How do I get the most from a greenhouse?
Ventilation, shading and hygiene — the three things that determine whether a greenhouse works or cooks.
- Difficulty
- intermediate
- Time
- 1 hr 30 min
- Read
- 2 min
Short answer
Ventilation is the main thing. A closed greenhouse in sun reaches temperatures that kill plants within hours, and trapped humid air causes most greenhouse diseases. Open it early and shade it in summer.
A greenhouse extends the season at both ends and makes tender crops possible. It also concentrates heat, humidity and pests, so managing those is most of the work.
What you'll need
- Thermometer
- Shading or shade paint (optional)
- Automatic vent opener (optional)
Step by step
- Ventilate generously, and earlier than feels necessary.A closed greenhouse can exceed 40°C on a sunny spring day. Open vents and the door in the morning rather than waiting until it feels hot.
- Fit an automatic vent opener.It works on a wax cylinder with no power and opens as the temperature rises. It is inexpensive and it prevents the most common greenhouse disaster.
- Shade in summer.Shade paint, netting or blinds. Without it, midsummer temperatures scorch leaves and stop fruit setting.
- Keep a maximum-minimum thermometer.It tells you what actually happened while you were out, which is usually more extreme than you expect.
- Water in the morning rather than the evening.It gives foliage time to dry, which substantially reduces fungal disease in the humid greenhouse environment.
- Clean it thoroughly once a year.Glass, frame and staging, in autumn or winter. It removes overwintering pests and disease and lets in far more light.
- Watch for pests constantly.Warm still conditions with no predators suit red spider mite, whitefly and aphids. Check weekly — early intervention is much easier.
- Use it through winter as well.Overwintering tender plants, hardy salads, forcing early crops. A greenhouse left empty in winter is wasted space.
Tips
- Biological controls — introduced predatory insects — work particularly well in a greenhouse because the environment is enclosed.
- Insulating with bubble wrap in winter reduces heat loss substantially if you are keeping anything frost-free.
- A water butt fed from the greenhouse roof provides rainwater at ambient temperature, which plants prefer to cold tap water.
Common mistakes
- Not ventilating early enough — A closed greenhouse can exceed 40°C within an hour of sun. Plants are damaged or killed before anyone notices.
- Watering in the evening — Foliage stays wet overnight in humid enclosed air, which is ideal for fungal disease. Morning watering avoids it.
- Never cleaning the glass — Algae and grime substantially reduce light transmission, and the frame and corners harbour overwintering pests.
Questions people ask
How hot can a greenhouse get?
Well over 40°C on a sunny day if unventilated, even in spring. That is enough to kill plants within hours, which is why an automatic vent opener is worth fitting.
When should I shade a greenhouse?
From late spring through summer, using shade paint, netting or blinds. Without it, high temperatures scorch foliage and prevent fruit setting on tomatoes and peppers.