter 🌥️

Between November and February, even with good orientation, days are short and light intensity is low. Some crops like tomatoes, peppers, or basil absolutely need 6 to 8 hours of direct light to grow properly.

Below this threshold, plants "vegetate" without really developing. This is perfectly normal, and it's not a problem with the greenhouse or the seeds. It's simply the physics of winter gardening.

Solutions:

• ☀️ Choose varieties adapted to low light for winter crops (corn salad, spinach, some lettuces)
• 💡 Install horticultural lighting (full-spectrum LED lamps) to compensate for short days
• 🌿 Concentrate the most light-demanding crops (tomatoes, cucumbers, peppers) from March to October

💧 Watering in a greenhouse: not too much, not too little

Watering in a greenhouse is a science in itself. And unlike an outdoor garden where rain sometimes "catches up" with forgotten watering, in a polycarbonate greenhouse, everything depends on you.

Overwatering: the #1 cause of plant death 🚿

We often think that yellowing or stagnant plants are lacking water. And sometimes, it's the opposite. Excessive watering suffocates the roots by saturating the air spaces in the soil. The roots can no longer absorb oxygen, they rot, and the plant slowly withers away.

Symptoms of overwatering:

• 🟡 Leaves yellowing starting from the bottom
• 😔 Plants "wilting" despite moist soil
• 🦠 Sour or rotten smell in the greenhouse
• 🍄 Appearance of mold on the soil surface

The golden rule: stick your finger 2-3 cm into the soil. If it's still moist, don't water. If it's dry, water. Simple, effective, and much more reliable than a fixed schedule.

Underwatering: stress that inhibits growth 🌵

Conversely, a greenhouse can dry out very quickly in hot weather. A plant subjected to even brief water stress can take several days to recover its normal growth rate. Repeated dry spells weaken the plants' immune defenses and make them more vulnerable to diseases and pests.

Solutions for controlled watering:

• 💧 Invest in a programmable drip irrigation system (it's a real game-changer, especially in summer)
• 🌅 Water preferably in the morning, so that the foliage and soil surface dry during the day
• 🪴 Use self-watering planters to buffer variations
• 🧪 Regularly check that water can drain properly: soil that retains too much water is often a sign of an unsuitable substrate

🌍 Substrate and soil fertility: the foundation of everything

We talk a lot about greenhouse structure, ventilation, watering… But the quality of the soil in which your plants grow is just as crucial. And it's a point that is often overlooked, especially when you're starting out.

Ordinary garden soil is not always suitable 🧱

Many gardeners installing their greenhouse for the first time simply use garden soil. This is understandable, but it's often a mistake. Garden soil, in a greenhouse, can quickly become compact, hydrophobic, and poor in nutrients after a few weeks of intensive cultivation.

In a polycarbonate greenhouse, cultivation cycles are faster and more numerous than outdoors. The soil therefore becomes depleted more quickly.

The right substrate mix 🌿

For successful greenhouse cultivation, here is the ideal mix we recommend:

This mix offers both good water retention, excellent aeration, and long-lasting fertility.

Soil fatigue: a real phenomenon ⚠️

After 2 to 3 seasons of intensive cultivation in the same beds or in the same greenhouse area, the soil can become "fatigued." This phenomenon, well-known to market gardeners, results in increasingly stunted plants despite the same conditions.

Solutions:

• 🔄 Practice crop rotation: do not plant the same species in the same spot two years in a row
• ♻️ Amend regularly with compost, well-decomposed manure, or organic fertilizers
• 🌱 Consider soilless cultivation (in pots, grow bags) for total control over the substrate
• 🧪 Have your soil analyzed if in doubt (simple kits are available at garden centers, or through specialized laboratories)

pH: the detail that changes everything 🔬

Soil pH directly influences plants' ability to absorb nutrients. Soil that is too acidic (pH < 6) or too alkaline (pH > 7.5) blocks the absorption of many minerals, even if they are present in sufficient quantities.

