Greenhouse Watering: Mistakes that kill your crops!

Greenhouse watering is undoubtedly the topic that generates the most questions among gardeners, whether beginners or experienced. And frankly, that's perfectly normal. Because watering correctly in a polycarbonate greenhouse is really not the same as watering outdoors. The rules change, needs evolve with the season, the weather no longer plays the same role, and mistakes can be costly, sometimes in just a few days. 🌧️

At AtmoSerre, we support hundreds of passionate gardeners every day, and the question of watering consistently comes up in discussions. That's why we wanted to write this truly comprehensive guide, rooted in practical experience, to give you concrete benchmarks that you can apply in your greenhouse tomorrow.

Whether you're growing tomatoes 🍅, cucumbers, herbs, strawberries 🍓, or simply your first seedlings, this guide is for you. You'll understand why to water in a certain way, and not just how. The nuance is important.

Let's go. 💡

💧 Why watering in a polycarbonate greenhouse is so different from outdoor gardening

When gardening outdoors, rain does a lot of the work. You can forget to water for a few days, and if a storm passes, the soil is replenished. In a greenhouse, you are solely responsible for water supply. There is no natural safety net.

A polycarbonate greenhouse creates a closed microclimate, with a higher temperature than outside, often between 5°C and 15°C warmer depending on the season. This thermal differential is great news for your crops, but it significantly accelerates water evaporation. Soil that would remain moist for three days outdoors can be dry in 24 hours under a well-exposed greenhouse in summer. ☀️

There's also another often overlooked phenomenon: condensation. At night, the polycarbonate panels cool down and humidity in the air condenses on the panels. This water runs off and falls to the ground, but not necessarily where you need it. Some gardeners believe their plants have been watered at night by this phenomenon. In reality, it's rarely enough, and it can even promote fungal diseases if the soil is already moist.

The multi-wall polycarbonate of AtmoSerre greenhouses plays a key role here. Thanks to its double wall, it regulates temperature fluctuations much better than glass or single plastic. The result: fewer thermal shocks for plants, better controlled condensation, and an overall more stable environment. But this does not exempt you from adapting your watering practice. 🧑🌾

🌡️ Factors influencing watering frequency in a greenhouse

There is no universal answer to the question "how often should you water in a greenhouse?". The correct frequency depends on several interacting variables, and you need to learn to read them.

☀️ Season and outdoor temperature

This is the number one factor. In summer, when outdoor temperatures exceed 30°C, the heat inside a greenhouse can easily reach 40°C to 50°C if ventilation is insufficient. In these conditions, some crops may require two waterings per day. In winter, with cooler temperatures and less sunshine, watering every two or three days may be sufficient for winter crops that don't need much water.

Spring and autumn are intermediate periods where you will frequently adjust. You can't get by with a fixed routine.

🪴 Type of crop and its developmental stage

Not all plants drink as much. Here's a real-world fact:

• 🍅 Tomatoes are very thirsty plants, especially during fruit formation. A mature plant in full production can absorb between 1 and 2 liters per day in mid-summer.
• 🌿 Aromatic herbs like basil or parsley appreciate cool soil, but hate having their feet constantly in water.
• 🥕 Carrots and radishes require regular but moderate watering, especially during the germination phase.
• 🍓 Strawberries in a greenhouse are particularly sensitive to excess moisture at the collar.
• 🌱 Seedlings are the most delicate: they must remain slightly moist at all times, but never soggy.

The developmental stage also changes everything. A freshly sprouted seedling does not have the same needs as a plant in full bloom. Young plants have a limited root system and cannot reach deep water. They depend entirely on your regularity.

🌍 Soil or substrate type

Clay soil retains water much longer than a sandy substrate or a very well-draining mix. In a pot or growing tray filled with potting soil, water evaporates quickly, especially if the container is small and exposed to heat. In a large in-ground growing bed, moisture is retained longer at depth.

