How do I disinfect a greenhouse after a disease outbreak?

After a bad year of botrytis, mildew, damping off or a soil disease, an ordinary clean is not enough. Spores and resting bodies survive in debris, in the aluminium channels, on canes, in seed trays and in the top layer of border soil. Disinfection is genuinely worth doing — but it is important to know that it cleans surfaces, and does very little to soil.

Safety

What you'll need

• Horticultural disinfectant, true
• Gloves and eye protection, true
• Stiff brushes and a sponge, true
• Bin bags for debris, true
• Thin plastic labels for the pane overlaps

Step by step

1. Identify what you are dealing with first.Airborne fungal problems such as botrytis and mildew live on debris and surfaces and respond well to a clean and disinfect. Soil-borne problems such as root rots, wilts and eelworm live in the border soil and are not solved by surface disinfection.
2. Choose the right time.Autumn, once the crop is out and before winter occupants go in. The greenhouse needs to be empty and it needs several dry hours to air afterwards.
3. Strip it completely and bag the debris.Everything out. All plant material, fallen leaves, old string, spent growbags and dead roots go into bags for the council green waste, not the compost heap. This step removes most of the inoculum before you use any product.
4. Wash before you disinfect.Disinfectants do not work through dirt. Brush down dry, then wash all surfaces with warm soapy water and rinse. Only then apply the disinfectant.
5. Apply the disinfectant to the label.Dilution rate, contact time and any rinsing instruction exactly as stated. Glass, frame, staging, floor, path and the inside of the door. Contact time matters more than scrubbing — most products need to stay wet on the surface for a stated number of minutes.
6. Get into the hiding places.The glazing bar channels, the joints in the staging, the pane overlaps, the corners of the base and the door track. These hold debris and are where spores overwinter.
7. Deal with pots, trays and canes.Wash and disinfect every pot and seed tray, or discard the cheap ones. Canes are a common carrier and are hard to clean properly — for a bad outbreak, discard them.
8. Replace border soil if the problem was soil-borne.Excavate to about 30cm and replace with fresh topsoil and compost, or switch to growing in pots, ring culture or growbags. This is the only reliable answer to soil sickness and root diseases in a greenhouse border.
9. Air it thoroughly and leave it empty.Vents and door open for at least a day. Let every surface dry completely before anything goes back in.

Tips

• Sulphur candles were once the standard greenhouse fumigant. Modern availability for amateur use is limited and varies — check what is currently approved for garden use rather than relying on old advice, and never burn one in a greenhouse containing plants.
• The most valuable single step is removing debris. If you do nothing else, strip the house completely and bag everything.
• Keep a note of which crop had which disease and where it was standing. Greenhouse disease problems are usually repeated in the same spot with the same crop.

Common mistakes

• Disinfecting over dirt and debris — Organic matter deactivates most disinfectants. Cleaning first is not optional preparation, it is what makes the product work at all.
• Expecting disinfectant to fix a soil problem — Surface disinfectants do not penetrate soil in any meaningful way and would kill the soil biology if they did. Soil-borne disease means replacing the soil or growing in containers.
• Mixing products for extra effect — Mixing bleach-based and acidic products releases chlorine gas. Never mix garden chemicals, and never use a product for a purpose not on its label.
• Composting the debris — A domestic compost heap does not reliably reach the temperature needed to kill fungal spores and resting bodies, so you spread the problem across the garden.

Questions people ask