Greenhouse Ventilation and Shading

Make heat management part of the daily greenhouse routine. Decide how you will check conditions and open the structure before a sunny spell arrives, especially if you will be away during the day.

By Simon Adams · Updated 25 October 2026

Read conditions inside

Use a maximum–minimum thermometer near crop height, protected from direct sun. Outdoor forecasts help you prepare, but do not show the temperature among your plants. Note the highest reading alongside the weather, vent positions and how plants looked. Aim to learn which conditions cause problems in your structure.

Give warm air a way out

Use roof vents, side vents and the door to create airflow. Automatic openers are useful, but respond with a delay; check them and provide other ventilation when necessary. Keep supports and plants clear of openings. Adjust ventilation to the crop and conditions rather than closing everything simply because it is not summer.

Add shade you can adjust

External shading intercepts sunlight before it enters. Use only the shade needed, then reduce it as conditions cool. Check compatibility before applying shading paint to polycarbonate or acrylic panels. Do not let blinds or netting obstruct vents.

When choosing a system, consider how you will secure it, remove it and reach the openings underneath it. A cheaper arrangement is only useful if it can be managed safely and reliably. Test the change and compare the next sunny day’s readings.

Look at the crop as well as the thermometer

Garden Organic identifies excessive greenhouse heat as a common cause of tomato greenback, where the stalk end fails to ripen properly. Its advice combines ventilation, shading and regular watering and feeding. Do not assume that extra fertiliser will solve a problem caused by the growing environment.

Keep a photograph and a short note when something goes wrong. Record the variety, affected part of the plant and recent conditions. That gives you useful evidence when comparing a symptom with a crop guide.

Avoid damp, congested growth

Grey mould thrives where plants are humid and overcrowded. Leave space between plants, remove decaying material promptly and improve airflow. Inspect fuzzy grey-brown growth on dying leaves, flowers or fruit rather than leaving it among healthy plants. Ventilation helps reduce the conditions that favour disease, but cannot replace removing infected material.

Make a plan for days away

Write down who will open and close the greenhouse, check water and respond to a hot forecast. If automatic equipment is doing the job, test it before relying on it. Your routine should cover a changeable spring day as well as a predictable summer heatwave.