Testing your soil's moisture before each watering is an essential habit. Simply push a finger 4 to 5 cm into the soil. If it's still cool, you can wait. If it's dry, it's high time to act. It's basic, but it's the most reliable method there is.

📅 Recommended watering frequency by season

Here's a summary table to help you get your bearings. These values are general guidelines, to be adapted to your situation.

These data correspond to standard crops in a well-insulated polycarbonate greenhouse. AtmoSerre greenhouses with 6 mm offer significantly superior thermal insulation compared to 4 mm, which better stabilizes internal temperatures and can slightly reduce watering frequency in winter. This is a detail that matters in the long run.

🛠️ Best watering techniques in a polycarbonate greenhouse

There are several ways to water in a greenhouse, and they don't all have the same effects on your plants. Here's what we truly observe in the field.

💧 Manual watering at the base of plants

This is the most common method among amateur gardeners, and it has its advantages. You control the exact amount of water supplied, you observe your plants at the same time, and you quickly detect problems (yellowed leaves, pests, diseases). It's also the ideal time to remove dead leaves or pinch your tomatoes.

The classic mistake: watering with a fine spray from above, onto the leaves. In a greenhouse, this promotes foliar humidity and thus fungal diseases like downy mildew or botrytis. Always water at the base, at the stem, directing the water directly to the soil.

Prefer a watering can with a thin spout or an adjustable watering wand that allows you to direct the flow precisely. 🪴

💡 Drip irrigation: the most effective technique

Drip irrigation is undoubtedly the most effective and recommended method for greenhouse watering. It delivers water directly to the roots, continuously or on a programmed schedule, without ever wetting the foliage.

The advantages are numerous:

• 💧 Significant water savings (up to 30 to 50% compared to manual watering according to INRAE studies)
• 🌿 Reduction of foliar diseases since leaves remain dry
• 📈 Better regularity of water supply, which is crucial to avoid deficiencies or water stress
• ⏱️ Considerable time saving, especially if you pair the system with a timer

At AtmoSerre, we offer an irrigation system specially designed to integrate into our polycarbonate garden greenhouses. This is the accessory we recommend first and foremost to anyone who wants to take their results to the next level.

Setting up drip irrigation takes a little time at first to lay the pipes and adjust the drippers. But once in place, you'll wonder how you ever managed without it. 🔥

🌧️ Overhead watering: to be avoided in greenhouses

Overhead watering consists of spraying water in a fine rain over the plants. Outdoors, it can be useful for certain crops. In a greenhouse, it's a bad idea in the vast majority of cases.

Here's why: in a closed greenhouse, the air is already laden with humidity. Adding water to the leaves creates an ideal environment for fungi and foliar diseases. Botrytis, downy mildew, powdery mildew... all these diseases love wet leaves in a warm, confined air. This is exactly what overhead watering produces.

There is one exception: light overhead watering in mid-summer to cool the atmosphere during high heat. But in this case, it must be done early in the morning, so that the leaves dry quickly during the day, and never in the evening.

🪣 Sub-irrigation and capillary action: for pots and containers

If you grow in pots or raised beds in your greenhouse, sub-irrigation is an interesting technique. The principle: water is placed in a reservoir under the pot, and the substrate draws it up by capillary action according to the plant's needs.

It's particularly suitable for herbs and certain leafy vegetables. It requires less monitoring and avoids humidity fluctuations. Some pots with integrated reservoirs can really simplify life in a greenhouse.

⏰ What time of day to water in a greenhouse?

The timing of watering is almost as important as the amount of water provided. This is a point that many gardeners underestimate.

