Damping Off With Seedlings.
Damping Off in Seedlings: How to Prevent This Common Problem
At GardenAdvice, one of the most common problems gardeners face when raising seedlings is damping off. This frustrating fungal disease can wipe out healthy-looking trays of seedlings almost overnight, particularly when conditions are cool, humid, and overcrowded.
A Real-Life Example
“I sow Lobelia ‘Crystal Palace’ every spring and germination is always good, but as soon as I prick the seedlings out they collapse. Within a fortnight they develop a fine white mould at the base and never produce a second set of leaves. What causes this?”
The answer? Damping off. It’s a fungal disease that attacks seedlings at soil level, causing them to rot at the base, wilt, and collapse. Once it takes hold, it’s almost impossible to save affected plants – but it is easy to prevent.
How to Prevent Damping Off
Damping off is caused by several soil-borne fungi (including Pythium, Rhizoctonia, and Fusarium), and prevention is far easier than cure. Here’s how to protect your seedlings:
1. Start Clean
- Wash all pots and trays thoroughly with hot, soapy water before use.
- Always use fresh, sterile seed compost.
2. Sow Thinly
- Avoid overcrowding seeds, which restricts airflow and promotes fungal growth.
- Prick out seedlings as soon as they produce their first true leaves.
3. Improve Airflow
- Once seeds have germinated, remove propagator lids or covers.
- Maintain a steady temperature and avoid excess humidity.
4. Water Wisely
- Water from below to avoid soaking the surface.
- Let the top layer of compost dry slightly between waterings.
5. Use Sulphur Dust
- At GardenAdvice, we recommend Vitax Yellow Sulphur Powder.
- Dust the compost lightly when sowing and again after pricking out.
- The sulphur acts as a gentle fungicide, helping to suppress fungal spores before they attack.
Information on Yellow Sulphur Powder here
What Happened to Cheshunt Compound?
For many years, gardeners relied on Cheshunt Compound, a fungicide containing copper sulphate and ammonium carbonate, to protect seedlings from damping off. However, it was withdrawn from sale following changes in pesticide regulations.
- The last legal sale date was in 2010, and the final permitted use date was November 30, 2011.
- It is no longer approved for use in the UK or EU.
- Gardeners should not use any remaining stock, as it is now considered an unauthorised product.
Fortunately, safer and more environmentally friendly alternatives like sulphur powder are widely available and effective when used correctly.
Why Don’t Commercial Growers Get Damping Off?
Most commercial growers raise seedlings in sterile, temperature-controlled environments with advanced hygiene protocols. Many buy in plug plants raised under laboratory conditions. While that may not be practical at home, following the simple steps above – especially sulphur dusting – can help you achieve professional results in your own
