Key Takeaways -

• Mould thrives in greenhouses due to high humidity, poor airflow, and excess moisture.
• Common types include grey mould, sooty mould, and powdery mildew, all of which harm plant health.
• Prevent mould by improving ventilation, controlling humidity, and avoiding overwatering.
• Regular maintenance, proper spacing, and prompt removal of mouldy plant material help keep your greenhouse mould-free.

Mould is a common problem in greenhouses, especially in the UK, where high humidity and poor air circulation create the perfect conditions for it to grow. If left untreated, mould can harm plants, reduce yields, and even damage the greenhouse structure.

This article will explain why mould develops in greenhouses, focusing on moisture, overcrowding, and temperature changes. It will also cover the most common types of mould, such as grey mould, sooty mould, and powdery mildew, and how they affect plant health.

Keep reading to learn simple and effective ways to prevent and manage mould. By understanding what causes it and how to control it, you can keep your greenhouse healthy and your plants thriving.

Understanding Mould and Why It Grows in Greenhouses

Mould is a fungus that thrives in damp, humid environments, making greenhouses an ideal place for it to grow. It spreads through tiny airborne spores that settle on surfaces, quickly multiplying when moisture and organic matter are present.

Once mould takes hold, it can spread rapidly across plants, soil, and greenhouse structures, feeding on decaying leaves and plant tissues. Without proper ventilation and moisture control, mould spores continue circulating, creating ongoing problems for plant health.

Understanding how mould spreads is key to preventing its growth and protecting your greenhouse from fungal infections.

Why Greenhouses Provide the Perfect Conditions for Mould Growth

High Humidity and Moisture Retention

Greenhouses trap moisture, especially during colder months, creating a humid environment where mould thrives. When relative humidity rises above 80%, condensation forms on plants and surfaces, providing the perfect conditions for fungal growth (1).

Without proper humidity control, mould spores spread quickly, affecting plant health and greenhouse structures.

Poor Air Circulation

Limited airflow inside a greenhouse allows moisture to build up, leading to condensation on leaves, plant stems, and greenhouse surfaces. Stagnant air prevents this moisture from evaporating, creating damp conditions where mould can flourish.

Proper ventilation is essential to keep air moving and reduce humidity levels.

Overcrowding and Overwatering

Placing plants too close together restricts airflow, increasing humidity and making it easier for mould to spread to other plants. Overwatering keeps soil and plant surfaces damp, further encouraging fungal growth. Watering plants carefully and maintaining adequate spacing can help reduce the risk of mould.

Temperature Fluctuations

Mould thrives in temperatures between 20–25°C (68–77°F), especially when combined with high humidity. Sudden drops in temperature can cause condensation to form on plants, increasing moisture levels and creating an ideal environment for fungal growth.

Keeping temperatures stable helps prevent mould development.

Presence of Decomposing Organic Matter

Dead leaves, rotting plant material, and other organic debris provide food for mould, allowing it to spread quickly. Regularly removing fallen leaves and maintaining clean greenhouse conditions can help prevent mould from taking hold and spreading to healthy plants.

Identifying the Most Common Types of Greenhouse Mould

Grey Mould (Botrytis cinerea)

Grey mould appears as an ash-grey fungus on leaves stems, and fruits, feeding on plant tissues and extracting nutrients. It spreads rapidly through airborne spores, especially in damp conditions, causing wilting, decay, and plant death.

Once established, it can quickly infect nearby plants. Cucumbers, tomatoes, strawberries, and ornamental flowers are particularly vulnerable. Managing humidity and removing infected plant material is crucial to controlling its spread.

Sooty Mould

Sooty mould forms as a black, soot-like coating on leaves, developing on the sugary honeydew secreted by aphids, whiteflies, and other sap-feeding insects. While it doesn’t infect plants directly, it blocks sunlight, reducing photosynthesis and weakening plant growth.

Managing insect infestations and regularly cleaning leaves can help prevent sooty mould from spreading.

Powdery Mildew

Powdery mildew appears as a white or greyish powder on stems, leaves, and fruit. It spreads quickly in humid conditions, coating plant surfaces and restricting growth. Over time, it weakens plant structure, reduces yields, and can persist if not treated.

Maintaining good airflow and reducing humidity levels can help prevent its spread.

How Mould Affects Greenhouse Plants and Soil

Damage to Plants

Mould can cause visible damage to leaves, stems, and fruits, leading to yellowing, wilting, or premature leaf drop. Infected stems may become discoloured, and fruits can rot, reducing their quality. If left unchecked, mould weakens plants, stunts growth, and lowers yields.

It also makes plants more vulnerable to other fungal infections, further compromising their health. Early detection and prompt removal of infected plant material are essential to prevent the spread of mould and protect overall plant productivity.

Mould in Soil and What It Means

Soil mould is often caused by overwatering, poor airflow, and decomposing organic matter. While white, fluffy mould on the soil surface is usually harmless, grey or black mould can indicate more serious fungal issues.

These harmful moulds may damage plant roots, leading to poor growth and increased disease risk. Maintaining proper drainage, avoiding excess moisture, and regularly aerating the soil can help prevent mould-related problems in greenhouse soil.

Frequently Asked Questions

Does the Type of Greenhouse Material Affect Mould Growth?

Yes, the material of your greenhouse can influence mould growth. Wooden greenhouses are more prone to mould because wood absorbs moisture, creating an ideal environment for fungal development.

Metal and plastic structures are less likely to retain moisture, but mould can still form on greenhouse surfaces if humidity levels are high. Regular cleaning and proper ventilation help prevent mould growth, regardless of the material.

Is a Winter Greenhouse More Prone to Mould Growth?

Yes, winter greenhouses are more susceptible to mould due to increased humidity and reduced ventilation. Cold temperatures cause condensation to form on plants and surfaces, creating damp conditions where mould thrives.

Limited airflow during winter can further trap moisture inside the greenhouse. Using dehumidifiers, proper ventilation, and temperature control can help reduce the risk of mould growth.

When Is the Best Time to Water Plants in the Greenhouse to Keep Mould Away?

The best time to water plants is in the morning when warm air helps dry excess moisture throughout the day. Watering early allows plant surfaces and soil to dry before nighttime, preventing prolonged damp conditions that encourage mould growth.

Avoid watering in the evening, as cooler temperatures and stagnant moisture create an ideal environment for fungal spores. Ensuring proper air circulation also helps reduce humidity levels and mould risks.

Can Mould in a Greenhouse Be Harmful to Humans?

Yes, mould in a greenhouse can be harmful to humans, especially those with allergies, asthma, or weakened immune systems. Mould spores can become airborne and cause respiratory issues, skin irritation, or allergic reactions when inhaled or touched.

Prolonged exposure to mould in an enclosed greenhouse can worsen symptoms over time. Regular cleaning, ventilation, and mould prevention measures help create a safer environment for both plants and people.

Conclusion

Mould in a greenhouse is a common issue caused by high humidity, poor ventilation, and excess moisture. If left unchecked, it can harm plants, reduce yields, and even affect the structure of the greenhouse.

Understanding the causes of mould growth and taking proactive steps—such as improving airflow, controlling humidity, and maintaining proper watering practices—can help prevent and manage outbreaks.

Regular maintenance, prompt removal of infected plant material, and proper greenhouse hygiene are essential for keeping mould under control. By following these strategies, you can create a healthier environment for your plants and ensure long-term greenhouse success.

Sources -

1. https://hartley-botanic.co.uk/magazine/do-you-have-mould-in-your-greenhouse-uk/