Controlling greenhouse pests and diseases

The easiest way to begin is to start off with a clean greenhouse before sowing seeds. Cleaning can be done in winter or early spring, when there isn’t much else to do in the garden. This involves removing any weeds, dead plants or rotting leaves then wash the glass with warm soapy water and if you had pest and disease problems last year, scrub down staging and pots with garden disinfectant.

Make sure you use fresh compost and clean pots and to avoid spreading soil disease, water seeds and seedlings with tap water from a clean watering can rather than water stored in a water butt – these steps help avoid damping off, a fungal disease that causes seedlings to collapse and die.

In the greenhouse you can control whitefly either with contact insecticides or by introducing a biological control such as encarsia formosa, but you can't mix the two methods as the biological control will be wiped out by the chemical one.

As the weather gets warmer, open the greenhouse windows and door to get the air circulating through the greenhouse. Fan heaters that blow cool air around are also useful as fungal diseases like mildew and mould are less likely to build up if the air is moving.

In summer when the greenhouse gets hot and dry, plants can get stressed and spider mite populations can build up rapidly. Fine webbing and yellow or bronze speckling on the leaves are a sign of spider mite. Where possible, plants should be moved outside in the summer. For those that remain, prevent spider mite by misting plants with water using a hand-held sprayer to keep the humidity high. You can also water the floor if it is paved, sand or gravel.