Protecting Greenhouse Plants During a Heatwave

Illustration of staging and potted crops inside a greenhouse

Affiliate disclosure: As an Amazon Associate I earn from qualifying purchases. This article contains affiliate links.

Protecting greenhouse plants during a heatwave requires a three-pronged approach: increasing ventilation, implementing shading, and managing humidity. The aim is to limit damaging leaf temperatures while keeping roots adequately moist. Around 27°C is an RHS prompt to check overheating risk, not a universal damage threshold for every crop. Because greenhouses trap solar radiation, they can quickly become lethal environments for tender plants if air circulation and light levels are not actively managed.

Table of Contents

Diagnosing heat stress in greenhouse plants

Before applying interventions, it is important to recognise the signs that your plants are struggling with excessive heat. Heat stress often manifests in several distinct ways:

Sun-flag: This is a partial collapse of the plant where leaves droop or fold, even if the soil is moist.
Leaf scorch: Brown or bleached patches appearing on the leaves where they have been directly exposed to intense sunlight.
Desiccation: The drying out of tender young shoots and new growth, which are more vulnerable than established foliage.
Wilting: This occurs when the rate of transpiration (water loss through leaf pores) exceeds the rate at which roots can absorb water from the soil.

To monitor these risks, use a maximum-minimum thermometer. This allows you to track the peak temperatures reached during the day, helping you decide when to increase shading or ventilation measures.

Maximising ventilation and airflow

Air movement has a direct cooling effect on leaf surfaces and helps replenish gases absorbed by plants. In a heatwave, all available openings should be utilised.

Primary ventilation points

Most greenhouses have three main areas for air exchange: the doors, the roof vents (ridge vents), and the side vents (often louvred). Roof vents are the most effective because hot air rises; side vents are generally less efficient.

Advanced ventilation tips

Automatic openers: These use wax cylinders that expand when heated to push vents open. However, because wax takes time to expand, you should keep doors or side vents open to prevent a temperature spike before the automatic vents respond.
Pane removal: If existing vents are inadequate, ask the greenhouse supplier about safe additional ventilation. Remove panes only where the structure permits it and safe handling and storage can be arranged; do not improvise with roof glass.
Night ventilation: If temperatures remain high after sunset, leave vents and doors open overnight.

Implementing shading strategies

Ventilation alone is often insufficient during a heatwave. Shading reduces the amount of solar energy entering the structure, helping to control temperature without assuming one fixed range suits every crop. However, because plants need light for growth, you should use the minimum amount of shade necessary.

Shading options and their effects

Method	Placement	Cooling Effect	Key Consideration
External Blinds	Outside glass	Maximum	Prevents rays from entering; can be expensive.
Internal Blinds	Inside glass	Moderate	Easier to automate; heat still enters the glass.
Shade Netting	Inside/Outside	Moderate	Polyethylene mesh; cost-effective but degrades over time.
Shading Paints	Outside glass	High	Applied in spring; washed off in autumn.

When choosing a method, consider your plant types. Sun-loving plants, such as succulents, may not require shading, whereas edible crops like tomatoes need a balance to ensure they receive enough light for fruiting while avoiding scorch.

Managing humidity and damping down

Humidity refers to the concentration of water vapour in the air. When humidity is low, plants transpire more quickly, increasing the risk of wilting. Maintaining humidity helps cool the environment and protects plants from heat stress.

The process of damping down

Damping down involves wetting the hard surfaces of the greenhouse, such as paths and sitting areas, rather than just watering the plants. This increases the local humidity and lowers the ambient temperature through evaporation.

Frequency: Damp suitable hard surfaces only as needed for crops that benefit from extra humidity. Avoid slippery puddles or persistent dampness, and keep water clear of electrical equipment.
Water choice: Use suitable clean water; do not spread washing-up water or other contaminated grey water over greenhouse paths.
Pest control: Increasing humidity through damping down can also reduce the risk of certain pests, such as the glasshouse red spider mite, which thrives in dry conditions.

Watering and plant spacing during heatwaves

Plants with adequate moisture at the roots are significantly more resilient to heat because transpiration provides a natural cooling effect for the leaves. If the roots are dry, this cooling mechanism fails.

Watering requirements

Check greenhouse plants at least daily during hot weather and more often for small pots and seedlings in extreme heat. Check the compost before adding water; wilting plants can also have saturated or damaged roots. Ensure that water reaches the root zone effectively to support the plant’s cooling process.

The importance of spacing

As plants grow, they should be given more space. Overcrowding restricts airflow around individual plants, creating pockets of stagnant, hot air and increasing the risk of fungal diseases like grey mould. Proper spacing ensures that the ventilation provided by open doors and vents actually reaches the foliage.

Summary of heatwave actions

To maintain a healthy greenhouse during a heatwave, follow this priority list:

Monitor: Check the max-min thermometer; ventilate before heat builds up and use crop-specific temperature guidance.
Ventilate: Open all doors, side vents, and roof vents.
Shade: Deploy blinds or netting to block direct sun while maintaining minimum light for growth.
Humidify: Use damping down only where appropriate for the crops and conditions; avoid persistent wetness.
Hydrate: Water according to root-zone moisture and crop needs, allowing excess to drain.

Useful products for this job

The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.

Maximum-minimum thermometer: Essential for monitoring peak temperatures to determine when shading and ventilation are required. Ensure it is designed for outdoor/greenhouse use and has a clear scale for tracking daily highs and lows. Search Amazon for Maximum-minimum thermometer.
Automatic vent opener: Ensures roof vents open automatically as temperatures rise, even when the gardener is absent. Check the compatibility with your specific vent frame and the temperature setting at which the wax expands. Search Amazon for Automatic vent opener.
Greenhouse shade netting: A cost-effective way to reduce light intensity and lower internal temperatures. Verify the percentage of shade provided to ensure plants still receive enough light for growth. Search Amazon for Greenhouse shade netting.

Related garden guides

Sources and further guidance

Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.