Most greenhouse vegetables thrive best with a pH between 6.2 and 6.8.

A pH test (less than 5€ at garden centers) can save you weeks of trouble. If the pH is too low, add horticultural lime. If it's too high, sulfur or peat moss can help correct it.

🧫 Diseases and pests in polycarbonate greenhouses: recognize and act quickly

The confined environment of a greenhouse is a double-edged sword. It protects against external weather conditions, but it also creates conditions that can favor certain diseases and pests. Here's a comprehensive overview.

Main fungal diseases 🍄

Botrytis (gray mold) is probably the most devastating fungus in greenhouses. It is recognized by its brown spots covered with gray mold. It thrives in conditions of high humidity and poor air circulation.

Powdery mildew appears as a white powder on leaves and stems. It significantly slows down photosynthesis and weakens plants.

Damping-off is the nightmare of greenhouse seedlings: young plantlets suddenly collapse at soil level. It is caused by fungi like Pythium or Rhizoctonia, favored by excess humidity and low temperatures.

Prevention:

• 🌬️ Ventilate daily, without exception
• 💦 Never wet the foliage when watering
• 🌿 Immediately remove dead or diseased leaves
• 🧴 Treat preventively with Bordeaux mixture or diluted baking soda

Common greenhouse pests 🐛

Polycarbonate greenhouses, although isolated from the outside world, are not impervious to harmful insects. Here they are, classified by frequency of appearance:

• 🐜 Aphids: they settle on young shoots and flower buds, in dense colonies. Treatment: diluted black soap, or introduction of ladybugs (biological control)
• 🦟 Whiteflies: they fly in clouds when plants are touched. Treatment: yellow sticky traps, neem oil
• 🕷️ Spider mites (red spiders): invisible to the naked eye but recognizable by webs and discolorations. Treatment: humidify the atmosphere, natural acaricide
• 🐌 Slugs: they enter at night through openings. Treatment: beer traps, iron phosphate pellets
• 🦂 Fungus gnats (small black flies): they lay eggs in moist soil and larvae eat seedling roots. Treatment: let the soil dry out between waterings

A field tip we often give: inspect your plants under the leaves, not just on top. The majority of pests (aphids, whiteflies, mites) colonize the underside of the foliage. That's often where it all starts.

🌱 Seedling errors in polycarbonate greenhouses

Your plants aren't growing? Sometimes, the problem doesn't come from the greenhouse itself, but from how the seedlings were started. Here are the most common mistakes.

Sowing too deep ⬇️

A seed sown too deeply takes much longer to germinate, and sometimes simply doesn't germinate at all. The basic rule: sow at a depth equal to twice the seed's diameter. For a tomato seed (about 3-4 mm in diameter), sow at a maximum depth of 6-8 mm.

Sowing in unsuitable substrate 🧱

Seedling pots should be filled with a special seedling mix: light, fine, lightly fertilized. A classic all-purpose potting mix is often too rich and too compact for seedlings, which can inhibit germination or burn young roots.

Sowing too early 📅

This is a very common mistake, especially among impatient gardeners (and we all recognize ourselves a bit in that!). In a polycarbonate greenhouse, the temptation to sow as early as January is strong. But if the nighttime temperature drops below 10°C, the seeds will remain dormant, and even if they germinate, the plants will be stunted and vulnerable.

For tomatoes, peppers, and eggplants, wait until the nighttime temperature is consistently above 12-15°C before sowing in your greenhouse. In practice, this often corresponds to March-April depending on the region, sometimes as early as February with supplementary heating.

Not giving enough importance to seed quality 🌰

Expired seeds germinate poorly, if at all. Seed viability varies by species: 3 years for peppers and onions, 4-5 years for tomatoes and cucumbers, up to 6 years for lettuces. After these periods, the germination rate drops drastically.

Tip: Test your seeds by placing them between two layers of damp paper indoors. If less than 70% of the seeds germinate in 7-10 days, get a new batch.