Early morning, between 7 and 9 am, is the ideal time. Here's why it's truly the best time slot:

• 🌱 Roots absorb water better when temperatures start to rise
• ☀️ Water that accidentally touches the leaves has time to dry in the morning
• 💡 Plants have water available throughout the entire photosynthetic activity period
• 🌡️ The greenhouse is not yet overheating, so water doesn't evaporate immediately

Watering in the evening is a common mistake in a greenhouse. At night, temperatures drop and evaporation stops. Water remains in the soil and on the walls, which favors pathogens. We also avoid watering in the middle of the day in summer: water evaporates too quickly before even reaching the roots, and thermal shocks on hot soil can stress plants.

In winter, with well-insulated AtmoSerre greenhouses, morning watering is even more important: it allows excess moisture to dissipate during the mildest hours of the day. ❄️

🌿 The ideal water for greenhouse watering: it really matters!

We rarely think about it, but the quality of the water used for watering has a real impact on your crops.

Tap water is hard in many French regions. This lime gradually deposits in the soil, alters the pH, and can block the assimilation of certain minerals by plants. In the long term, this is a source of deficiencies that manifest as yellowed leaves or plants that "die" for no apparent reason.

Collected rainwater is by far the best option. It is soft, at room temperature, slightly acidic, and plants literally love it. If you have the option of installing a collection tank connected to your greenhouse or home, it's an investment that quickly pays off. 🌧️

If you have no other choice but tap water, let it sit overnight in a watering can or container before use. This allows chlorine to dissipate and the water to warm up to room temperature. Watering with cold water directly from the tap in summer can cause thermal shock to the roots.

The ideal watering water temperature is close to the soil temperature, generally between 15°C and 20°C. Neither too cold nor too hot.

❌ The 8 most common watering mistakes in a polycarbonate greenhouse

Here's what we observe most often. These mistakes are avoidable once you are aware of them. 🔥

1. 💦 Watering too often and too little at a time

This is the number one mistake among beginners. We water a little bit every day, the soil remains moist on the surface, but the deep roots remain dry. Result: plants develop shallow and fragile root systems.

What to do: water less often, but provide a significant amount of water each time. The goal is for the water to penetrate 15 to 20 cm deep. This encourages roots to grow deeper, making plants more robust and resilient to stress.

2. 🌿 Wetting the leaves when watering

We've mentioned this before, but it's worth repeating because it's so common. Wet leaves in a warm, confined atmosphere are the perfect breeding ground for diseases. Water only at the base, no exceptions. 🧑🌾

3. ❄️ Watering in the evening in winter

In the evening, temperatures drop, condensation forms on the walls, and already moist soil cannot evaporate. The result in the morning: soggy soil, suffocated roots, risk of rot. This is a common problem seen in greenhouses in winter.

4. 📅 Watering according to a fixed schedule without observing the plants

"I water every Monday and Friday": this kind of rigid routine does not take into account the weather reality, temperature variations, or the specific needs of each plant. A cloudy week in July does not require the same amount of water as a week with 35°C every day.

Observe your plants. Leaves that start to sag slightly in the middle of the day (temporary wilting) are a sign of early water deficiency. Yellow leaves on the lower parts can indicate an excess. Learn to read these signals.

5. 🔥 Neglecting ventilation when watering in summer

In summer, after abundant watering, the humidity in the greenhouse skyrockets. If the vents are closed, this humidity cannot escape, and conditions become favorable for diseases. Opening windows and the door after morning watering is a habit that must be adopted. 🌬️

This is also one of the reasons why we always emphasize the importance of good ventilation in our greenhouses. If you want to delve into this topic, our article on garden greenhouse ventilation will give you all the keys to optimize airflow in your growing space.

6. 🌱 Treating all seedlings the same way as adult plants

A seedling tray does not have the same needs as a 1.50 m tomato plant. Seedlings need constant, light, surface moisture. They cannot tolerate sudden changes. Heavy watering can drown seeds or small seedlings before they even have time to root.

For seedlings, it is best to use a fine mister or a watering can with a very fine rose, and check the moisture twice a day without hesitation.

