section heading hidden

What Damping Off Looks Like — and Why It Happens

You sow a tray of tomatoes or sweet peas, they germinate beautifully, and then within a few days the seedlings flop over at soil level as though someone snipped them with scissors. The stems look pinched, water-soaked and dark, and sometimes a faint fuzz of mould appears on the compost surface. That is damping off, and it is one of the most disheartening problems in the small garden or allotment greenhouse.

It is not a single disease but a group of soil-borne fungi and fungus-like organisms — Pythium, Rhizoctonia, Phytophthora and sometimes Botrytis — that are always present in low numbers and only cause trouble when conditions favour them. In practice, damping off is a management problem, not a mystery. Get the three fundamentals right — cleanliness, fresh compost and moving air — and you will very rarely see it.

Start With Scrupulously Clean Containers

Reused seed trays are the single most common source of infection. Old compost clinging to the corners, a ridge of dried algae along the drainage holes, a stack of pots that has sat in a damp shed since last spring — all of it carries spores over from one season to the next.

• Wash every tray, module, pot and propagator insert in hot soapy water, using a small brush to get into the corners and drainage slots.
• Follow with a disinfecting soak: roughly one part thin household bleach to nine parts water, or a proprietary garden disinfectant used at the label rate. Ten minutes is plenty.
• Rinse thoroughly in clean water — bleach residues burn young roots — and dry in full sun or a warm, airy place before stacking.
• Never stand trays directly on bare, previously used soil. Use a clean bench, a gravel tray or a sheet of plastic.

Wooden seed boxes and terracotta pots are porous and much harder to clean properly, so keep them for established plants rather than for germinating seed.

Use Fresh Seed Compost — and Treat It Gently

Seed compost is formulated to be low in nutrients, free-draining and finely textured, and it is usually sterilised during manufacture. That sterility is your best defence, so don't squander it. Never reuse compost from last year's pots, never mix in garden soil, and don't bulk it out with old grow-bag contents however tempting it looks.

• Buy fresh each season and store it sealed, off the ground, in a dry place. A split bag left open in a damp shed will grow mould before you have sown a single seed.
• Peat-free seed composts work perfectly well but often need a little more care with watering, as they can hold moisture unevenly.
• Bring the compost to room temperature a day before sowing. Cold, clammy compost straight from an unheated shed is a gift to fungal problems.
• Firm it lightly — just enough that the surface doesn't slump when watered. Over-compacting shuts out the air that roots and beneficial microbes need.

Some gardeners swear by a thin top-dressing of vermiculite or horticultural grit over the seeds. It keeps the surface drier, reflects light and makes it much harder for mould to establish. A weak chamomile tea used for the first watering is a traditional and harmless extra precaution.

Water From Below and Keep the Surface Dry

Damping off thrives in wet, stagnant conditions, and the biggest culprit is overhead watering from a cold tap. Water sitting in the leaf axils and splashing onto the compost surface spreads spores from plant to plant in seconds.

• Stand trays in a shallow bath of water and let them drink from the base for ten to twenty minutes, then drain fully. The surface stays comparatively dry while the roots get what they need.
• Use water at room temperature. Fill your can or jug the day before and let it stand so the chill and any chlorine dissipate.
• Never leave trays standing in a puddle. Empty saucers and gravel trays within half an hour.
• Water in the morning, not the evening, so any surplus has the day to evaporate.

If the compost surface is already green with algae or dotted with mould, that is a signal you are watering too often, not that you need a fungicide.

Improve Air Circulation

Fungal spores need still, humid air to settle and germinate. A closed propagator on a warm windowsill is the perfect incubator — for the seedlings and for the problem.

• Remove the propagator lid the moment the first seedlings show. Those few extra days of "help" cause more losses than enough cold nights ever will.
• Wipe condensation off lids daily while you are waiting for germination, and prop lids open at an angle once the majority are through.
• Space trays out on the bench rather than crowding them together, and don't stack them in a corner behind other pots.
• On still days, open the greenhouse door or window for an hour, or run a small fan on its lowest setting a metre away — not blasting the seedlings, just stirring the air.
• Sow thinly and thin promptly. Seedlings whose leaves touch stay damp for longer and pass problems along the row.

A Quick Prevention Checklist

Before you sow, run through this in your head:

• Containers washed, disinfected and dried.
• Fresh seed compost, stored dry and brought to room temperature.
• Clean water at ambient temperature, applied from below.
• Lids off at first germination, condensation wiped daily.
• Trays spaced out with moving air around them and good light to keep growth sturdy.

If you do lose a patch — and most of us do at some point — remove the affected seedlings and the compost immediately rather than trying to save them. The healthy ones on the edge of the tray often pull through. Many gardeners find that thinning the remaining seedlings, dusting the surface with a little dry vermiculite and moving the tray somewhere cooler and airier stops the trouble dead. It is a small setback, not a lost season.