🧑🌾 Planting density: too many plants in the greenhouse

This is a common mistake, especially when you're enthusiastic and want to grow everything at once. Planting too densely in a polycarbonate greenhouse is counterproductive at all levels.

Why spacing is crucial 📏

Plants that are too crowded compete for light, nutrients, and water. The result: they become leggy (long, thin stems), produce little, and are much more vulnerable to diseases because air no longer circulates between them.

Here are some reference planting distances for common greenhouse crops:

These distances are not approximate suggestions. They are tested distances that allow each plant to fully develop its root system and foliage.

The solution: plan before planting 📋

Before planting, draw a plan of your greenhouse on paper. Note the dimensions, identify the sunniest areas (often the south side), and calculate how many plants you can actually accommodate. This is 30 minutes well spent that will save you months of disappointment.

🪴 Plant nutrition in the greenhouse: understanding deficiencies

A plant in a greenhouse that isn't growing is often a plant suffering from a nutritional deficiency. And contrary to what you might think, adding more fertilizer isn't always the solution. Sometimes, it's quite the opposite.

Reading leaf signals 🌿

Plants communicate through their foliage. Learning to "read" these signals is to gain a real diagnostic power.

• 🟡 Yellow leaves starting from the bottom → nitrogen (N) deficiency or overwatering
• 🟤 Brown and dry leaf edges → potassium (K) deficiency, especially in tomatoes
• 🟣 Leaves turning a purplish tint → phosphorus (P) deficiency, often in cold weather
• ⬜ Young leaves yellow between green veins → iron (Fe) or magnesium (Mg) deficiency
• 🍂 Browning tips → salt burn (too much fertilizer) or calcium deficiency

The basics of effective fertilization 🌱

Greenhouse fertilization must be regular, measured, and adapted to the growth stage:

• Vegetative growth phase (leaf and stem development): prioritize a nitrogen-rich fertilizer (high N)
• Flowering and fruiting phase: switch to a potassium and phosphorus-rich fertilizer (high K and P), which promotes fruit formation
• Every 15 days during the active period: a supply of diluted liquid fertilizer at the base of the plants is generally sufficient

Be careful with dosages: over-fertilization burns roots and can kill a plant in a few days. Always follow the manufacturer's recommendations, or even reduce by 20-30% if you are a beginner. It is better to under-dose and adjust than the other way around.

🏗️ The greenhouse structure: its indirect but real role in growth

We sometimes forget, but the quality of the greenhouse itself influences growing conditions. A poorly built greenhouse, with ill-fitting panels that let in cold or humidity, or a structure that vibrates in the wind and stresses the plants, can indirectly affect growth.

At AtmoSerre, all our polycarbonate greenhouses are designed to offer stable and optimal conditions for your crops. Our structures resist winds up to 180 km/h and can support snow loads up to 90 cm (approximately 150 kg/m²). This level of robustness guarantees a greenhouse that doesn't move, doesn't leak, and maintains a constant thermal envelope.

Our model L'Intemporelle, for example, is available with 4 mm panels (ideal for mild to moderate climates) or 6 mm (recommended for regions subject to winter cold). This choice of thickness has a direct impact on nighttime thermal insulation, and therefore on the growth of your plants.

A well-built greenhouse is also a greenhouse that stays clean longer. High-quality polycarbonate panels have integrated UV protection that prevents yellowing and maintains optimal light transmission over time.

If you are looking to compare available models, our complete collection is accessible on the our garden greenhouses page: you will find all options with their detailed characteristics.