7. 💧 Not considering the type of substrate

Potting mix for seedlings saturates with water much faster than soil in open ground. A gardener who waters a tray of potting mix with the same amount of water as a garden bed will quickly have problems with overwatering. Always adapt the quantity to the substrate and the container.

8. 🧪 Forgetting the impact of fertilizers and amendments on water needs

Soil enriched with well-rotted compost has better water retention and better-fed plants, which manage their water reserves more efficiently. Conversely, exhausted soil, without organic matter, drains very quickly. Regular soil amendment is inseparable from good watering management. It's a whole package.

🍅 Specific water needs depending on crops: the practical guide

Each plant has its own personality when it comes to hydration. Here's a summary of the major crop families often found in polycarbonate greenhouses.

🍅 Solanaceae: tomatoes, peppers, eggplants

These plants are very water-intensive, especially during fruiting. Regularity is crucial: erratic watering (alternating dry/soaked) is the primary cause of blossom end rot in tomatoes, those black spots at the end of the fruit that many gardeners have encountered.

• Watering: regular and deep, at the base only
• In summer: 1 to 2 liters per plant per day
• Tip: mulch the base of tomatoes to limit evaporation and maintain stable humidity 🌿

🥒 Cucurbits: cucumbers, zucchini, melons

These plants transpire heavily through their large leaves. They need constant but not excessive moisture. Cucumbers, in particular, are very sensitive to drought, which can cause bitter fruits.

• Watering: frequent (daily in summer)
• Never let the soil dry out completely
• Ventilate after watering to avoid excessive foliar humidity

🥗 Salads and leafy greens

Less demanding than solanaceous plants, but they need regular moisture, especially for germination. "Bolted" lettuce is often due to water stress.

• Watering: moderate and regular
• Ideal with low-pressure drip irrigation
• Avoid wet leaves, which promote botrytis

🌿 Aromatic herbs

Basil likes water but hates having its roots in soggy soil. Thyme and rosemary prefer drier soil. Knowing the needs of each species avoids many disappointments.

• Basil: regular watering, cool but well-drained soil
• Thyme, rosemary, sage: infrequent watering, let the soil dry between waterings
• Mint: heavy water consumer, monitor regularly

💡 Automating greenhouse watering: what you need to know

Watering automation is one of the questions we get asked most often. And the short answer: yes, it's definitely worth it, provided it's set up correctly.

A watering programmer coupled with a drip system allows you to define time slots and watering durations. You can go on vacation with peace of mind, your plants will continue to be watered correctly. It's a huge convenience.

But beware: automation does not replace observation. A clogged hose, a misplaced dripper, compacted substrate... these small problems go unnoticed if you don't check regularly. Automation frees up your time, but it doesn't exempt you from a daily glance.

Some tips for properly configuring an automatic system:

• 🕗 Program watering early in the morning (between 6 AM and 9 AM)
• ⏱️ Prefer several short waterings to a single long watering (better absorption)
• 📊 Adjust programming with each change of season
• 🧪 Test the system manually for the first few weeks before letting it run on its own

Moisture sensors connected to the soil are an interesting option to go even further: they trigger watering only when the soil truly needs it. This is the best of the best in terms of precision and water saving. 📈

🌱 Mulching and water retention in the greenhouse: a winning combination

Mulching is often underestimated in greenhouses. And yet, it is one of the simplest and most effective techniques to reduce watering frequency while improving plant health.

By covering the soil between your plants with mulch (straw, dead leaves, ramial chipped wood, bark...), you limit surface evaporation. Result: the soil stays cool longer, you can space out waterings, and weeds have difficulty germinating. 🌿

In a polycarbonate greenhouse, mulching offers an additional advantage: it regulates temperature fluctuations at ground level. Under a well-exposed greenhouse in summer, the surface of unmulched soil can reach 40°C or more, which stresses superficial roots. A 5 to 8 cm layer of mulch keeps the soil at a much more pleasant temperature.

