A greenhouse is a wonderful addition to almost any garden. It enables you to grow tender plants earlier, ripen crops that need more warmth and keep harvesting later in the season. Unlike other garden protection such as cloches and row covers which need removing whenever you want to weed or water, greenhouses make tending your plants much easier. However, it’s not just plants that enjoy the protection of a greenhouse – pests and diseases love them too, avoiding the harsh weather and frosts that usually kill them off! So how do you stop them from enjoying a sheltered existence where they can strike at your new sown plants? The answer is a good spring clean...
Cleaning has never really been my strong point (as my wife would gladly tell you) but when it comes to the greenhouse it is an essential part of the gardening year. Once Christmas is out of the way and days start to lengthen, cleaning the greenhouse is a great way to feel connected with the garden again, even if the temperatures are too low to be sowing seeds.
![Jeremy in his greenhouse]()
So what are the pests and diseases that try to invade the greenhouse?
- Slugs: The number one pest for gardeners, they love to hibernate in hidden cracks, under old pots and any damp sheltered places. As soon as Spring arrives you can be sure that young seedlings will be their favourite snack if they are left.
- Aphids: These lay eggs before the winter becomes harsh, often on existing plants. So it is important to remove all old plant material, inspect the underside of leaves of overwintering plants and check that there aren’t any weeds or leaves under benching. Aphids are one of the main ways that viruses are transmitted between plants.
- Red spider mite: The hot dry conditions in greenhouses over summer are ideal for red spider mite which multiply rapidly, reducing the crop of many plants. The tiny red female insects can overwinter on greenhouse walls and pots so it is important to clean them out thoroughly.
- Mealy bug: Colonies of mealy bugs resemble blobs of sticky cotton wool and are largely a greenhouse pest, sucking the sap from plants like aphids. They can often be found under the rim of pots or similar places, which need to be cleaned thoroughly.
- Mildew: Good hard frosts will usually kill off mildews but they can survive on old plant material, particularly the leaves of cucurbits such as cucumbers.
- Botrytis (Grey Mould): This fungal infection can attack the foliage of plants left in damp conditions over winter. It is important to fully ventilate the greenhouse and remove all dead plant material to prevent this.
- Blight: The spores of blight infection can easily spread to tomatoes and potatoes from plant debris left in the greenhouse or garden, so it must be removed and disposed of or burnt.
![Pots outside a greenhouse in winter]()
The solution to eradicating these potential problems is to do a deep clean of the greenhouse before the new sowing season begins. I usually follow these steps:
- Remove everything from the greenhouse. Slugs can hide anywhere and the best way to find them is to take out any removable benching, pots, tools etc so that every corner can be reached. All old plant material should also be removed to the compost heap, or burnt if it suffered any diseases last season. This year I found at least 15 large slugs hiding in the gap of a breeze-block which was propping up my bench. Needless to say, they were removed far away! I also leave out a beer filled slug-trap for a few weeks before sowing new plants as an extra precaution and always start the most tender plants off in pots on a bench.
- Clean the glass. Light levels in the UK are usually poor around early spring and plants need as much sunlight as possible if they are to grow well without becoming straggly. It’s amazing just how much algae can build up in the warm moist conditions, so a good clean inside and out with warm water and a sponge is essential. Some people like to add some disinfectant but if you do, make sure that it is based on natural ingredients and is suitable for use near plants – many garden catalogs sell such products. Use an old plastic plant label to reach the grime that forms between glass pains. [Don’t forget to switch off any electrical supply before you start using water!]
- Scrub the surfaces: Again, all traces of mildews, grime and pests must be removed from any staging where your plants will be placed. Any capillary matting should usually be replaced. The greenhouse should then be well ventilated to allow everywhere to dry.
- Clean Pots and Trays: Emptying out all compost, I usually leave these a few days to dry out so that most of the dirt can be ‘dusted out’ before rinsing them.
- Replenish Soil: If, like me, you grow tomatoes in the same greenhouse bed every year then it is all too easy for the soil to become depleted of nutrients or diseases, such as blight, to persist. The best solution is to dig out some of the spent compost each year and replace it with a fresh supply. I usually cover this with a layer of sterilised potting compost/soil to keep weeds at bay.
Many people heat their greenhouses and over-winter tender plants in them. In this case you will need to wait until the milder weather in order to do a cleanout, or find somewhere sheltered for the plants to stay while you work. However, I like to completely empty the greenhouse and give it a few weeks of frosty ventilation to finally sterilize everything. It’s not the most enthralling task in the gardening year but there is a certain satisfaction in knowing that the greenhouse, my operations centre for the new season’s growth, is well prepared for the coming year. If only I could get such satisfaction from indoor cleaning...!