💡 Summary: the 10 main reasons why your greenhouse plants aren't growing

To clearly summarize everything we've covered, here are the 10 most frequent causes, ranked by importance:

1. 🌡️ Unsuitable temperature (too hot or too cold)
2. 💨 Insufficient ventilation (stuffy and stagnant air)
3. 💧 Incorrect watering (too much or not enough)
4. ☀️ Lack or excess of light
5. 🌍 Unsuitable or depleted soil
6. 🧫 Presence of fungal diseases (botrytis, powdery mildew)
7. 🐛 Pest infestation (aphids, mites, whiteflies)
8. 📅 Sowing too early or expired seeds
9. 🌿 Too high planting density
10. 🧪 Nutrient deficiencies (lack or imbalance of fertilizer)

Most of these problems can be resolved with observation, a bit of method, and the right reflexes. And the good news is that a well-managed polycarbonate greenhouse remains the most powerful gardening tool for year-round cultivation in France.

📊 Summary table: quick diagnosis and solutions

❓ FAQ: your most frequent questions about greenhouse plants

Why aren't my tomato plants flowering in the greenhouse? 🍅

The main cause is often temperature: above 35°C, tomatoes stop flowering. Also check for pollination: in a closed greenhouse, insects cannot enter. Gently shake the flowering stems each morning to simulate wind, or open the greenhouse during sunny hours to allow pollinators to enter.

Is a polycarbonate greenhouse sufficient without heating in winter? ❄️

It depends on your region and what you want to grow. A 6mm polycarbonate greenhouse generally maintains an indoor temperature 5 to 10°C higher than outside. This is sufficient for many winter crops (spinach, lamb's lettuce, parsley, lettuce). For more demanding plants (tomatoes, basil), auxiliary heating is still necessary.

My greenhouse seedlings rot before germinating. Why? 🌱

This is typically damping-off, caused by soil fungi. Favorable factors include: excess humidity, too low a temperature, unsterilized substrate. Use a quality special seedling compost, water very moderately, and even ventilate lightly. If the problem persists, a preventive treatment with diluted copper can help.

How many times a week should I water in a greenhouse? 💧

There is no universal answer: it depends on the season, heat, substrate type, and crops. In summer, during hot weather, daily watering may be necessary. In winter, once every 5 to 7 days is often sufficient. The finger rule (2 cm into the soil) remains the best indicator. And if you want to know more about mistakes to avoid in the greenhouse from the start, our article Garden greenhouse: 7 mistakes that ruin your harvests is for you 📖.

My greenhouse soil is compact and hard. How can I improve it? 🧱

Compact soil in a greenhouse is often due to the use of unamended garden soil, or an accumulation of mineral salts after years of watering. The solution: remove the soil to a depth of 20-30 cm and replace it with a compost/potting soil/coarse sand mixture. If you cannot completely replace the soil, work it on the surface with a claw and incorporate mature compost and perlite.

Can homemade compost be used in a polycarbonate greenhouse? ♻️

Yes, absolutely, provided the compost is well-rotted (at least 6 months of decomposition, homogeneous texture, no ammonia smell). Fresh compost can burn roots and introduce pathogens. Mixed at a maximum of 25% with quality potting soil, homemade compost is an excellent organic amendment.

🌿 Conclusion: your polycarbonate greenhouse has everything to succeed

If you've read this article so far, you now have a complete and concrete understanding of what can cause your plants to stagnate in a polycarbonate greenhouse and, most importantly, how to remedy it.

The truth is, a well-managed polycarbonate greenhouse is one of the best gardening tools available. It offers you climate control that you would never have outdoors, extends your seasons by several months, and dramatically multiplies your harvests. But like any powerful tool, it requires a minimum of understanding to get the most out of it.

Observation is your best ally. Get into the habit of spending a few minutes in your greenhouse each morning, observing your plants from all angles, noting the temperatures, and feeling the soil. Over time, you will develop a true intuition as a greenhouse gardener.

And if you are considering investing in a quality greenhouse to maximize your chances of success from the start, do not hesitate to explore our range on AtmoSerre. Our greenhouses are designed by gardening enthusiasts, for gardening enthusiasts, with a warranty of up to 10 years and structures designed to last even in the most demanding climatic conditions 🌱🔥.

Happy growing to all, and may your plants grow tall and strong! 🌿🍅🥕