Some rules for proper mulching in a greenhouse:

• 🌾 Recommended thickness: 5 to 8 cm
• 🌱 Leave a clear space around the collar of plants to prevent rot
• 🔄 Renew the mulch during the season if it decomposes too quickly
• 🍂 Avoid overly dense mulches on seedlings: they can hinder emergence

🔬 Relative humidity in the greenhouse: the number to watch

The relative humidity (RH) of the air is a parameter often overlooked by greenhouse gardeners. And yet, it is directly linked to watering and plant health.

The ideal for most greenhouse vegetables is an RH between 60 and 75%. Below 50%, plants transpire excessively, dehydrate faster, and can suffer even if the soil is moist. Above 85%, conditions become favorable for fungal diseases.

A simple hygrometer, which can be found for a few euros, allows you to monitor this parameter daily. It's an indispensable tool that we recommend to all our customers.

If humidity is too low in summer (which happens during high heat with lots of ventilation), you can:

• Lightly water the aisles (not the plants) 💧
• Place trays of water in the greenhouse that will evaporate naturally
• Lightly mist the foliage only in the morning and never in the evening

If humidity is too high (especially in autumn and winter), the priority is ventilation. Opening the greenhouse windows in mid-morning, even briefly, is often enough to bring the RH back to an acceptable range.

🏆 Which watering system for which greenhouse size?

The size of your greenhouse directly influences the most suitable watering solution. Here are our recommendations based on the most common configurations.

For a small greenhouse (6 to 12 m²): Daily manual watering is perfectly manageable. Investing in a good watering can with a long, fine spout is sufficient to start. If you often go away or lack time, a small drip system with a programmer remains the best option even on a small scale. 🪴

For a medium greenhouse (12 to 24 m²): Drip irrigation becomes truly beneficial at this stage. Manually watering an 18 or 24 m² greenhouse in the middle of summer, twice a day, is time-consuming and tiring. A well-designed irrigation system does the work for you with a precision that you cannot manually match.

For a large greenhouse (24 m² and more): At this scale, automation is no longer a luxury, it's a necessity. A complete system with a programmer, sectoral valves, and humidity sensors will allow you to manage different cultivation areas with specific needs. This is what professional market gardeners do, and it is entirely accessible to amateur gardeners who have invested in a beautiful structure.

Our L'Intemporelle greenhouse, available from 6 to 36 m², is designed to easily accommodate integrated irrigation systems. Its spacious structure allows for neat hose routing and easy access to all your plants. This is one of the reasons it is highly valued by gardeners who want to move to professional greenhouse management.

🌼 Fertilization and watering: two inseparable practices

You cannot talk about watering without mentioning fertigation, a technique that involves dissolving fertilizers directly in the irrigation water. It is a very effective method, especially when coupled with a drip system.

Fertigation offers several advantages:

• 💡 Nutrients are delivered directly to the roots, in an immediately absorbable form
• 🌿 Doses are very precise, which limits excesses (often problematic)
• 📈 Plant growth is generally more regular and faster
• 💧 It saves water and fertilizer compared to separate application

If you use liquid fertilizers in your greenhouse, integrate them into your watering routine. Generally, you fertigate once or twice a week during active growth phases, alternating with plain waterings to prevent the accumulation of mineral salts in the soil.

⚠️ One mistake to absolutely avoid: foliar fertilizers (sprayed on leaves) in a closed greenhouse. In a humid and warm atmosphere, this can cause burns or promote diseases. Reserve this practice for outdoors or ventilate heavily before and after.

🧑🌾 A few words about our approach at AtmoSerre

At AtmoSerre, we don't just sell greenhouses. We support gardening enthusiasts in their project to cultivate all year round, with complete peace of mind. Our polycarbonate garden greenhouses are designed to last, to withstand the most demanding climatic conditions, and to create the best possible environment for your plants.

