Problems, pests, and how to stay ahead

Greenhouses and cold frames concentrate warmth, which means they also concentrate problems. The most common is poor ventilation. A greenhouse that stays above 27°C for hours becomes a stress chamber. Plants stop growing, aphids explode, and fungal diseases take hold. The solution is simple: open vents and doors before it gets hot, not after. If you cannot be home during the day, automatic vent openers are essential.

Red spider mite is the summer plague. They are almost invisible — tiny rust-coloured specks under leaves — but the damage is obvious: stippled, yellowing foliage that eventually browns and dies. They thrive in hot, dry air. The best prevention is humidity. Mist paths and staging with water on hot mornings; it raises humidity and makes the environment less hospitable. If you already have an infestation, spray with a fatty acid or plant oil solution every three days for two weeks. Biological control with predatory mites works but requires consistent temperatures.

Whitefly are slower but just as frustrating. They cluster under leaves, flying up in clouds when disturbed. They weaken plants and excrete sticky honeydew that attracts sooty mould. Yellow sticky traps reduce numbers but rarely eliminate them. The best control is biological: a parasitic wasp called Encarsia formosa, which lays eggs inside whitefly scales. Introduce it early, when you first spot whitefly, and it will keep numbers low all season.

Botrytis — grey mould — appears in cold, damp conditions, especially on winter salads and overwintering plants. It starts as soft brown patches, then develops a grey fuzz. Remove affected leaves immediately and increase ventilation. Water in the morning so foliage dries before nightfall. In a cold frame, prop the lid open for an hour each day, even in winter, to let moisture escape.

Damping off kills seedlings at the soil line. They topple over, blackened and rotten. It is a fungal problem caused by overwatering, poor air circulation, or reused compost. Always use fresh seed compost; water from below rather than overhead; sow thinly so seedlings are not crowded; and improve airflow by removing any covering as soon as the first shoot appears.