Do I need to clean pots and seed trays between seasons?

The reason to clean is specific. Damping-off organisms, vine weevil eggs, fungus gnat larvae and moss and algae spores all overwinter in the crevices of dirty trays, and seedlings are the least able to withstand any of them. An established shrub going into a pot with a bit of last year's soil on it does not care.

Safety

What you'll need

• Stiff brush
• Warm water and washing-up liquid
• Bucket or trug
• Garden disinfectant (optional)
• Old towel or rack to dry on (optional)

Step by step

1. Prioritise rather than doing everything.Seed trays, module trays, small pots for pricking out and anything used for cuttings — clean these properly. Large pots for shrubs and perennials — knock the soil off and move on.
2. Do it in autumn, not in spring.In November there is time and mild weather. In March you want to be sowing, and dirty trays get used because the alternative is delay.
3. Knock off the dry soil first.Most of the work is mechanical. A tap on a hard surface and a brush removes the bulk before anything gets wet.
4. Wash in warm soapy water with a brush.A stiff brush into the corners and around the rim. Physical removal of the soil film is what actually reduces disease carry-over, and this alone deals with most of it.
5. Pay attention to the crevices.The rim, the join between cells in a module tray, and the drainage holes. That is where eggs and spores sit and where a quick rinse never reaches.
6. Use a disinfectant only where it is justified.For trays that carried a damping-off outbreak, or pots from a plant that died of a soil-borne disease, a horticultural disinfectant used at the label rate is worthwhile. For general cleaning it adds little over soap and a brush.
7. Rinse thoroughly and dry properly.Residue harms seedlings. Rinse in clean water and let everything dry fully in air and light before stacking, because stacked damp trays grow mould.
8. Treat terracotta differently.Scrub off the white salt deposits with a brush and water, and soak stubborn pots in water rather than in a strong chemical, because terracotta absorbs whatever you soak it in and releases it to the next plant.
9. Inspect as you go.Cracked trays, brittle sun-degraded plastic and pots with blocked drainage holes are worth discarding now rather than discovering in March.
10. Store them dry and out of the light.Ultraviolet degrades plastic. Trays stacked in a shed or under a bench last many years; trays left outdoors last two.
11. Look for vine weevil grubs when you empty pots.Fat, creamy-white, C-shaped grubs with brown heads. Finding them at emptying time is the easiest control there is, and the compost from that pot should not be reused in a container.

Tips

• A bucket of warm soapy water and an hour on a mild autumn afternoon covers a whole season's trays.
• It is the brush that does the work, not the disinfectant.
• Wooden seed trays and labels need scrubbing rather than soaking, and old wooden trays are worth replacing if they are soft and rotting.

Common mistakes

• Soaking everything in bleach as a matter of routine — It is unnecessary for general cleaning, it degrades some plastics, and residue harms seedlings. Save disinfectant for trays with a known disease history.
• Reusing seed trays with last year's compost still in them — Damping-off organisms and fungus gnat larvae carry over, and seedlings are the most vulnerable thing you grow.
• Stacking trays while damp — They grow mould between the layers and you unstack a worse problem than you put away.
• Mixing cleaning products — Bleach with anything acidic or ammonia-based releases toxic gas. Use one product, follow its label, and ventilate.

Questions people ask