The polycarbonate we use in all our greenhouses diffuses light uniformly, without creating shade or hot spots. This is crucial for evaporation and therefore for watering management: you have a more homogeneous, more predictable, and easier to manage environment on a daily basis. 🌱

Our greenhouses are also equipped with ventilation windows that play a direct role in humidity management. By correctly regulating the air inside, you better control evaporation and thus your plants' water needs.

If you are just starting out or looking to improve your practice, our team is available by email and phone to advise you. We know our greenhouses inside out, and we also know the questions you ask, because our customers ask us every day. 📞

❓ FAQ: Your questions about greenhouse watering, our concrete answers

🌱 How many times a week should I water a polycarbonate greenhouse?

It all depends on the season and the crops. In summer, with high temperatures, daily watering (or even twice a day) may be necessary for tomatoes or cucumbers. In winter, for winter crops, two to three waterings per week are often sufficient. Observing the soil remains the best guide: if the first 5 centimeters are dry, it's time to water.

💧 Can I water in the evening in a greenhouse?

No, this is a mistake to avoid. In the evening, temperatures drop and condensation sets in. The water supplied does not evaporate, which creates excess nocturnal humidity conducive to fungal diseases. Always prefer watering early in the morning, between 7 and 9 am.

🍅 Why do my greenhouse tomatoes develop black spots on the end of the fruit?

This is most likely blossom end rot, linked to calcium deficiency. But contrary to popular belief, it's not a lack of calcium in the soil; it's irregular watering that prevents the roots from assimilating available calcium. Regularize your watering and you will see rapid improvement in subsequent fruits.

🌿 What type of water should I use for watering in a greenhouse?

Rainwater is ideal: soft, at room temperature, slightly acidic. If you use tap water, let it sit overnight to remove chlorine and warm to room temperature. In very calcareous regions, a filter or water softener may be useful in the long term.

🛠️ Is a drip system easy to install in an AtmoSerre greenhouse?

Yes, absolutely. The structure of our greenhouses allows hoses to be run along the walls or on the ground without difficulty. Our Irrigation System available on our website is directly compatible with our greenhouses and can be installed in a few hours. It is the accessory investment that changes the daily life of gardeners the most, according to our customer feedback.

❄️ How to adapt watering in winter in a polycarbonate greenhouse?

In winter, plants have reduced water needs and the greenhouse retains moisture better. Space out waterings (every 3 to 7 days depending on crops), water early in the morning to allow evaporation during the day, and check soil moisture with your finger before each watering. Absolutely avoid soggy soil, especially at night, which causes root rot.

To go further on managing your greenhouse in winter, our article on garden greenhouse ventilation will give you valuable advice for controlling humidity and air flow even in cold weather. 🌬️

🌡️ What ideal relative humidity should be maintained in a garden greenhouse?

For most greenhouse-grown vegetables, a relative humidity between 60 and 75% is ideal. Below that, plants dehydrate. Above 80-85%, the risk of fungal diseases increases sharply. A hygrometer placed at crop height gives you this information in real time. It is an essential accessory for serious management of your growing space.

🌱 Conclusion: mastering watering means mastering your greenhouse

Watering in a polycarbonate greenhouse is a skill that is acquired progressively. In the first few weeks, you will inevitably experiment, observe, and adjust. This is normal and is even part of the pleasure of greenhouse gardening.

Remember the basics: water at the base, in the morning, deeply, regularly, and by constantly observing your plants. These five simple principles are enough to avoid 90% of the most common mistakes.

If you want to go further and truly optimize your results, invest in a drip system with a timer. This is the decision that transforms a beautiful greenhouse into a truly efficient production tool, all year round.

And if you are still looking for the ideal greenhouse to realize your gardening project, we invite you to discover all our models on AtmoSerre. Every greenhouse in our range is designed to last, to integrate harmoniously into your garden, and to give you the best possible conditions for growing everything you love. 🌼🔥

Happy growing to all! 🧑🌾🌿