Greenhouse Gardening – infestations

Once you get an infestation in the greenhouse it seems to spread like wildfire, so the best way to avoid them is to spring clean the greenhouse before sowing seeds. Thorough cleaning can be done either in winter or early spring, when there isn’t much going on in the greenhouse because you’ll need to move all the plants around to eradicate any weeds, dead plants or rotting leaves, wash all the glass with warm soapy water and if you had pest and disease problems last year, scrub down the staging and pots with garden disinfectant. You can also use weed treatment on the paths or decking at this point, or re-stain wood.

When you begin to plant again, ensure you use fresh compost and clean (washed and sterilised or new) 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. Mature plants prefer to be watered from a rain butt, so you need to keep seedlings and mature plants separated.

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 - fungal diseases like mildew and mould are less likely to build up if the air is moving, although avoid creating draughts which can harm plants too.

In summer when the greenhouse gets hot and dry, plants can get stressed and spider mite populations can build up rapidly. 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, or made of sand or gravel.

Greenhouse gardening infestation photograph by L. Marie, used under a creative commons attribution licence