How do I use a greenhouse properly?

A greenhouse extends the season at both ends and lets you grow crops that will not ripen outdoors. It also concentrates heat, humidity and pests, so it needs active management — an unventilated greenhouse in July is a genuinely hostile environment for plants.

What you'll need

• Greenhouse with opening vents
• Watering can
• Shade netting or shade paint (optional)
• Max-min thermometer (optional)
• Horticultural disinfectant (optional)

Step by step

1. Ventilate early and generously on sunny days.Open vents and the door before it heats up. Temperatures inside can climb far higher than most plants tolerate.
2. Fit an automatic vent opener if you are out during the day.They work on wax expansion, need no power, and prevent the most common greenhouse disaster.
3. Apply shading from late spring to late summer.Shade netting or shade paint on the outside. Direct midday sun through glass scorches leaves.
4. Water in the morning rather than the evening.Evening watering leaves humid air overnight, which encourages fungal disease like botrytis.
5. Keep humidity down by ventilating rather than by watering less.Plants still need water. Airflow is what manages humidity.
6. Damp down the floor on hot days to cool the air.Water on the path evaporates and cools, without wetting the foliage.
7. Check for pests weekly — greenhouses concentrate whitefly, red spider mite and aphids.Warm, still, protected conditions suit pests as much as plants. Biological controls work well in an enclosed space.
8. Clean the glass and structure thoroughly each winter.Wash the glass inside and out, and disinfect the frame and staging. It removes overwintering pests and disease and restores light levels.

Tips

• A max-min thermometer tells you what actually happened while you were out, which is far more useful than the current reading.
• Grow bags and containers dry out extremely fast in a greenhouse. Check them daily in summer, sometimes twice.
• Red spider mite thrives in hot dry conditions. Keeping humidity moderate by damping down suppresses it considerably.

Common mistakes

• Not ventilating enough — Closed greenhouses reach temperatures that scorch and kill plants within hours on a sunny day.
• Watering in the evening — It leaves humid, still air overnight, which is ideal for fungal disease.
• Skipping the winter clean — Pests and disease overwinter in the frame and staging, and dirty glass measurably reduces winter light.

Questions people ask